Fix TLS certificate validation in C# and ASP.NET Core
=====================================================

Never return true from a certificate validation callback. Leave validation to
SslStream and change only the trust anchor: set
SocketsHttpHandler.SslOptions.CertificateChainPolicy to CustomRootTrust with
the partner's CA in CustomTrustStore, and RevocationMode NoCheck if the
private CA has no CRL. SslStream still checks the chain, the dates and the
host name, with no callback.

Page: https://softwaresecurity.com/fix/tls-certificate-validation/csharp-aspnetcore/

What is in this archive
-----------------------
tls-certificate-validation/csharp-aspnetcore/   the project: three implementations of the same code and one test suite
  src/Rates/impl/VulnerablePartnerTls.cs   (vulnerable)
  src/Rates/impl/FixedPartnerTls.cs   (fixed)
  src/Rates/impl/WrongPartnerTls.cs   (a plausible fix that still fails)
run-all.mjs   runs the tests in all three modes and checks the results (Node.js only)
LICENSE-CODE.txt

Run it
------
  Tested with .NET 10.0.12 (SDK 10.0.112). run-all.mjs needs Node.js 22 or
  later. Supported versions: https://softwaresecurity.com/fix/versions/

  From the tls-certificate-validation/csharp-aspnetcore folder, install the
  dependencies once (this needs network access):

    dotnet restore tests/Rates.Tests --locked-mode

  Then run the tests. IMPL chooses the implementation the tests load:

    # every test passes
    IMPL=fixed dotnet test --project tests/Rates.Tests --no-restore --results-directory reports --report-xunit-junit --report-xunit-junit-filename junit.xml
    # an exploit test fails
    IMPL=vulnerable dotnet test --project tests/Rates.Tests --no-restore --results-directory reports --report-xunit-junit --report-xunit-junit-filename junit.xml
    # a test fails
    IMPL=wrong dotnet test --project tests/Rates.Tests --no-restore --results-directory reports --report-xunit-junit --report-xunit-junit-filename junit.xml

  The JUnit report is written to reports/junit.xml. To run all three modes and
  check the results in one step, run the harness from this top folder (it needs
  only Node.js; --install runs the install step first):

    node run-all.mjs --install tls-certificate-validation/csharp-aspnetcore
    node run-all.mjs tls-certificate-validation/csharp-aspnetcore

Verified
--------
  11 October 2026 on macOS 27.0 (Darwin 27.0.0, arm64), with dotnetSdk 10.0.112,
  dotnetRuntime 10.0.12, aspnetMvcTesting 10.0.12, xunitV3 4.0.1. Versions and
  results hold for that date: run the tests with your own versions before
  relying on the fix.

Licence
-------
  The code is MIT licensed (LICENSE-CODE.txt). The page text is CC BY 4.0.
