CheckTrustedIssuer: Fixes for invalid chains

NickCraver · NickCraver · commit f6ef08277932 · 2024-03-07T09:24:21.000-05:00
This issue was brought to my attention last night (thanks reporter!): dotnet/dotnet-api-docs#6660 This changeset ensures that we do not honor self-signed certs or partial/broken chains as a result of `X509VerificationFlags.AllowUnknownCertificateAuthority` downstream and adds a few tests and utilities to generate test certificates (currently valid for ~9000 days). Instead we are checking that the certificate we're being told to trust is explicitly in the chain, given that the result of `.Build()` cannot be trusted for this case.
diff --git a/src/StackExchange.Redis/ConfigurationOptions.cs b/src/StackExchange.Redis/ConfigurationOptions.cs
@@ -296,15 +296,25 @@ 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? __, 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
+                return sslPolicyError == SslPolicyErrors.RemoteCertificateChainErrors
+                       && certificate is X509Certificate2 v2
+                       && CheckTrustedIssuer(v2, issuer);
+            };
         }
 
         private static bool CheckTrustedIssuer(X509Certificate2 certificateToValidate, X509Certificate2 authority)
         {
-            // reference: https://stackoverflow.com/questions/6497040/how-do-i-validate-that-a-certificate-was-created-by-a-particular-certification-a
+            // 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
             X509Chain chain = new X509Chain();
             chain.ChainPolicy.RevocationMode = X509RevocationMode.NoCheck;
             chain.ChainPolicy.RevocationFlag = X509RevocationFlag.ExcludeRoot;
@@ -313,7 +323,23 @@ private static bool CheckTrustedIssuer(X509Certificate2 certificateToValidate, X
             chain.ChainPolicy.UrlRetrievalTimeout = new TimeSpan(0, 0, 0);
 
             chain.ChainPolicy.ExtraStore.Add(authority);
-            return chain.Build(certificateToValidate);
+            // This only verifies that the chain is valid, but with AllowUnknownCertificateAuthority could trust
+            // self-signed or partial chained vertificates
+            var chainIsVerfiied = chain.Build(certificateToValidate);
+            if (chainIsVerfiied)
+            {
+                // 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
+                foreach (var chainElement in chain.ChainElements)
+                {
+                    if (chainElement.Certificate.RawData.SequenceEqual(authority.RawData))
+                    {
+                        return true;
+                    }
+                }
+            }
+            // If we didn't find the trusted issuer in the chain at all - we do not trust the result.
+            return false;
         }
 
