C#: Fix cs/web/xss false positive on Razor tag-helper attribute values

csharp/ql/lib/change-notes/2026-09-18-razor-tag-helper-attribute-xss-fp.md

@@ -0,0 +1,4 @@
+---
+category: majorAnalysis
+---
+* Fixed a false positive in `cs/web/xss` for ASP.NET Core Razor Pages/MVC views: `WriteLiteral` calls generated for tag helper attribute values (for example, `asp-for`) capture the value into an internal buffer instead of writing it directly to the response, so they are no longer treated as XSS sinks.

csharp/ql/lib/semmle/code/csharp/frameworks/microsoft/AspNetCore.qll

@@ -491,6 +491,16 @@ class MicrosoftAspNetCoreMvcRazorPageBase extends Class {
 
   /** Gets the `WriteLiteral` method. */
   Method getWriteLiteralMethod() { result = this.getAMethod("WriteLiteral") }
+
+  /** Gets the `BeginWriteTagHelperAttribute` method. */
+  Method getBeginWriteTagHelperAttributeMethod() {
+    result = this.getAMethod("BeginWriteTagHelperAttribute")
+  }
+
+  /** Gets the `EndWriteTagHelperAttribute` method. */
+  Method getEndWriteTagHelperAttributeMethod() {
+    result = this.getAMethod("EndWriteTagHelperAttribute")
+  }
 }
 
 /** A class deriving from `Microsoft.AspNetCore.Http.HttpRequest`, implements `HttpRequest` in ASP.NET Core. */

csharp/ql/lib/semmle/code/csharp/security/dataflow/flowsinks/Html.qll

@@ -177,14 +177,83 @@ class MicrosoftAspNetCoreMvcHtmlHelperRawSink extends AspNetCoreHtmlSink {
   }
 }
 
+/**
+ * Holds if `writeLiteral` is a call to `RazorPageBase.WriteLiteral` whose argument is captured
+ * between a matching pair of `BeginWriteTagHelperAttribute()`/`EndWriteTagHelperAttribute()`
+ * calls on `page`, in the same basic block, with no other such calls in between.
+ *
+ * The Razor source generator emits this bracketing for every literal or expression segment of an
+ * HTML attribute value on an element that also carries a tag helper (for example `asp-for`). Such
+ * a `WriteLiteral` call does not write directly to the response: `WriteLiteral` appends to an
+ * internal string buffer, and `EndWriteTagHelperAttribute()` returns that buffer as a tag helper
+ * attribute value rather than as page markup. This is therefore not a direct-write sink, unlike an
+ * unbracketed `WriteLiteral` call, whose argument is written straight to the response.
+ *
+ * Because a basic block cannot contain a branch, requiring `beginCall`, `writeLiteral`, and
+ * `endCall` to appear (in that order) in the same basic block, with no other
+ * `Begin`/`EndWriteTagHelperAttribute` call from `page` strictly between `beginCall` and
+ * `writeLiteral`, or between `writeLiteral` and `endCall`, guarantees that `beginCall`/`endCall`
+ * are the immediately enclosing bracket around `writeLiteral` on every path that reaches it (that
+ * is, the bracket opened by `beginCall` is still open, and not yet closed by some other `endCall`,
