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Detect HTTPS interception (#1430)
* WIP: Implement HTTPS interception detection by Durumeric, et. al.
Special thanks to @FiloSottile for guidance with the custom listener.
* Add {{.IsMITM}} context action and {mitm} placeholder
* Improve MITM detection heuristics for Firefox and Edge
* Add tests for MITM detection heuristics
* Improve Safari heuristics for interception detection
* Read ClientHello during first Read() instead of during Accept()
As far as I can tell, reading the ClientHello during Accept() prevents
new connections from being accepted during the read. Since Read() should
be called in its own goroutine, this keeps Accept() non-blocking.
* Clean up MITM detection handler; make possible to close connection
* Use standard lib cipher suite values when possible
* Improve Edge heuristics and test cases
* Refactor MITM checking logic; add some debug statements for now
* Fix bug in MITM heuristic tests and actual heuristic code
* Fix gofmt
* Remove debug statements; preparing for merge
This commit is contained in:
Matt Holt
GitHub
@@ -0,0 +1,560 @@
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package httpserver
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import (
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"bytes"
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"context"
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"crypto/tls"
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"io"
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"net"
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"net/http"
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"strings"
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"sync"
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"time"
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)
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// tlsHandler is a http.Handler that will inject a value
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// into the request context indicating if the TLS
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// connection is likely being intercepted.
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type tlsHandler struct {
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next http.Handler
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listener *tlsHelloListener
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closeOnMITM bool // whether to close connection on MITM; TODO: expose through new directive
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}
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// ServeHTTP checks the User-Agent. For the four main browsers (Chrome,
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// Edge, Firefox, and Safari) indicated by the User-Agent, the properties
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// of the TLS Client Hello will be compared. The context value "mitm" will
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// be set to a value indicating if it is likely that the underlying TLS
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// connection is being intercepted.
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//
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// Note that due to Microsoft's decision to intentionally make IE/Edge
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// user agents obscure (and look like other browsers), this may offer
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// less accuracy for IE/Edge clients.
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//
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// This MITM detection capability is based on research done by Durumeric,
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// Halderman, et. al. in "The Security Impact of HTTPS Interception" (NDSS '17):
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// https://jhalderm.com/pub/papers/interception-ndss17.pdf
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func (h *tlsHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
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h.listener.helloInfosMu.RLock()
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info := h.listener.helloInfos[r.RemoteAddr]
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h.listener.helloInfosMu.RUnlock()
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ua := r.Header.Get("User-Agent")
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var checked, mitm bool
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if r.Header.Get("X-BlueCoat-Via") != "" || // Blue Coat (masks User-Agent header to generic values)
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r.Header.Get("X-FCCKV2") != "" || // Fortinet
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info.advertisesHeartbeatSupport() { // no major browsers have ever implemented Heartbeat
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checked = true
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mitm = true
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} else if strings.Contains(ua, "Edge") || strings.Contains(ua, "MSIE") ||
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strings.Contains(ua, "Trident") {
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checked = true
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mitm = !info.looksLikeEdge()
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} else if strings.Contains(ua, "Chrome") {
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checked = true
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mitm = !info.looksLikeChrome()
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} else if strings.Contains(ua, "Firefox") {
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checked = true
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mitm = !info.looksLikeFirefox()
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} else if strings.Contains(ua, "Safari") {
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checked = true
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mitm = !info.looksLikeSafari()
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}
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if checked {
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r = r.WithContext(context.WithValue(r.Context(), CtxKey("mitm"), mitm))
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}
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if mitm && h.closeOnMITM {
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// TODO: This termination might need to happen later in the middleware
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// chain in order to be picked up by the log directive, in case the site
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// owner still wants to log this event. It'll probably require a new
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// directive. If this feature is useful, we can finish implementing this.
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r.Close = true
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return
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}
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h.next.ServeHTTP(w, r)
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}
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type clientHelloConn struct {
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net.Conn
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readHello bool
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listener *tlsHelloListener
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}
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func (c *clientHelloConn) Read(b []byte) (n int, err error) {
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if !c.readHello {
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// Read the header bytes.
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hdr := make([]byte, 5)
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n, err := io.ReadFull(c.Conn, hdr)
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if err != nil {
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return n, err
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}
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// Get the length of the ClientHello message and read it as well.
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length := uint16(hdr[3])<<8 | uint16(hdr[4])
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hello := make([]byte, int(length))
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n, err = io.ReadFull(c.Conn, hello)
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if err != nil {
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return n, err
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}
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// Parse the ClientHello and store it in the map.
