JS: Handle default 'content-type' header in Response() objects #20918
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November 27, 2025 10:58
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Pull request overview
This PR improves the JavaScript XSS detection by recognizing that the Response() constructor defaults to text/plain;charset=utf-8 content-type when no explicit content-type header is set. This means that new Response(data) without an explicit HTML content-type header is not an XSS sink, reducing false positives.
Key Changes:
- Updated the
ResponseArgumentHeadersclass to model the default content-type header behavior - Removed XSS alerts from test cases where no content-type or non-HTML content-type is used
- Added new test cases to verify the behavior with unrelated headers
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.
Show a summary per file
| File | Description |
|---|---|
| javascript/ql/lib/semmle/javascript/frameworks/WebResponse.qll | Implements logic to add a default text/plain;charset=utf-8 content-type when no explicit content-type header is defined |
| javascript/ql/test/query-tests/Security/CWE-079/ReflectedXss/response-object.js | Updates test cases by removing XSS alert annotations from Response calls with default or plain-text content-type, and adds new test cases |
| javascript/ql/test/query-tests/Security/CWE-079/ReflectedXss/ReflectedXss.expected | Removes expected XSS alerts that are now correctly not detected due to default content-type |
| javascript/ql/test/query-tests/Security/CWE-079/ReflectedXss/ReflectedXssWithCustomSanitizer.expected | Removes expected XSS alerts consistent with the main test expectations |
| javascript/ql/src/change-notes/2025-11-26-response-default-content-type.md | Documents the analysis improvement in the release notes |
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javascript/ql/test/query-tests/Security/CWE-079/ReflectedXss/response-object.js
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| --- | ||
| category: minorAnalysis | ||
| --- | ||
| * `new Response(x)` is not longer seen as a reflected XSS sink when no`content-type` header |
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Typo: "is not longer" should be "is no longer".
Suggested change
| * `new Response(x)` is not longer seen as a reflected XSS sink when no`content-type` header | |
| * `new Response(x)` is no longer seen as a reflected XSS sink when no`content-type` header |
Co-authored-by: Copilot <[email protected]>
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When using
new Response()to construct HTTP responses, thecontent-typeheader defaults totext/plain;charset=utf-8unless explicitly set to something else.This means its argument is not an html-injection sink when the header is omitted.