CheckTrustedIssuer: Fixes for invalid chains

docs/ReleaseNotes.md

@@ -9,6 +9,8 @@ Current package versions:
 ## Unreleased
 
 - Add new `LoggingTunnel` API; see https://stackexchange.github.io/StackExchange.Redis/Logging ([#2660 by mgravell](https://github.com/StackExchange/StackExchange.Redis/pull/2660))
+- **Potentiallty Breaking**: Fix `CheckTrustedIssuer` certificate validation for broken chain scenarios ([#2665 by NickCraver](https://github.com/StackExchange/StackExchange.Redis/pull/2665))
+  - Users inadvertently trusting a remote cert with a broken chain could not be failing custom validation before this change. This is only in play if you are using `ConfigurationOptions.TrustIssuer` at all.
 
 ## 2.7.27
 

src/StackExchange.Redis/ConfigurationOptions.cs

@@ -6,6 +6,7 @@
 using System.Net.Security;
 using System.Net.Sockets;
 using System.Security.Authentication;
+using System.Security.Cryptography;
 using System.Security.Cryptography.X509Certificates;
 using System.Text;
 using System.Threading;
@@ -279,13 +280,13 @@ public bool CheckCertificateRevocation
         }
 
         /// <summary>
-        /// Create a certificate validation check that checks against the supplied issuer even if not known by the machine.
+        /// Create a certificate validation check that checks against the supplied issuer even when not known by the machine.
         /// </summary>
         /// <param name="issuerCertificatePath">The file system path to find the certificate at.</param>
         public void TrustIssuer(string issuerCertificatePath) => CertificateValidationCallback = TrustIssuerCallback(issuerCertificatePath);
 
         /// <summary>
-        /// Create a certificate validation check that checks against the supplied issuer even if not known by the machine.
+        /// Create a certificate validation check that checks against the supplied issuer even when not known by the machine.
         /// </summary>
         /// <param name="issuer">The issuer to trust.</param>
         public void TrustIssuer(X509Certificate2 issuer) => CertificateValidationCallback = TrustIssuerCallback(issuer);
@@ -296,24 +297,65 @@ private static RemoteCertificateValidationCallback TrustIssuerCallback(X509Certi
         {
             if (issuer == null) throw new ArgumentNullException(nameof(issuer));
 
-            return (object _, X509Certificate? certificate, X509Chain? __, SslPolicyErrors sslPolicyError)
-                => sslPolicyError == SslPolicyErrors.RemoteCertificateChainErrors
-                    && certificate is X509Certificate2 v2
-                    && CheckTrustedIssuer(v2, issuer);
+            return (object _, X509Certificate? certificate, X509Chain? certificateChain, SslPolicyErrors sslPolicyError) =>
+            {
+                // If we're already valid, there's nothing further to check
+                if (sslPolicyError == SslPolicyErrors.None)
+                {
+                    return true;
+                }
+                // If we're not valid due to chain errors - check against the trusted issuer
+                // Note that we're only proceeding at all here if the *only* issue is chain errors (not more flags in SslPolicyErrors)
+                return sslPolicyError == SslPolicyErrors.RemoteCertificateChainErrors
+                       && certificate is X509Certificate2 v2
+                       && CheckTrustedIssuer(v2, certificateChain, issuer);
+            };
         }
 
-        private static bool CheckTrustedIssuer(X509Certificate2 certificateToValidate, X509Certificate2 authority)
+        private static bool CheckTrustedIssuer(X509Certificate2 certificateToValidate, X509Chain? chainToValidate, X509Certificate2 authority)
         {
-            // reference: https://stackoverflow.com/questions/6497040/how-do-i-validate-that-a-certificate-was-created-by-a-particular-certification-a
-            X509Chain chain = new X509Chain();
+            // Reference:
+            // https://stackoverflow.com/questions/6497040/how-do-i-validate-that-a-certificate-was-created-by-a-particular-certification-a
+            // https://github.com/stewartadam/dotnet-x509-certificate-verification
+            using X509Chain chain = new X509Chain();
             chain.ChainPolicy.RevocationMode = X509RevocationMode.NoCheck;
-            chain.ChainPolicy.RevocationFlag = X509RevocationFlag.ExcludeRoot;
             chain.ChainPolicy.VerificationFlags = X509VerificationFlags.AllowUnknownCertificateAuthority;
-            chain.ChainPolicy.VerificationTime = DateTime.Now;
+            chain.ChainPolicy.VerificationTime = chainToValidate?.ChainPolicy?.VerificationTime ?? DateTime.Now;
             chain.ChainPolicy.UrlRetrievalTimeout = new TimeSpan(0, 0, 0);
 
