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crypto/tls: enforce TLS 1.3 (and TLS 1.2) downgrade protection checks

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Fixes #37763

Change-Id: Ic6bcc9af0d164966f4ae31087998e5b546540038
Reviewed-on: https://go-review.googlesource.com/c/go/+/231038
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
This commit is contained in:
Filippo Valsorda 2020-04-29 17:54:24 -04:00
parent dac6a2bce8
commit b65e9fa492
4 changed files with 63 additions and 2 deletions
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14
handshake_client.go
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@ -54,7 +54,7 @@ func (c *Conn) makeClientHello() (*clientHelloMsg, ecdheParameters, error) {
return nil, nil, errors.New("tls: no supported versions satisfy MinVersion and MaxVersion")
}
clientHelloVersion := supportedVersions[0]
clientHelloVersion := config.maxSupportedVersion()
// The version at the beginning of the ClientHello was capped at TLS 1.2
// for compatibility reasons. The supported_versions extension is used
// to negotiate versions now. See RFC 8446, Section 4.2.1.
@ -181,6 +181,18 @@ func (c *Conn) clientHandshake() (err error) {
return err
}
// If we are negotiating a protocol version that's lower than what we
// support, check for the server downgrade canaries.
// See RFC 8446, Section 4.1.3.
maxVers := c.config.maxSupportedVersion()
tls12Downgrade := string(serverHello.random[24:]) == downgradeCanaryTLS12
tls11Downgrade := string(serverHello.random[24:]) == downgradeCanaryTLS11
if maxVers == VersionTLS13 && c.vers <= VersionTLS12 && (tls12Downgrade || tls11Downgrade) ||
maxVers == VersionTLS12 && c.vers <= VersionTLS11 && tls11Downgrade {
c.sendAlert(alertIllegalParameter)
return errors.New("tls: downgrade attempt detected, possibly due to a MitM attack or a broken middlebox")
}
if c.vers == VersionTLS13 {
hs := &clientHandshakeStateTLS13{
c: c,