.NET / C# source review

When it applies

Reviewing C#/.NET source (ASP.NET Core, MVC, or legacy WebForms). The headline risks are unsafe deserialization, string-built SQL, XXE, and Razor's raw-output escape hatch.

Why it works

.NET ships powerful-but-dangerous serializers (BinaryFormatter, Json.NET with TypeNameHandling, LosFormatter/ViewState) that instantiate arbitrary types, and several XML APIs resolve DTDs by default on older frameworks. Reviews find where these meet untrusted input.

Sinks & patterns (grep, then trace to user input)

• Deserialization: BinaryFormatter, LosFormatter, SoapFormatter, NetDataContractSerializer, JsonConvert with TypeNameHandling != None, Js.NET/fastJSON polymorphic types → RCE gadgets.
• SQLi: string-concatenated SqlCommand/ExecuteReader/FromSqlRaw(EF Core) vs parameters.
• Command exec: Process.Start with concatenated arguments / UseShellExecute.
• XXE: XmlDocument/XmlReader/XmlTextReader without DtdProcessing=Prohibit (legacy default resolves entities); XmlSerializer with a user-controlled type.
• XSS: Razor @Html.Raw(...), MvcHtmlString, WebForms <%= %> with unencoded input; Response.Write.
• Other: path traversal via Path.Combine(root, input), LdapConnection filter injection, reflection (Type.GetType/Activator.CreateInstance) on input, insecure ViewState (no MAC).

Framework specifics

• ASP.NET Core: model binding over-posting/mass assignment (bind whole entity), [AllowAnonymous] on sensitive actions, disabled antiforgery on POST APIs, exposed dev endpoints, open redirect via Redirect(returnUrl) without Url.IsLocalUrl.
• WebForms: ViewState deserialization (machineKey), event-validation off.

Method

1. Run security-code-scan/CodeQL; treat as leads.
2. rg 'BinaryFormatter|TypeNameHandling|FromSqlRaw|Html\.Raw|XmlDocument|Process\.Start' → trace to input.
3. Check controller [Authorize]/antiforgery coverage and model-binding scope.
4. Confirm with web-deserialization, web-sqli, web-xxe, web-command-injection.

Gotchas

• Json.NET is safe by default — the bug is an explicit TypeNameHandling.All/Auto.
• EF Core parameterises LINQ; FromSqlRaw/ExecuteSqlRaw with interpolation is where SQLi returns.
• ViewState RCE needs the machineKey (leaked/weak) — note the precondition.

References

OWASP .NET security cheat sheet; ysoserial.net gadget research; Microsoft secure-coding guidance.