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rawParsed := parseRawClientHello(hello)
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c.listener.helloInfosMu.Lock()
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c.listener.helloInfos[c.Conn.RemoteAddr().String()] = rawParsed
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c.listener.helloInfosMu.Unlock()
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// Since we buffered the header and ClientHello, pretend we were
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// never here by lining up the buffered values to be read with a
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// custom connection type, followed by the rest of the actual
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// underlying connection.
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mr := io.MultiReader(bytes.NewReader(hdr), bytes.NewReader(hello), c.Conn)
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mc := multiConn{Conn: c.Conn, reader: mr}
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c.Conn = mc
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c.readHello = true
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}
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return c.Conn.Read(b)
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}
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// multiConn is a net.Conn that reads from the
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// given reader instead of the wire directly. This
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// is useful when some of the connection has already
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// been read (like the TLS Client Hello) and the
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// reader is a io.MultiReader that starts with
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// the contents of the buffer.
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type multiConn struct {
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net.Conn
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reader io.Reader
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}
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// Read reads from mc.reader.
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func (mc multiConn) Read(b []byte) (n int, err error) {
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return mc.reader.Read(b)
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}
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// parseRawClientHello parses data which contains the raw
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// TLS Client Hello message. It extracts relevant information
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// into info. Any error reading the Client Hello (such as
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// insufficient length or invalid length values) results in
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// a silent error and an incomplete info struct, since there
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// is no good way to handle an error like this during Accept().
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// The data is expected to contain the whole ClientHello and
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// ONLY the ClientHello.
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//
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// The majority of this code is borrowed from the Go standard
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// library, which is (c) The Go Authors. It has been modified
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// to fit this use case.
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func parseRawClientHello(data []byte) (info rawHelloInfo) {
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if len(data) < 42 {
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return
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}
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sessionIdLen := int(data[38])
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if sessionIdLen > 32 || len(data) < 39+sessionIdLen {
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return
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}
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data = data[39+sessionIdLen:]
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if len(data) < 2 {
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return
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}
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// cipherSuiteLen is the number of bytes of cipher suite numbers. Since
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// they are uint16s, the number must be even.
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cipherSuiteLen := int(data[0])<<8 | int(data[1])
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if cipherSuiteLen%2 == 1 || len(data) < 2+cipherSuiteLen {
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return
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}
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numCipherSuites := cipherSuiteLen / 2
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// read in the cipher suites
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info.cipherSuites = make([]uint16, numCipherSuites)
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for i := 0; i < numCipherSuites; i++ {
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info.cipherSuites[i] = uint16(data[2+2*i])<<8 | uint16(data[3+2*i])
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}
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data = data[2+cipherSuiteLen:]
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if len(data) < 1 {
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return
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}
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// read in the compression methods
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compressionMethodsLen := int(data[0])
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if len(data) < 1+compressionMethodsLen {
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return
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}
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info.compressionMethods = data[1 : 1+compressionMethodsLen]
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data = data[1+compressionMethodsLen:]
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// ClientHello is optionally followed by extension data
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if len(data) < 2 {
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return
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}
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extensionsLength := int(data[0])<<8 | int(data[1])
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data = data[2:]
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if extensionsLength != len(data) {
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return
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}
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// read in each extension, and extract any relevant information
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// from extensions we care about
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for len(data) != 0 {
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if len(data) < 4 {
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return
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}
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extension := uint16(data[0])<<8 | uint16(data[1])
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length := int(data[2])<<8 | int(data[3])
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data = data[4:]
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if len(data) < length {
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return
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}
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// record that the client advertised support for this extension
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info.extensions = append(info.extensions, extension)
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switch extension {
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case extensionSupportedCurves:
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// http://tools.ietf.org/html/rfc4492#section-5.5.1
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if length < 2 {
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return
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}
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l := int(data[0])<<8 | int(data[1])
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if l%2 == 1 || length != l+2 {
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return
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}
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numCurves := l / 2
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info.curves = make([]tls.CurveID, numCurves)
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d := data[2:]
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for i := 0; i < numCurves; i++ {
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info.curves[i] = tls.CurveID(d[0])<<8 | tls.CurveID(d[1])
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d = d[2:]
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}
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case extensionSupportedPoints:
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// http://tools.ietf.org/html/rfc4492#section-5.5.2
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if length < 1 {
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return
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}
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l := int(data[0])
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if length != l+1 {
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return
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}
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info.points = make([]uint8, l)
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copy(info.points, data[1:])
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}
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data = data[length:]
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}
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return
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}
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// newTLSListener returns a new tlsHelloListener that wraps ln.
