An incomplete fix for [GHSA-58c5-g7wp-6w37](https://github.com/angular/angular/security/advisories/GHSA-58c5-g7wp-6w37) (`CVE-2025-66035`) in `@angular/common`'s HTTP Client (`HttpClient`) allows Cross-Site Request Forgery (XSRF) tokens to be leaked to attacker-controlled cross-origin domains when requests use URLs that exploit parsing discrepancies between regular expressions and the browser's WHATWG URL parser. ### Technical Description To address `CVE-2025-66035`, `xsrfInterceptorFn` introduced a regular expression (`const ABSOLUTE_URL_REGEX = /^(?:https?:)?\/\//i`) to detect and skip absolute and protocol-relative URLs before attaching the `X-XSRF-TOKEN` header. However, browsers resolve request URLs passed to `fetch()` and `XMLHttpRequest.open()` using the **WHATWG URL parser**. For `http:` and `https:` schemes, the WHATWG URL standard performs several normalizations prior to resolving the authority/origin: * Backslashes (`\`) are treated as forward slashes (`/`) — e.g., `/\evil.example/x`, `\\evil.example/x`, `\/evil.example/x`, `https:\\evil.example/x`, `https:/\evil.example/x`. * Leading and trailing C0 control characters and spaces (`<= 0x20`) are stripped — e.g., `' //evil.example/x'`, `'\0//evil.example/x'`, `'\x1f//evil.example/x'`. * ASCII tab (`\t`), newline (`\n`), and carriage return (`\r`) characters are removed anywhere in the input — e.g., `'/\t/evil.example/x'`, `'/\n/evil.example/x'`, `'htt\tps://evil.example/x'`. Because these URLs do not match `/^(?:https?:)?\/\//i`, `xsrfInterceptorFn` treats them as relative same-origin URLs and attaches the `X-XSRF-TOKEN` header, while the browser resolves and dispatches the request to the cross-origin host (`evil.example`). Note: Angular `21.0.3+` and `22.x` replaced the regular expression with a WHATWG `new URL(req.url, locationOrigin)` origin comparison in [#65683](https://github.com/angular/angular/pull/65683) and were already unaffected. However, the `20.3.x` LTS branch retained the regular expression until `20.3.34`. ### Impact The token leakage bypasses Angular's built-in XSRF protection on affected versions, allowing an attacker to capture the user's valid `X-XSRF-TOKEN` header if their server permits the request and custom header via CORS. Once obtained, the attacker can perform Cross-Site Request Forgery (CSRF) attacks against the victim user's session. ### Attack Preconditions 1. The victim's Angular application must have **XSRF protection enabled** (enabled by default in `HttpClient`). 2. The attacker must be able to make the application send a state-changing HTTP request (e.g., `POST`, `PUT`, `DELETE`, `PATCH`) to a URL they control (such as `/\attacker.example/api` or ` //attacker.example/api`). ### Patches The issue is resolved by parsing and comparing the request URL's origin against the application's current origin using the standard `URL` constructor (`new URL(req.url, locationOrigin)`). * `21.0.3` (and all `22.x` releases) * `20.3.34` ### Workarounds If you cannot immediately upgrade to a patched version: * Validate and normalize outgoing request URLs using `new URL(url, window.location.origin)` and verify that `parsedUrl.origin === window.location.origin` before passing dynamic URLs to mutating `HttpClient` requests. * Avoid passing untrusted or user-controlled URLs to `HttpClient` mutating methods.