Fix unrestricted file upload in C# and ASP.NET Core
===================================================

Decide the type from the bytes, and never let the client pick the name. Limit
the endpoint with WithFormOptions(multipartBodyLengthLimit), require the
extension, IFormFile.ContentType and the PNG or JPEG signature to agree, save
under RandomNumberGenerator.GetHexString() outside the web root, and serve
with Results.File(..., fileDownloadName) plus X-Content-Type-Options: nosniff.

Page: https://softwaresecurity.com/fix/file-upload/csharp-aspnetcore/

What is in this archive
-----------------------
file-upload/csharp-aspnetcore/   the project: three implementations of the same code and one test suite
  src/Avatars/impl/VulnerableAvatarEndpoints.cs   (vulnerable)
  src/Avatars/impl/FixedAvatarEndpoints.cs   (fixed)
  src/Avatars/impl/WrongAvatarEndpoints.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 file-upload/csharp-aspnetcore folder, install the dependencies once
  (this needs network access):

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

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

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

Verified
--------
  11 October 2026 on macOS 27.0 (Darwin 27.0.0, arm64), with dotnetSdk 10.0.112,
  dotnetRuntime 10.0.12, mvcTesting 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.
