Test failure: System.Net.Security.Tests.TlsSessionTests.ClientSession_ExternalCertificateValidation_AcceptWithDefaultValidation_FailsOnUntrustedCert fails with "Assert.NotNull() Failure: Value is null" #135628

Build Information

Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1631737
Build error leg or test failing: System.Net.Security.Tests.TlsSessionTests.ClientSession_ExternalCertificateValidation_AcceptWithDefaultValidation_FailsOnUntrustedCert
Pull request: #134978

Error Message

{
  "ErrorMessage": [
    "ClientSession_ExternalCertificateValidation_AcceptWithDefaultValidation_FailsOnUntrustedCert [FAIL]",
    "Assert.NotNull() Failure: Value is null"
  ],
  "ErrorPattern": "",
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

Error Details

Helix work item System.Net.Security.Tests, queue windows.10.amd64.open.rt (Windows Server 2016, 10.0.14393).
Console log: https://helixr1107v0xdcypoyl9e7f.blob.core.windows.net/dotnet-runtime-refs-pull-134978-merge-10223389250843eb9f/System.Net.Security.Tests/1/console.9d84ff3f.log?helixlogtype=result

System.Net.Security.Tests.TlsSessionTests.ClientSession_ExternalCertificateValidation_AcceptWithDefaultValidation_FailsOnUntrustedCert [FAIL]
  Assert.NotNull() Failure: Value is null
  Stack Trace:
    TlsSessionTests.cs(1841,0): at ...<ClientSession_ExternalCertificateValidation_AcceptWithDefaultValidation_FailsOnUntrustedCert>b__2()
    TlsSessionTests.cs(2068,0): at TlsSessionTests.DriveHandshakeWithExternalValidationAsync(TlsBufferSession session, Stream transport, Action onSuspend)
    TlsSessionTests.cs(1836,0): at ...<ClientSession_ExternalCertificateValidation_AcceptWithDefaultValidation_FailsOnUntrustedCert>b__0()
    TlsSessionTests.cs(1855,0): at TlsSessionTests.ClientSession_ExternalCertificateValidation_AcceptWithDefaultValidation_FailsOnUntrustedCert()

The test asserts session.GetRemoteCertificates() is non-null in the onSuspend callback (line 1841, added in #134570).
GetRemoteCertificates() is documented to return null when no intermediates were received, so on this Windows
machine the client apparently saw no peer-sent intermediates. One occurrence so far; the PR (#134978) does not touch System.Net.Security.
Searched dotnet/runtime for the test name, assertion text and the TlsSessionTests class: no existing KBE, tracker, or open PR covers it.

Known issue validation

Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=1631737
Error message validated: [ClientSession_ExternalCertificateValidation_AcceptWithDefaultValidation_FailsOnUntrustedCert [FAIL] Assert.NotNull() Failure: Value is null]
Result validation: ✅ Known issue matched with the provided build.
Validation performed at: 10/12/2026 3:51:14 PM UTC

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