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restrict RSA keys in certificates to <= 8192 bits
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marten-seemann committed Aug 2, 2023
1 parent a42b12e commit d46f370
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Showing 3 changed files with 90 additions and 0 deletions.
8 changes: 8 additions & 0 deletions handshake_client.go
Original file line number Diff line number Diff line change
Expand Up @@ -950,6 +950,10 @@ 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 {
Expand All @@ -961,6 +965,10 @@ 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
}
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78 changes: 78 additions & 0 deletions handshake_client_test.go
Original file line number Diff line number Diff line change
Expand Up @@ -2714,3 +2714,81 @@ 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-----
MIIInjCCBIWgAwIBAgIBAjANBgkqhkiG9w0BAQsFADASMRAwDgYDVQQDEwd0ZXN0
aW5nMB4XDTIzMDYwNzIxMjMzNloXDTIzMDYwNzIzMjMzNlowEjEQMA4GA1UEAxMH
dGVzdGluZzCCBCIwDQYJKoZIhvcNAQEBBQADggQPADCCBAoCggQBAWdHsf6Rh2Ca
n2SQwn4t4OQrOjbLLdGE1pM6TBKKrHUFy62uEL8atNjlcfXIsa4aEu3xNGiqxqur
ZectlkZbm0FkaaQ1Wr9oikDY3KfjuaXdPdO/XC/h8AKNxlDOylyXwUSK/CuYb+1j
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u58=
-----END CERTIFICATE-----`

func TestHandshakeRSATooBig(t *testing.T) {
testCert, _ := pem.Decode([]byte(largeRSAKeyCertPEM))

c := &Conn{conn: &discardConn{}, config: fromConfig(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)
}
}
4 changes: 4 additions & 0 deletions handshake_server.go
Original file line number Diff line number Diff line change
Expand Up @@ -849,6 +849,10 @@ 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) {
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