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func newTLSListener(ln net.Listener, config *tls.Config, readTimeout time.Duration) *tlsHelloListener {
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return &tlsHelloListener{
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Listener: ln,
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config: config,
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readTimeout: readTimeout,
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helloInfos: make(map[string]rawHelloInfo),
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}
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}
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// tlsHelloListener is a TLS listener that is specially designed
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// to read the ClientHello manually so we can extract necessary
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// information from it. Each ClientHello message is mapped by
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// the remote address of the client, which must be removed when
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// the connection is closed (use ConnState).
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type tlsHelloListener struct {
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net.Listener
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config *tls.Config
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readTimeout time.Duration
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helloInfos map[string]rawHelloInfo
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helloInfosMu sync.RWMutex
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}
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// Accept waits for and returns the next connection to the listener.
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// After it accepts the underlying connection, it reads the
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// ClientHello message and stores the parsed data into a map on l.
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func (l *tlsHelloListener) Accept() (net.Conn, error) {
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conn, err := l.Listener.Accept()
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if err != nil {
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return nil, err
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}
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helloConn := &clientHelloConn{Conn: conn, listener: l}
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return tls.Server(helloConn, l.config), nil
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}
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// rawHelloInfo contains the "raw" data parsed from the TLS
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// Client Hello. No interpretation is done on the raw data.
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//
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// The methods on this type implement heuristics described
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// by Durumeric, Halderman, et. al. in
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// "The Security Impact of HTTPS Interception":
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// https://jhalderm.com/pub/papers/interception-ndss17.pdf
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type rawHelloInfo struct {
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cipherSuites []uint16
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extensions []uint16
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compressionMethods []byte
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curves []tls.CurveID
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points []uint8
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}
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// advertisesHeartbeatSupport returns true if info indicates
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// that the client supports the Heartbeat extension.
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func (info rawHelloInfo) advertisesHeartbeatSupport() bool {
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for _, ext := range info.extensions {
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if ext == extensionHeartbeat {
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return true
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}
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}
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return false
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}
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// looksLikeFirefox returns true if info looks like a handshake
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// from a modern version of Firefox.
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func (info rawHelloInfo) looksLikeFirefox() bool {
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// "To determine whether a Firefox session has been
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// intercepted, we check for the presence and order
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// of extensions, cipher suites, elliptic curves,
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// EC point formats, and handshake compression methods."
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// We check for the presence and order of the extensions.
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// Note: Sometimes padding (21) is present, sometimes not.
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// Note: Firefox 51+ does not advertise 0x3374 (13172, NPN).
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// Note: Firefox doesn't advertise 0x0 (0, SNI) when connecting to IP addresses.
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requiredExtensionsOrder := []uint16{23, 65281, 10, 11, 35, 16, 5, 65283, 13}
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if !assertPresenceAndOrdering(requiredExtensionsOrder, info.extensions, true) {
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return false
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}
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// We check for both presence of curves and their ordering.
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expectedCurves := []tls.CurveID{29, 23, 24, 25}
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if len(info.curves) != len(expectedCurves) {
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return false
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}
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for i := range expectedCurves {
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if info.curves[i] != expectedCurves[i] {
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return false
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}
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}
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// We check for order of cipher suites but not presence, since
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// according to the paper, cipher suites may be not be added
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// or reordered by the user, but they may be disabled.
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expectedCipherSuiteOrder := []uint16{
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tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, // 0xc02b
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tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, // 0xc02f
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tls.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305, // 0xcca9
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tls.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305, // 0xcca8
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tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, // 0xc02c
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tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384, // 0xc030
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tls.TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA, // 0xc00a
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tls.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA, // 0xc009
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tls.TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, // 0xc013
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tls.TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA, // 0xc014
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TLS_DHE_RSA_WITH_AES_128_CBC_SHA, // 0x33
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TLS_DHE_RSA_WITH_AES_256_CBC_SHA, // 0x39
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tls.TLS_RSA_WITH_AES_128_CBC_SHA, // 0x2f
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tls.TLS_RSA_WITH_AES_256_CBC_SHA, // 0x35
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tls.TLS_RSA_WITH_3DES_EDE_CBC_SHA, // 0xa
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}
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return assertPresenceAndOrdering(expectedCipherSuiteOrder, info.cipherSuites, false)
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}
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// looksLikeChrome returns true if info looks like a handshake
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// from a modern version of Chrome.