+ * at the point `writeLiteral` executes).
+ *
+ * `beginCall`, `writeLiteral`, and `endCall` are additionally required to have an implicit `this`
+ * qualifier, which is how the Razor source generator always emits these calls. This ensures all
+ * three calls act on the same page instance, so a bracket on one page cannot be mistaken for a
+ * bracket around a `WriteLiteral` call on a different page.
+ */
+private predicate isBracketedForTagHelperAttribute(Call writeLiteral) {
+  exists(
+    MicrosoftAspNetCoreMvcRazorPageBase page, MethodCall beginCall, MethodCall endCall, int i,
+    int j, int k
+  |
+    writeLiteral = page.getWriteLiteralMethod().getACall() and
+    beginCall = page.getBeginWriteTagHelperAttributeMethod().getACall() and
+    endCall = page.getEndWriteTagHelperAttributeMethod().getACall() and
+    writeLiteral.(QualifiableExpr).hasImplicitThisQualifier() and
+    beginCall.hasImplicitThisQualifier() and
+    endCall.hasImplicitThisQualifier() and
+    writeLiteral.getBasicBlock().getNode(i) = beginCall.getControlFlowNode() and
+    writeLiteral.getBasicBlock().getNode(j) = writeLiteral.getControlFlowNode() and
+    writeLiteral.getBasicBlock().getNode(k) = endCall.getControlFlowNode() and
+    i < j and
+    j < k and
+    not exists(int i2, Call other |
+      (
+        other = page.getBeginWriteTagHelperAttributeMethod().getACall() or
+        other = page.getEndWriteTagHelperAttributeMethod().getACall()
+      ) and
+      writeLiteral.getBasicBlock().getNode(i2) = other.getControlFlowNode() and
+      i < i2 and
+      i2 < j
+    ) and
+    not exists(int k2, Call other |
+      (
+        other = page.getBeginWriteTagHelperAttributeMethod().getACall() or
+        other = page.getEndWriteTagHelperAttributeMethod().getACall()
+      ) and
+      writeLiteral.getBasicBlock().getNode(k2) = other.getControlFlowNode() and
+      j < k2 and
+      k2 < k
+    )
+  )
+}
+
 /**
  * An expression that is used as an argument to `Page.WriteLiteral` in ASP.NET 6.0 razor page, typically in
  * a `.cshtml` file.
+ *
+ * `WriteLiteral` calls whose argument is captured for a tag helper attribute value (see
+ * `isBracketedForTagHelperAttribute`) are excluded, since such calls buffer the value as a tag
+ * helper attribute rather than writing it directly to the response.
  */
 class MicrosoftAspNetRazorPageWriteLiteralSink extends AspNetCoreHtmlSink {
   MicrosoftAspNetRazorPageWriteLiteralSink() {
-    this.getExpr() =
-      any(MicrosoftAspNetCoreMvcRazorPageBase h).getWriteLiteralMethod().getACall().getAnArgument()
+    exists(Call writeLiteral |
+      writeLiteral = any(MicrosoftAspNetCoreMvcRazorPageBase h).getWriteLiteralMethod().getACall() and
+      this.getExpr() = writeLiteral.getAnArgument() and
+      not isBracketedForTagHelperAttribute(writeLiteral)
+    )
   }
 }
 

