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crypto/tls: add GODEBUG to control max RSA key size

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Add a new GODEBUG setting, tlsmaxrsasize, which allows controlling the
maximum RSA key size we will accept during TLS handshakes.

Change-Id: I52f060be132014d219f4cd438f59990011a35c96
Reviewed-on: https://go-review.googlesource.com/c/go/+/517495
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
This commit is contained in:
Roland Shoemaker 2023-08-08 18:25:59 -07:00 • committed by Gopher Robot
parent 7bb9115281
commit 59cc5ee2c8
3 changed files with 35 additions and 8 deletions
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5
conn.go
View file

@ -1467,6 +1467,11 @@ func (c *Conn) closeNotify() error {
//
// For control over canceling or setting a timeout on a handshake, use
// HandshakeContext or the Dialer's DialContext method instead.
//
// In order to avoid denial of service attacks, the maximum RSA key size allowed
// in certificates sent by either the TLS server or client is limited to 8192
// bits. This limit can be overridden by setting tlsmaxrsasize in the GODEBUG
// environment variable (e.g. GODEBUG=tlsmaxrsasize=4096).
func (c *Conn) Handshake() error {
return c.HandshakeContext(context.Background())
}

29
handshake_client.go
View file

@ -17,8 +17,10 @@ import (
"errors"
"fmt"
"hash"
"internal/godebug"
"io"
"net"
"strconv"
"strings"
"time"
)
@ -936,9 +938,23 @@ func (hs *clientHandshakeState) sendFinished(out []byte) error {
return nil
}
// maxRSAKeySize is the maximum RSA key size in bits that we are willing
// defaultMaxRSAKeySize is the maximum RSA key size in bits that we are willing
// to verify the signatures of during a TLS handshake.
const maxRSAKeySize = 8192
const defaultMaxRSAKeySize = 8192
var tlsmaxrsasize = godebug.New("tlsmaxrsasize")
func checkKeySize(n int) (max int, ok bool) {
if v := tlsmaxrsasize.Value(); v != "" {
if max, err := strconv.Atoi(v); err == nil {
if (n <= max) != (n <= defaultMaxRSAKeySize) {
tlsmaxrsasize.IncNonDefault()
}
return max, n <= max
}
}
return defaultMaxRSAKeySize, n <= defaultMaxRSAKeySize
}
// verifyServerCertificate parses and verifies the provided chain, setting
// c.verifiedChains and c.peerCertificates or sending the appropriate alert.
@ -951,9 +967,12 @@ func (c *Conn) verifyServerCertificate(certificates [][]byte) error {
c.sendAlert(alertBadCertificate)
return errors.New("tls: failed to parse certificate from server: " + err.Error())
}
if cert.cert.PublicKeyAlgorithm == x509.RSA && cert.cert.PublicKey.(*rsa.PublicKey).N.BitLen() > maxRSAKeySize {
c.sendAlert(alertBadCertificate)
return fmt.Errorf("tls: server sent certificate containing RSA key larger than %d bits", maxRSAKeySize)
if cert.cert.PublicKeyAlgorithm == x509.RSA {
n := cert.cert.PublicKey.(*rsa.PublicKey).N.BitLen()
if max, ok := checkKeySize(n); !ok {
c.sendAlert(alertBadCertificate)
return fmt.Errorf("tls: server sent certificate containing RSA key larger than %d bits", max)
}
}
activeHandles[i] = cert
certs[i] = cert.cert

9
handshake_server.go
View file

@ -864,9 +864,12 @@ func (c *Conn) processCertsFromClient(certificate Certificate) error {
c.sendAlert(alertBadCertificate)
return errors.New("tls: failed to parse client certificate: " + err.Error())
}
if certs[i].PublicKeyAlgorithm == x509.RSA && certs[i].PublicKey.(*rsa.PublicKey).N.BitLen() > maxRSAKeySize {
c.sendAlert(alertBadCertificate)
return fmt.Errorf("tls: client sent certificate containing RSA key larger than %d bits", maxRSAKeySize)
if certs[i].PublicKeyAlgorithm == x509.RSA {
n := certs[i].PublicKey.(*rsa.PublicKey).N.BitLen()
if max, ok := checkKeySize(n); !ok {
c.sendAlert(alertBadCertificate)
return fmt.Errorf("tls: client sent certificate containing RSA key larger than %d bits", max)
}
}
}