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func (info rawHelloInfo) looksLikeChrome() bool {
|
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// "We check for ciphers and extensions that Chrome is known
|
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// to not support, but do not check for the inclusion of
|
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// specific ciphers or extensions, nor do we validate their
|
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// order. When appropriate, we check the presence and order
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// of elliptic curves, compression methods, and EC point formats."
|
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// Not in Chrome 56, but present in Safari 10 (Feb. 2017):
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// TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 (0xc024)
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// TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 (0xc023)
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// TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA (0xc00a)
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// TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA (0xc009)
|
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// TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384 (0xc028)
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// TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256 (0xc027)
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// TLS_RSA_WITH_AES_256_CBC_SHA256 (0x3d)
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// TLS_RSA_WITH_AES_128_CBC_SHA256 (0x3c)
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|
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// Not in Chrome 56, but present in Firefox 51 (Feb. 2017):
|
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// TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA (0xc00a)
|
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// TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA (0xc009)
|
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// TLS_DHE_RSA_WITH_AES_128_CBC_SHA (0x33)
|
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// TLS_DHE_RSA_WITH_AES_256_CBC_SHA (0x39)
|
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|
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// Selected ciphers present in Chrome mobile (Feb. 2017):
|
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// 0xc00a, 0xc014, 0xc009, 0x9c, 0x9d, 0x2f, 0x35, 0xa
|
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chromeCipherExclusions := map[uint16]struct{}{
|
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TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384: {}, // 0xc024
|
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TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256: {}, // 0xc023
|
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TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384: {}, // 0xc028
|
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tls.TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256: {}, // 0xc027
|
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TLS_RSA_WITH_AES_256_CBC_SHA256: {}, // 0x3d
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tls.TLS_RSA_WITH_AES_128_CBC_SHA256: {}, // 0x3c
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TLS_DHE_RSA_WITH_AES_128_CBC_SHA: {}, // 0x33
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TLS_DHE_RSA_WITH_AES_256_CBC_SHA: {}, // 0x39
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}
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for _, ext := range info.cipherSuites {
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if _, ok := chromeCipherExclusions[ext]; ok {
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return false
|
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}
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}
|
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|
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// Chrome does not include curve 25 (CurveP521) (as of Chrome 56, Feb. 2017).
|
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for _, curve := range info.curves {
|
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if curve == 25 {
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return false
|
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}
|
||||
}
|
||||
|
||||
return true
|
||||
}
|
||||
|
||||
// looksLikeEdge returns true if info looks like a handshake
|
||||
// from a modern version of MS Edge.
|
||||
func (info rawHelloInfo) looksLikeEdge() bool {
|
||||
// "SChannel connections can by uniquely identified because SChannel
|
||||
// is the only TLS library we tested that includes the OCSP status
|
||||
// request extension before the supported groups and EC point formats
|
||||
// extensions."
|
||||
//
|
||||
// More specifically, the OCSP status request extension appears
|
||||
// *directly* before the other two extensions, which occur in that
|
||||
// order. (I contacted the authors for clarification and verified it.)
|
||||
for i, ext := range info.extensions {
|
||||
if ext == extensionOCSPStatusRequest {
|
||||
if len(info.extensions) <= i+2 {
|
||||
return false
|
||||
}
|
||||
if info.extensions[i+1] != extensionSupportedCurves ||
|
||||
info.extensions[i+2] != extensionSupportedPoints {
|
||||
return false
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
for _, cs := range info.cipherSuites {
|
||||
// As of Feb. 2017, Edge does not have 0xff, but Avast adds it
|
||||
if cs == scsvRenegotiation {
|
||||
return false
|
||||
}
|
||||
// Edge and modern IE do not have 0x4 or 0x5, but Blue Coat does
|
||||
if cs == TLS_RSA_WITH_RC4_128_MD5 || cs == tls.TLS_RSA_WITH_RC4_128_SHA {
|
||||
return false
|
||||
}
|
||||
}
|
||||
|
||||
return true
|
||||
}
|
||||
|
||||
// looksLikeSafari returns true if info looks like a handshake
|
||||
// from a modern version of MS Safari.
|
||||
func (info rawHelloInfo) looksLikeSafari() bool {
|
||||
// "One unique aspect of Secure Transport is that it includes
|
||||
// the TLS_EMPTY_RENEGOTIATION_INFO_SCSV (0xff) cipher first,
|
||||
// whereas the other libraries we investigated include the
|
||||
// cipher last. Similar to Microsoft, Apple has changed
|
||||
// TLS behavior in minor OS updates, which are not indicated
|
||||
// in the HTTP User-Agent header. We allow for any of the
|
||||
// updates when validating handshakes, and we check for the
|
||||
// presence and ordering of ciphers, extensions, elliptic
|
||||
// curves, and compression methods."