csharp/ql/test/query-tests/Security Features/CWE-079/XSS/RazorTagHelperAttribute.cshtml

@@ -0,0 +1 @@
+// empty

csharp/ql/test/query-tests/Security Features/CWE-079/XSS/RazorTagHelperAttribute.cshtml.g.cs

@@ -0,0 +1,89 @@
+// A hand-written test file that mimics the output of compiling a `.cshtml` file that has an
+// element with a tag helper (e.g. `asp-for`) and an HTML attribute whose value contains a
+// tainted expression, for example: <input asp-for="Value" value="@Model.Value">
+#pragma checksum "RazorTagHelperAttribute.cshtml" "{ff1816ec-aa5e-4d10-87f7-6f4963833460}" "c4ae76542f1958092cebd8f57beef899d20fc548"
+// <auto-generated/>
+#pragma warning disable 1591
+[assembly: global::Microsoft.AspNetCore.Razor.Hosting.RazorCompiledItemAttribute(typeof(dotnetweb.Pages.Pages_RazorTagHelperAttribute), @"mvc.1.0.razor-page", @"RazorTagHelperAttribute.cshtml")]
+namespace dotnetweb.Pages
+{
+    #line hidden
+    using System;
+    using System.Collections.Generic;
+    using System.Linq;
+    using System.Threading.Tasks;
+    using Microsoft.AspNetCore.Mvc;
+    using Microsoft.AspNetCore.Mvc.Rendering;
+    using Microsoft.AspNetCore.Mvc.ViewFeatures;
+#nullable restore
+using dotnetweb;
+
+#line default
+#line hidden
+#nullable disable
+    [global::Microsoft.AspNetCore.Razor.Hosting.RazorSourceChecksumAttribute(@"SHA1", @"c4ae76542f1958092cebd8f57beef899d20fc548", @"RazorTagHelperAttribute.cshtml")]
+    public class Pages_RazorTagHelperAttribute : global::Microsoft.AspNetCore.Mvc.RazorPages.Page
+    {
+        #pragma warning disable 1998
+        public async override global::System.Threading.Tasks.Task ExecuteAsync()
+        {
+#nullable restore
+#line 3 "RazorTagHelperAttribute.cshtml"
+
+    var model = Request.Query["m"]; // $ Source=model
+
+#line default
+#line hidden
+#nullable disable
+            WriteLiteral("<input type=\"text\" ");
+            // GOOD: this `WriteLiteral` call is generated for the value of an HTML attribute on
+            // an element with a tag helper (e.g. `asp-for`). It writes into an internal string
+            // buffer via `BeginWriteTagHelperAttribute()`/`EndWriteTagHelperAttribute()`, and the
+            // buffered text is HTML-attribute-encoded later, when the tag helper attribute value
+            // is rendered. It is not a direct, unencoded write, so it must not be flagged.
+            BeginWriteTagHelperAttribute();
+            WriteLiteral(model);
+            var __tagHelperAttribute_1 = EndWriteTagHelperAttribute();
+            WriteLiteral(" />");
+
+            // BAD: a `WriteLiteral` call that is not bracketed by
+            // `BeginWriteTagHelperAttribute()`/`EndWriteTagHelperAttribute()` writes directly,
+            // unencoded, to the response and must still be flagged.
+            WriteLiteral(model); // $ Alert=model
+
+            // GOOD: two independent tag helper attribute brackets that occur one after another
+            // in the same basic block must each be matched with their own nearest
+            // `Begin`/`EndWriteTagHelperAttribute` call, and neither should leak into the other.
+            BeginWriteTagHelperAttribute();
+            WriteLiteral("literal-prefix-");
+            var __tagHelperAttribute_2 = EndWriteTagHelperAttribute();
+            BeginWriteTagHelperAttribute();
+            WriteLiteral(model);
+            var __tagHelperAttribute_3 = EndWriteTagHelperAttribute();
+
+            // BAD: a bare `WriteLiteral` call sandwiched between two unrelated tag helper
+            // attribute brackets, in the same basic block, must not be mistaken for being
+            // captured by either neighboring bracket.
+            BeginWriteTagHelperAttribute();
+            WriteLiteral("prefix");
+            var __tagHelperAttribute_4 = EndWriteTagHelperAttribute();
+            WriteLiteral(model); // $ Alert=model
+            BeginWriteTagHelperAttribute();
+            WriteLiteral("suffix");
+            var __tagHelperAttribute_5 = EndWriteTagHelperAttribute();
+        }
+        #pragma warning restore 1998
+        [global::Microsoft.AspNetCore.Mvc.Razor.Internal.RazorInjectAttribute]
+        public global::Microsoft.AspNetCore.Mvc.ViewFeatures.IModelExpressionProvider ModelExpressionProvider { get; private set; }
+        [global::Microsoft.AspNetCore.Mvc.Razor.Internal.RazorInjectAttribute]
+        public global::Microsoft.AspNetCore.Mvc.IUrlHelper Url { get; private set; }
+        [global::Microsoft.AspNetCore.Mvc.Razor.Internal.RazorInjectAttribute]
+        public global::Microsoft.AspNetCore.Mvc.IViewComponentHelper Component { get; private set; }
+        [global::Microsoft.AspNetCore.Mvc.Razor.Internal.RazorInjectAttribute]
+        public global::Microsoft.AspNetCore.Mvc.Rendering.IJsonHelper Json { get; private set; }
+        [global::Microsoft.AspNetCore.Mvc.Razor.Internal.RazorInjectAttribute]
+        public global::Microsoft.AspNetCore.Mvc.Rendering.IHtmlHelper<string> Html { get; private set; }
+        public global::Microsoft.AspNetCore.Mvc.ViewFeatures.ViewDataDictionary<string> ViewData => (global::Microsoft.AspNetCore.Mvc.ViewFeatures.ViewDataDictionary<string>)PageContext?.ViewData;
+    }
+}
+#pragma warning restore 1591

