Fix server-side template injection in C# and ASP.NET Core
=========================================================

Don't run user input as code: CSharpScript.EvaluateAsync compiles the
'template' as C#. Render user-authored templates with a secure engine such as
Fluid, whose member access is limited to the types you register, so it cannot
reach code, reflection or the environment. For plain values, bind them to a
constant template. Do not denylist Process or GetType;
Environment.GetEnvironmentVariable slips past.

Page: https://softwaresecurity.com/fix/template-injection/csharp-aspnetcore/

What is in this archive
-----------------------
template-injection/csharp-aspnetcore/   the project: three implementations of the same code and one test suite
  src/Templating/impl/VulnerableTemplateRenderer.cs   (vulnerable)
  src/Templating/impl/FixedTemplateRenderer.cs   (fixed)
  src/Templating/impl/WrongTemplateRenderer.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 template-injection/csharp-aspnetcore folder, install the dependencies
  once (this needs network access):

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

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

    # every test passes
    SSTI_SECRET=TMPL-INJECTION-PROOF-4d9f2a7c IMPL=fixed dotnet test --project tests/Templating.Tests --no-restore --results-directory reports --report-xunit-junit --report-xunit-junit-filename junit.xml
    # an exploit test fails
    SSTI_SECRET=TMPL-INJECTION-PROOF-4d9f2a7c IMPL=vulnerable dotnet test --project tests/Templating.Tests --no-restore --results-directory reports --report-xunit-junit --report-xunit-junit-filename junit.xml
    # a test fails
    SSTI_SECRET=TMPL-INJECTION-PROOF-4d9f2a7c IMPL=wrong dotnet test --project tests/Templating.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 template-injection/csharp-aspnetcore
    node run-all.mjs template-injection/csharp-aspnetcore

Verified
--------
  11 October 2026, with dotnetSdk 10.0.112, dotnetRuntime 10.0.12, fluidCore
  2.40.0, roslynScripting 5.6.0, aspnetMvcTesting 10.0.12, 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.
