CVE-2026-27738

Description

An Open Redirect vulnerability exists in the internal URL processing logic in Angular SSR. The logic normalizes URL segments by stripping leading slashes; however, it only removes a single leading slash. When an Angular SSR application is deployed behind a proxy that passes the `X-Forwarded-Prefix` header, an attacker can provide a value starting with three slashes (e.g., `///evil.com`). 1. The application processes a redirect (e.g., from a router `redirectTo` or i18n locale switch). 2. Angular receives `///evil.com` as the prefix. 3. It strips one slash, leaving `//evil.com`. 4. The resulting string is used in the `Location` header. 5. Modern browsers interpret `//` as a protocol-relative URL, redirecting the user from `https://your-app.com` to `https://evil.com`. ### Impact This vulnerability allows attackers to conduct large-scale phishing and SEO hijacking: - **Scale:** A single request can poison a high-traffic route, impacting all users until the cache expires. - **SEO Poisoning:** Search engine crawlers may follow and index these malicious redirects, causing the legitimate site to be delisted or associated with malicious domains. - **Trust:** Because the initial URL belongs to the trusted domain, users and security tools are less likely to flag the redirect as malicious. ### Attack Preconditions - The application must use Angular SSR. - The application must have routes that perform internal redirects. - The infrastructure (Reverse Proxy/CDN) must pass the `X-Forwarded-Prefix` header to the SSR process without sanitization. - The cache must not vary on the `X-Forwarded-Prefix` header. ### Patches - 21.2.0-rc.1 - 21.1.5 - 20.3.17 - 19.2.21 ### Workarounds Until the patch is applied, developers should sanitize the `X-Forwarded-Prefix` header in their`server.ts` before the Angular engine processes the request: ```ts app.use((req, res, next) => { const prefix = req.headers['x-forwarded-prefix']?.trim(); if (prefix) { // Sanitize by removing all leading slashes req.headers['x-forwarded-prefix'] = prefix.replace(/^[/\\]+/, '/'); } next(); }); ``` ### Resources - [Report](https://github.com/angular/angular-cli/issues/32501) - [Fix](https://github.com/angular/angular-cli/pull/32521)

How to fix CVE-2026-27738

To remediate CVE-2026-27738, upgrade the affected package to a fixed version below.

• —upgrade to 21.2.0-rc.1 or later