Fix insecure deserialization in C# and ASP.NET Core
===================================================

Do not let the payload choose the type. With Newtonsoft.Json keep
TypeNameHandling at None and bind to a concrete class; a SerializationBinder
blocklist is not enough and even quiets the analyzer. For real polymorphism
use System.Text.Json with [JsonDerivedType], which allowlists the subtypes,
instead of Type.GetType. BinaryFormatter is removed in modern .NET.

Page: https://softwaresecurity.com/fix/deserialization/csharp-aspnetcore/

What is in this archive
-----------------------
deserialization/csharp-aspnetcore/   the project: three implementations of the same code and one test suite
  src/Session/impl/VulnerableSessionService.cs   (vulnerable)
  src/Session/impl/FixedSessionService.cs   (fixed)
  src/Session/impl/WrongSessionService.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 deserialization/csharp-aspnetcore folder, install the dependencies
  once (this needs network access):

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

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

    # every test passes
    IMPL=fixed dotnet test --project tests/Session.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/Session.Tests --no-restore --results-directory reports --report-xunit-junit --report-xunit-junit-filename junit.xml
    # a test fails
    IMPL=wrong dotnet test --project tests/Session.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 deserialization/csharp-aspnetcore
    node run-all.mjs deserialization/csharp-aspnetcore

Verified
--------
  11 October 2026, with dotnetSdk 10.0.112, dotnetRuntime 10.0.12,
  newtonsoftJson 13.0.4, xunitV3 4.0.2. 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.