|
||||
|
||||
// Note that any C lib (e.g. curl) compiled on macOS
|
||||
// will probably use Secure Transport which will also
|
||||
// share the TLS handshake characteristics of Safari.
|
||||
|
||||
// Let's do the easy check first... should be sufficient in many cases.
|
||||
if len(info.cipherSuites) < 1 {
|
||||
return false
|
||||
}
|
||||
if info.cipherSuites[0] != scsvRenegotiation {
|
||||
return false
|
||||
}
|
||||
|
||||
// We check for the presence and order of the extensions.
|
||||
requiredExtensionsOrder := []uint16{10, 11, 13, 13172, 16, 5, 18, 23}
|
||||
if !assertPresenceAndOrdering(requiredExtensionsOrder, info.extensions, true) {
|
||||
return false
|
||||
}
|
||||
|
||||
// We check for order and presence of cipher suites
|
||||
expectedCipherSuiteOrder := []uint16{
|
||||
tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, // 0xc02c
|
||||
tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, // 0xc02b
|
||||
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384, // 0xc024
|
||||
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256, // 0xc023
|
||||
tls.TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA, // 0xc00a
|
||||
tls.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA, // 0xc009
|
||||
tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384, // 0xc030
|
||||
tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, // 0xc02f
|
||||
TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384, // 0xc028
|
||||
tls.TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256, // 0xc027
|
||||
tls.TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA, // 0xc014
|
||||
tls.TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, // 0xc013
|
||||
tls.TLS_RSA_WITH_AES_256_GCM_SHA384, // 0x9d
|
||||
tls.TLS_RSA_WITH_AES_128_GCM_SHA256, // 0x9c
|
||||
TLS_RSA_WITH_AES_256_CBC_SHA256, // 0x3d
|
||||
TLS_RSA_WITH_AES_128_CBC_SHA256, // 0x3c
|
||||
tls.TLS_RSA_WITH_AES_256_CBC_SHA, // 0x35
|
||||
tls.TLS_RSA_WITH_AES_128_CBC_SHA, // 0x2f
|
||||
}
|
||||
return assertPresenceAndOrdering(expectedCipherSuiteOrder, info.cipherSuites, true)
|
||||
}
|
||||
|
||||
// assertPresenceAndOrdering will return true if candidateList contains
|
||||
// the items in requiredItems in the same order as requiredItems.
|
||||
//
|
||||
// If requiredIsSubset is true, then all items in requiredItems must be
|
||||
// present in candidateList. If requiredIsSubset is false, then requiredItems
|
||||
// may contain items that are not in candidateList.
|
||||
//
|
||||
// In all cases, the order of requiredItems is enforced.
|
||||
func assertPresenceAndOrdering(requiredItems, candidateList []uint16, requiredIsSubset bool) bool {
|
||||
superset := requiredItems
|
||||
subset := candidateList
|
||||
if requiredIsSubset {
|
||||
superset = candidateList
|
||||
subset = requiredItems
|
||||
}
|
||||
|
||||
var j int
|
||||
for _, item := range subset {
|
||||
var found bool
|
||||
for j < len(superset) {
|
||||
if superset[j] == item {
|
||||
found = true
|
||||
break
|
||||
}
|
||||
j++
|
||||
}
|
||||
if j == len(superset) && !found {
|
||||
return false
|
||||
}
|
||||
}
|
||||
return true
|
||||
}
|
||||
|
||||
const (
|
||||
extensionOCSPStatusRequest = 5
|
||||
extensionSupportedCurves = 10 // also called "SupportedGroups"
|
||||
extensionSupportedPoints = 11
|
||||
extensionHeartbeat = 15
|
||||
|
||||
scsvRenegotiation = 0xff
|
||||
|
||||
// cipher suites missing from the crypto/tls package,
|
||||
// in no particular order here
|
||||
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 = 0xc024
|
||||
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 = 0xc023
|
||||
TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384 = 0xc028
|
||||
TLS_RSA_WITH_AES_128_CBC_SHA256 = 0x3c
|
||||
TLS_RSA_WITH_AES_256_CBC_SHA256 = 0x3d
|
||||
TLS_DHE_RSA_WITH_AES_128_CBC_SHA = 0x33
|
||||
TLS_DHE_RSA_WITH_AES_256_CBC_SHA = 0x39
|
||||
TLS_RSA_WITH_RC4_128_MD5 = 0x4
|
||||
)
|
||||
Reference in New Issue
Block a user