             chain.ChainPolicy.ExtraStore.Add(authority);
-            return chain.Build(certificateToValidate);
+            try
+            {
+                // This only verifies that the chain is valid, but with AllowUnknownCertificateAuthority could trust
+                // self-signed or partial chained vertificates
+                var chainIsVerified = chain.Build(certificateToValidate);
+                if (chainIsVerified)
+                {
+                    // Our method is "TrustIssuer", which means any intermediate cert we're being told to trust
+                    // is a valid thing to trust, up until it's a root CA
+                    bool found = false;
+                    byte[] authorityData = authority.RawData;
+                    foreach (var chainElement in chain.ChainElements)
+                    {
+#if NET8_0_OR_GREATER
+#error TODO: use RawDataMemory (needs testing)
+#endif
+                        using var chainCert = chainElement.Certificate;
+                        if (!found && chainCert.RawData.SequenceEqual(authorityData))
+                        {
+                            found = true;
+                        }
+                    }
+                    return found;
+                }
+            }
+            catch (CryptographicException)
+            {
+                // We specifically don't want to throw during validation here and would rather exit out gracefully
+            }
+
+            // If we didn't find the trusted issuer in the chain at all - we do not trust the result.
+            return false;
         }
 
         /// <summary>

tests/StackExchange.Redis.Tests/Certificates/CertValidationTests.cs

@@ -0,0 +1,58 @@
+using System;
+using System.IO;
+using System.Net.Security;
+using System.Security.Cryptography.X509Certificates;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace StackExchange.Redis.Tests;
+
+public class CertValidationTests : TestBase
+{
+    public CertValidationTests(ITestOutputHelper output) : base(output) { }
+
+    [Fact]
+    public void CheckIssuerValidity()
+    {
+        // The endpoint cert is the same here
+        var endpointCert = LoadCert(Path.Combine("Certificates", "device01.foo.com.pem"));
+
+        // Trusting CA explicitly
+        var callback = ConfigurationOptions.TrustIssuerCallback(Path.Combine("Certificates", "ca.foo.com.pem"));
+        Assert.True(callback(this, endpointCert, null, SslPolicyErrors.None));
+        Assert.True(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateChainErrors));
+        Assert.False(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateNameMismatch));
+        Assert.False(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateNotAvailable));
+        Assert.False(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateChainErrors | SslPolicyErrors.RemoteCertificateNameMismatch));
+        Assert.False(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateChainErrors | SslPolicyErrors.RemoteCertificateNotAvailable));
+
+        // Trusting the remote endpoint cert directly
+        callback = ConfigurationOptions.TrustIssuerCallback(Path.Combine("Certificates", "device01.foo.com.pem"));
+        Assert.True(callback(this, endpointCert, null, SslPolicyErrors.None));
+        Assert.True(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateChainErrors));
+        Assert.False(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateNameMismatch));
+        Assert.False(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateNotAvailable));
+        Assert.False(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateChainErrors | SslPolicyErrors.RemoteCertificateNameMismatch));
+        Assert.False(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateChainErrors | SslPolicyErrors.RemoteCertificateNotAvailable));
+
+        // Attempting to trust another CA (mismatch)
+        callback = ConfigurationOptions.TrustIssuerCallback(Path.Combine("Certificates", "ca2.foo.com.pem"));
+        Assert.True(callback(this, endpointCert, null, SslPolicyErrors.None));
+        Assert.False(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateChainErrors));
+        Assert.False(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateNameMismatch));
+        Assert.False(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateNotAvailable));
+        Assert.False(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateChainErrors | SslPolicyErrors.RemoteCertificateNameMismatch));
+        Assert.False(callback(this, endpointCert, null, SslPolicyErrors.RemoteCertificateChainErrors | SslPolicyErrors.RemoteCertificateNotAvailable));
+    }
+
+    private static X509Certificate2 LoadCert(string certificatePath) => new X509Certificate2(File.ReadAllBytes(certificatePath));
+
+    [Fact]
+    public void CheckIssuerArgs()
+    {
+        Assert.ThrowsAny<Exception>(() => ConfigurationOptions.TrustIssuerCallback(""));
+
+        var opt = new ConfigurationOptions();
+        Assert.Throws<ArgumentNullException>(() => opt.TrustIssuer((X509Certificate2)null!));
+    }
+}

tests/StackExchange.Redis.Tests/Certificates/README.md

@@ -0,0 +1,4 @@
+The certificates here are randomly generated for testing only.
+They are not valid and only used for test validation.
+
+Please do not file security issue noise - these have no impact being public at all.