         /// <summary>
diff --git a/tests/StackExchange.Redis.Tests/Certificates/CertValidationTests.cs b/tests/StackExchange.Redis.Tests/Certificates/CertValidationTests.cs
@@ -0,0 +1,58 @@
+﻿using System;
+using System.IO;
+using System.Net.Security;
+using System.Security.Cryptography;
+using System.Security.Cryptography.X509Certificates;
+using System.Text;
+using System.Text.RegularExpressions;
+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));
+
+        // 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));
+
+        // 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));
+    }
+
+    private static X509Certificate2 LoadCert(string certificatePath) =>
+        new X509Certificate2(StripPEM(File.ReadAllBytes(certificatePath)));
+    private static byte[] StripPEM(byte[] pemData) =>
+        Convert.FromBase64String(Regex.Replace(Encoding.UTF8.GetString(pemData), "(-+BEGIN CERTIFICATE-+)|(-+END CERTIFICATE-+)", "").Trim());
+
+    [Fact]
+    public void CheckIssuerArgs()
+    {
+        Assert.ThrowsAny<Exception>(() => ConfigurationOptions.TrustIssuerCallback(""));
+
+        var opt = new ConfigurationOptions();
+        Assert.Throws<ArgumentNullException>(() => opt.TrustIssuer((X509Certificate2)null!));
+    }
+}
diff --git a/tests/StackExchange.Redis.Tests/Certificates/README.md b/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.
diff --git a/tests/StackExchange.Redis.Tests/Certificates/ca.foo.com.pem b/tests/StackExchange.Redis.Tests/Certificates/ca.foo.com.pem
@@ -0,0 +1,20 @@
+-----BEGIN CERTIFICATE-----
+MIIDTTCCAjWgAwIBAgIUU9SR3QMGVO8yN/mr8SQJ1p/OgIAwDQYJKoZIhvcNAQEL
+BQAwNjETMBEGA1UEAwwKY2EuZm9vLmNvbTESMBAGA1UECgwJTXlEZXZpY2VzMQsw
+CQYDVQQGEwJVUzAeFw0yNDAzMDcxNDAxMzJaFw00ODEwMjcxNDAxMzJaMDYxEzAR
+BgNVBAMMCmNhLmZvby5jb20xEjAQBgNVBAoMCU15RGV2aWNlczELMAkGA1UEBhMC
+VVMwggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEKAoIBAQCqk8FT5dHU335oSEuY
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+T1Pjv3Vxku1O7q6FQrslDWfSBxzwep2q8fDXwD4C7VgVRM2EGg/vVee2juuTCmMU
+Z1LwJrOkBczW6b3ZvUThFGOvZkuI138EuR2gqPHMQIiQcPyX1syT7yhJAyDQRYOG
+cHSRojNciYVotSTgyYguUJdU7vfRJ+MLfoZClzJgvNav8yUC+sSrb5RD5vQlvxzG
+KrJ8Hh+hpIFsmQKj5dcochKvLLd1Z748b2+FB0jtxraU
+-----END CERTIFICATE-----
diff --git a/tests/StackExchange.Redis.Tests/Certificates/ca2.foo.com.pem b/tests/StackExchange.Redis.Tests/Certificates/ca2.foo.com.pem
@@ -0,0 +1,20 @@
+-----BEGIN CERTIFICATE-----
+MIIDTzCCAjegAwIBAgIUYXv168vvB1NPg3PfoRzcNFEMaC8wDQYJKoZIhvcNAQEL
+BQAwNzEUMBIGA1UEAwwLY2EyLmZvby5jb20xEjAQBgNVBAoMCU15RGV2aWNlczEL
+MAkGA1UEBhMCVVMwHhcNMjQwMzA3MTQwMTMyWhcNNDgxMDI3MTQwMTMyWjA3MRQw
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+nOyWd6d95rZwbeYyN908PjERQT3EMo8/O0eOpoG9I79vjbcD6LAIbxS9XdI8kK4+
+D4e/DX/R85KoWSprB+VRXGqsChY0Y+4x5x2q/IoCi6+tywhzjqIlaGDYrlc688HO
+/+4iR9L945gpg4NT1hLnCwDYcdZ5vjv4NfgXDbGPUcEheYnfz3cHE2mYxEG9KXta
+f8hSj/MNNv31BzNcj4XKcDqp4Ke3ow4lAZsPPlixOxxRaLnpsKZmEYYQcLI8KVNk
+gdAUOSPZgzRqAag0rvVfrpyvfvlu0D9xeiBLdhaJeZCq1/s=
+-----END CERTIFICATE-----
diff --git a/tests/StackExchange.Redis.Tests/Certificates/create_certificates.sh b/tests/StackExchange.Redis.Tests/Certificates/create_certificates.sh
@@ -0,0 +1,39 @@
+#!/bin/bash
+set -eu
+# Adopted 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"
diff --git a/tests/StackExchange.Redis.Tests/Certificates/device01.foo.com.pem b/tests/StackExchange.Redis.Tests/Certificates/device01.foo.com.pem
@@ -0,0 +1,18 @@
+-----BEGIN CERTIFICATE-----
+MIIC5jCCAc4CAQEwDQYJKoZIhvcNAQELBQAwNjETMBEGA1UEAwwKY2EuZm9vLmNv
+bTESMBAGA1UECgwJTXlEZXZpY2VzMQswCQYDVQQGEwJVUzAeFw0yNDAzMDcxNDAx
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+3dzUEaGexGqvA2MCapIQ2vCCMDF33ismQts=
+-----END CERTIFICATE-----
diff --git a/tests/StackExchange.Redis.Tests/StackExchange.Redis.Tests.csproj b/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>
