[release-branch.go1.20] crypto/tls: restrict RSA keys in certificates to <= 8192...

Extremely large RSA keys in certificate chains can cause a client/server
to expend significant CPU time verifying signatures. Limit this by
restricting the size of RSA keys transmitted during handshakes to <=
8192 bits.

Based on a survey of publicly trusted RSA keys, there are currently only
three certificates in circulation with keys larger than this, and all
three appear to be test certificates that are not actively deployed. It
is possible there are larger keys in use in private PKIs, but we target
the web PKI, so causing breakage here in the interests of increasing the
default safety of users of crypto/tls seems reasonable.

Thanks to Mateusz Poliwczak for reporting this issue.

Updates #61460
Fixes #61580
Fixes CVE-2023-29409

Change-Id: Ie35038515a649199a36a12fc2c5df3af855dca6c
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1912161
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
Run-TryBot: Roland Shoemaker <bracewell@google.com>
(cherry picked from commit d865c715d92887361e4bd5596e19e513f27781b7)
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1965747
TryBot-Result: Security TryBots <security-trybots@go-security-trybots.iam.gserviceaccount.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/514900
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

diff --git a/src/crypto/tls/handshake_client.go b/src/crypto/tls/handshake_client.go
index 63d86b9f3a7ef1b4155b45f559ac41a787f4b5cb..0ef87d4d775aab7cb661b842e4373149abbe312e 100644 (file)
--- a/src/crypto/tls/handshake_client.go
+++ b/src/crypto/tls/handshake_client.go
@@ -857,6 +857,10 @@ func (hs *clientHandshakeState) sendFinished(out []byte) error {
        return nil
 }
 
+// maxRSAKeySize is the maximum RSA key size in bits that we are willing
+// to verify the signatures of during a TLS handshake.
+const maxRSAKeySize = 8192
+
 // verifyServerCertificate parses and verifies the provided chain, setting
 // c.verifiedChains and c.peerCertificates or sending the appropriate alert.
 func (c *Conn) verifyServerCertificate(certificates [][]byte) error {
@@ -868,6 +872,10 @@ 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)
+               }
                activeHandles[i] = cert
                certs[i] = cert.cert
        }

diff --git a/src/crypto/tls/handshake_client_test.go b/src/crypto/tls/handshake_client_test.go
index 22be38faff46870d2a8f41f064aeeff3cb4ef691..6b5b61aed503ba8ea46822a56678313f85910367 100644 (file)
--- a/src/crypto/tls/handshake_client_test.go
+++ b/src/crypto/tls/handshake_client_test.go
@@ -2616,3 +2616,81 @@ func TestClientHandshakeContextCancellation(t *testing.T) {
                t.Error("Client connection was not closed when the context was canceled")
        }
 }
+
+// discardConn wraps a net.Conn but discards all writes, but reports that they happened.
+type discardConn struct {
+       net.Conn
+}
+
+func (dc *discardConn) Write(data []byte) (int, error) {
+       return len(data), nil
+}
+
+// largeRSAKeyCertPEM contains a 8193 bit RSA key
+const largeRSAKeyCertPEM = `-----BEGIN CERTIFICATE-----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+4xp0Zv7qZPLwnu0jsqB4jD8Ll9yPB02ndsoV6U5PeHzTkVhPml19jKUAwFfs7TJg
+kXy+/xFhYVUCAwEAATANBgkqhkiG9w0BAQsFAAOCBAIAAQnZY77pMNeypfpba2WK
+aDasT7dk2JqP0eukJCVPTN24Zca+xJNPdzuBATm/8SdZK9lddIbjSnWRsKvTnO2r
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+qILJ+xjcrvms5ZPDNahWkfRx3KCg8Q+/at2n5p7XYjMPYiLKHnDC+RE2b1qT20IZ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+-----END CERTIFICATE-----`
+
+func TestHandshakeRSATooBig(t *testing.T) {
+       testCert, _ := pem.Decode([]byte(largeRSAKeyCertPEM))
+
+       c := &Conn{conn: &discardConn{}, config: testConfig.Clone()}
+
+       expectedErr := "tls: server sent certificate containing RSA key larger than 8192 bits"
+       err := c.verifyServerCertificate([][]byte{testCert.Bytes})
+       if err == nil || err.Error() != expectedErr {
+               t.Errorf("Conn.verifyServerCertificate unexpected error: want %q, got %q", expectedErr, err)
+       }
+
+       expectedErr = "tls: client sent certificate containing RSA key larger than 8192 bits"
+       err = c.processCertsFromClient(Certificate{Certificate: [][]byte{testCert.Bytes}})
+       if err == nil || err.Error() != expectedErr {
+               t.Errorf("Conn.processCertsFromClient unexpected error: want %q, got %q", expectedErr, err)
+       }
+}

diff --git a/src/crypto/tls/handshake_server.go b/src/crypto/tls/handshake_server.go
index e22f284cfb428a63cdd7b2db0314a3b9e1b29605..9def927314815e64f4577d6e50d19f6c14613c1e 100644 (file)
--- a/src/crypto/tls/handshake_server.go
+++ b/src/crypto/tls/handshake_server.go
@@ -818,6 +818,10 @@ 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 len(certs) == 0 && requiresClientCert(c.config.ClientAuth) {