tests/StackExchange.Redis.Tests/Certificates/ca.foo.com.pem

@@ -0,0 +1,20 @@
+-----BEGIN CERTIFICATE-----
+MIIDTTCCAjWgAwIBAgIUU9SR3QMGVO8yN/mr8SQJ1p/OgIAwDQYJKoZIhvcNAQEL
+BQAwNjETMBEGA1UEAwwKY2EuZm9vLmNvbTESMBAGA1UECgwJTXlEZXZpY2VzMQsw
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+KrJ8Hh+hpIFsmQKj5dcochKvLLd1Z748b2+FB0jtxraU
+-----END CERTIFICATE-----

tests/StackExchange.Redis.Tests/Certificates/ca2.foo.com.pem

@@ -0,0 +1,20 @@
+-----BEGIN CERTIFICATE-----
+MIIDTzCCAjegAwIBAgIUYXv168vvB1NPg3PfoRzcNFEMaC8wDQYJKoZIhvcNAQEL
+BQAwNzEUMBIGA1UEAwwLY2EyLmZvby5jb20xEjAQBgNVBAoMCU15RGV2aWNlczEL
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+gdAUOSPZgzRqAag0rvVfrpyvfvlu0D9xeiBLdhaJeZCq1/s=
+-----END CERTIFICATE-----

tests/StackExchange.Redis.Tests/Certificates/create_certificates.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+set -eu
+# Adapted from https://github.com/stewartadam/dotnet-x509-certificate-verification/blob/main/create_certificates.sh
+
+base_dir="certificates"
+
+create_ca() {
+  local CA_CN="$1"
+  local certificate_output="${base_dir}/${CA_CN}.pem"
+
+  openssl genrsa -out "${base_dir}/${CA_CN}.key.pem" 2048 # Generate private key
+  openssl req -x509 -new -nodes -key "${base_dir}/${CA_CN}.key.pem" -sha256 -days 9000 -out "${certificate_output}" -subj "/CN=${CA_CN}/O=MyDevices/C=US" # Generate root certificate
+
+  echo -e "\nCertificate for CA ${CA_CN} saved to ${certificate_output}\n\n"
+}
+
+create_leaf_cert_req() {
+  local DEVICE_CN="$1"
+
+  openssl genrsa -out "${base_dir}/${DEVICE_CN}.key.pem" 2048 # new private key
+  openssl req -new -key "${base_dir}/${DEVICE_CN}.key.pem" -out "${base_dir}/${DEVICE_CN}.csr.pem" -subj "/CN=${DEVICE_CN}/O=MyDevices/C=US" # generate signing request for the CA
+}
+
+sign_leaf_cert() {
+  local DEVICE_CN="$1"
+  local CA_CN="$2"
+  local certificate_output="${base_dir}/${DEVICE_CN}.pem"
+
+  openssl x509 -req -in "${base_dir}/${DEVICE_CN}.csr.pem" -CA ""${base_dir}/${CA_CN}.pem"" -CAkey "${base_dir}/${CA_CN}.key.pem" -set_serial 01 -out "${certificate_output}" -days 8999 -sha256 # sign the CSR
+
+  echo -e "\nCertificate for ${DEVICE_CN} saved to ${certificate_output}\n\n"
+}
+
+mkdir -p "${base_dir}"
+
+# Create one self-issued CA + signed cert
+create_ca "ca.foo.com"
+create_leaf_cert_req "device01.foo.com"
+sign_leaf_cert "device01.foo.com" "ca.foo.com"

tests/StackExchange.Redis.Tests/Certificates/device01.foo.com.pem

@@ -0,0 +1,18 @@
+-----BEGIN CERTIFICATE-----
+MIIC5jCCAc4CAQEwDQYJKoZIhvcNAQELBQAwNjETMBEGA1UEAwwKY2EuZm9vLmNv
+bTESMBAGA1UECgwJTXlEZXZpY2VzMQswCQYDVQQGEwJVUzAeFw0yNDAzMDcxNDAx
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+3dzUEaGexGqvA2MCapIQ2vCCMDF33ismQts=
+-----END CERTIFICATE-----

tests/StackExchange.Redis.Tests/StackExchange.Redis.Tests.csproj

@@ -13,6 +13,7 @@
     <None Update="*.json" CopyToOutputDirectory="Always" />
     <EmbeddedResource Include="*Config.json" />
     <None Update="redislabs_ca.pem" CopyToOutputDirectory="PreserveNewest" />
+    <None Update="Certificates\*.pem" CopyToOutputDirectory="PreserveNewest" />
   </ItemGroup>
 
   <ItemGroup>