csharp/ql/test/query-tests/Security Features/CWE-079/XSS/XSS.expected

@@ -1,5 +1,7 @@
 #select
 | Index.cshtml:14:16:14:22 | call to operator implicit conversion | Index.cshtml:5:19:5:31 | access to property Query : IQueryCollection | Index.cshtml:14:16:14:22 | call to operator implicit conversion | $@ flows to here and is written to HTML or JavaScript: Microsoft.AspNetCore.Mvc.ViewFeatures.HtmlHelper.Raw() method. | Index.cshtml:5:19:5:31 | access to property Query : IQueryCollection | User-provided value |
+| RazorTagHelperAttribute.cshtml.g.cs:52:26:52:30 | call to operator implicit conversion | RazorTagHelperAttribute.cshtml:4:17:4:29 | access to property Query : IQueryCollection | RazorTagHelperAttribute.cshtml.g.cs:52:26:52:30 | call to operator implicit conversion | $@ flows to here and is written to HTML or JavaScript: Microsoft.AspNetCore.Mvc.Razor.RazorPageBase.WriteLiteral() method. | RazorTagHelperAttribute.cshtml:4:17:4:29 | access to property Query : IQueryCollection | User-provided value |
+| RazorTagHelperAttribute.cshtml.g.cs:70:26:70:30 | call to operator implicit conversion | RazorTagHelperAttribute.cshtml:4:17:4:29 | access to property Query : IQueryCollection | RazorTagHelperAttribute.cshtml.g.cs:70:26:70:30 | call to operator implicit conversion | $@ flows to here and is written to HTML or JavaScript: Microsoft.AspNetCore.Mvc.Razor.RazorPageBase.WriteLiteral() method. | RazorTagHelperAttribute.cshtml:4:17:4:29 | access to property Query : IQueryCollection | User-provided value |
 | XSSAspNet.cs:26:30:26:34 | access to local variable sayHi | XSSAspNet.cs:19:25:19:43 | access to property QueryString : NameValueCollection | XSSAspNet.cs:26:30:26:34 | access to local variable sayHi | $@ flows to here and is written to HTML or JavaScript: System.Web.WebPages.WebPage.WriteLiteral() method. | XSSAspNet.cs:19:25:19:43 | access to property QueryString : NameValueCollection | User-provided value |
 | XSSAspNet.cs:36:40:36:44 | access to local variable sayHi | XSSAspNet.cs:19:25:19:43 | access to property QueryString : NameValueCollection | XSSAspNet.cs:36:40:36:44 | access to local variable sayHi | $@ flows to here and is written to HTML or JavaScript: System.Web.WebPages.WebPage.WriteLiteralTo() method. | XSSAspNet.cs:19:25:19:43 | access to property QueryString : NameValueCollection | User-provided value |
 | XSSAspNet.cs:44:28:44:33 | access to local variable sayHi2 | XSSAspNet.cs:43:26:43:44 | access to property QueryString : NameValueCollection | XSSAspNet.cs:44:28:44:33 | access to local variable sayHi2 | $@ flows to here and is written to HTML or JavaScript. | XSSAspNet.cs:43:26:43:44 | access to property QueryString : NameValueCollection | User-provided value |
@@ -13,6 +15,9 @@
 edges
 | Index.cshtml:5:9:5:15 | access to local variable message : StringValues | Index.cshtml:14:16:14:22 | call to operator implicit conversion | provenance |  |
 | Index.cshtml:5:19:5:31 | access to property Query : IQueryCollection | Index.cshtml:5:9:5:15 | access to local variable message : StringValues | provenance |  |
+| RazorTagHelperAttribute.cshtml:4:9:4:13 | access to local variable model : StringValues | RazorTagHelperAttribute.cshtml.g.cs:52:26:52:30 | call to operator implicit conversion | provenance |  |
+| RazorTagHelperAttribute.cshtml:4:9:4:13 | access to local variable model : StringValues | RazorTagHelperAttribute.cshtml.g.cs:70:26:70:30 | call to operator implicit conversion | provenance |  |
+| RazorTagHelperAttribute.cshtml:4:17:4:29 | access to property Query : IQueryCollection | RazorTagHelperAttribute.cshtml:4:9:4:13 | access to local variable model : StringValues | provenance |  |
 | XSSAspNet.cs:19:17:19:21 | access to local variable sayHi : String | XSSAspNet.cs:26:30:26:34 | access to local variable sayHi | provenance |  |
 | XSSAspNet.cs:19:17:19:21 | access to local variable sayHi : String | XSSAspNet.cs:36:40:36:44 | access to local variable sayHi | provenance |  |
 | XSSAspNet.cs:19:25:19:43 | access to property QueryString : NameValueCollection | XSSAspNet.cs:19:17:19:21 | access to local variable sayHi : String | provenance |  |
@@ -49,6 +54,10 @@ nodes
 | Index.cshtml:5:9:5:15 | access to local variable message : StringValues | semmle.label | access to local variable message : StringValues |
 | Index.cshtml:5:19:5:31 | access to property Query : IQueryCollection | semmle.label | access to property Query : IQueryCollection |
 | Index.cshtml:14:16:14:22 | call to operator implicit conversion | semmle.label | call to operator implicit conversion |
+| RazorTagHelperAttribute.cshtml.g.cs:52:26:52:30 | call to operator implicit conversion | semmle.label | call to operator implicit conversion |
+| RazorTagHelperAttribute.cshtml.g.cs:70:26:70:30 | call to operator implicit conversion | semmle.label | call to operator implicit conversion |
+| RazorTagHelperAttribute.cshtml:4:9:4:13 | access to local variable model : StringValues | semmle.label | access to local variable model : StringValues |
+| RazorTagHelperAttribute.cshtml:4:17:4:29 | access to property Query : IQueryCollection | semmle.label | access to property Query : IQueryCollection |
 | XSSAspNet.cs:19:17:19:21 | access to local variable sayHi : String | semmle.label | access to local variable sayHi : String |
 | XSSAspNet.cs:19:25:19:43 | access to property QueryString : NameValueCollection | semmle.label | access to property QueryString : NameValueCollection |
 | XSSAspNet.cs:19:25:19:52 | access to indexer : String | semmle.label | access to indexer : String |