CVE-2026-27739 is a critical SSRF vulnerability in Angular's Server-Side Rendering (SSR) pipeline affecting angular-cli versions before 21.2.0-rc.1, 21.1.5, 20.3.17, and 19.2.21. The vulnerability stems from Angular's internal URL reconstruction logic that directly trusts user-controlled HTTP headers, specifically the Host and X-Forwarded-* headers, to determine the application's base origin without validating the destination domain, path, characters, or port. This lack of validation enables attackers to craft malicious requests that cause the SSR server to make arbitrary internal HTTP requests. The vulnerability manifests through two main vectors: implicit relative URL resolution in HttpClient requests and explicit manual URL construction using the unvalidated REQUEST object headers. For exploitation, the target application must use Angular SSR, perform HttpClient requests with relative URLs or manual URL construction from headers, have a server accessible to attackers who can influence these headers, and lack upstream infrastructure sanitization of these headers. Exploiting this SSRF can lead to internal network scanning, exfiltration of sensitive credentials, and breaches of confidentiality within internal services. Angular has addressed this issue in versions 21.2.0-rc.1, 21.1.5, 20.3.17, and 19.2.21 by adding validation and sanitization of these headers. Workarounds include avoiding reliance on req.headers for URL construction and implementing middleware to enforce strict hostname and numeric port validation. The CVSS 4.0 score is 9.2 (critical), reflecting the vulnerability's high impact and ease of exploitation without authentication or user interaction. No known exploits have been reported in the wild as of now.

The impact of CVE-2026-27739 is significant for organizations using Angular SSR in vulnerable versions. Successful exploitation allows attackers to perform SSRF attacks, enabling them to send arbitrary HTTP requests from the server to internal or external systems. This can lead to internal network reconnaissance, exposing sensitive infrastructure details, and potentially pivoting attacks deeper into the network. Credential exfiltration is possible if internal services expose sensitive tokens or secrets accessible via SSRF. Confidentiality breaches may occur if internal-only services or data stores are accessed. Since SSRF can bypass perimeter defenses by leveraging the server's network access, it poses a high risk to cloud-hosted applications and microservice architectures. Organizations relying on Angular SSR without proper header validation or infrastructure sanitization are at risk of data leakage, service disruption, and further compromise. The vulnerability's critical severity and no requirement for authentication or user interaction make it a high-priority issue for affected environments.

To mitigate CVE-2026-27739, organizations should immediately upgrade Angular CLI and Angular SSR to versions 21.2.0-rc.1, 21.1.5, 20.3.17, or 19.2.21 or later, where the vulnerability is patched. If immediate upgrade is not feasible, implement middleware in the server.ts file to strictly validate and sanitize incoming Host and X-Forwarded-* headers, enforcing allowed hostnames and numeric port ranges. Avoid constructing URLs for HttpClient requests using untrusted headers such as req.headers; instead, use hardcoded or environment-configured trusted base URLs. Ensure that front-facing proxies, load balancers, CDNs, or cloud infrastructure sanitize or reject suspicious or malformed headers to prevent attacker-controlled header injection. Conduct thorough code reviews to identify any manual URL constructions relying on unvalidated headers and refactor them accordingly. Employ network segmentation and internal service authentication to limit the impact of SSRF if exploited. Monitor logs for unusual internal requests originating from SSR components. Finally, educate developers about the risks of trusting client-supplied headers in server-side rendering contexts.