CVE-2026-27739 is a critical SSRF vulnerability in Angular's Server-Side Rendering (SSR) component within angular-cli versions before 21.2.0-rc.1, 21.1.5, 20.3.17, and 19.2.21. Angular SSR reconstructs URLs internally to perform HttpClient requests during server-side rendering. The vulnerability stems from Angular's internal URL reconstruction logic that directly trusts user-controlled HTTP headers, specifically the Host and X-Forwarded-* headers, without validating the destination domain, path, characters, or port. This lack of validation allows attackers to influence the base origin used for relative URL resolution or manual URL construction, effectively enabling SSRF attacks. SSRF allows an attacker to coerce the server to make arbitrary HTTP requests to internal or external systems, potentially bypassing network segmentation and exposing sensitive internal resources. Exploitation requires that the victim application uses Angular SSR, performs HttpClient requests with relative URLs or manual URL construction from unvalidated headers, and that the attacker can influence these headers without front-end proxy validation or sanitization by cloud/CDN/load balancer infrastructure. The vulnerability can lead to internal network scanning, credential exfiltration, and confidentiality breaches. Angular has patched this vulnerability in versions 21.2.0-rc.1, 21.1.5, 20.3.17, and 19.2.21. Workarounds include avoiding use of req.headers for URL construction and implementing middleware to validate hostnames and enforce numeric ports. The CVSS 4.0 score is 9.2, indicating critical severity with network attack vector, no privileges or user interaction required, and high impact on confidentiality.

The impact of CVE-2026-27739 is significant for organizations using Angular SSR in their web applications. 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 services that are otherwise inaccessible externally. Attackers may exfiltrate credentials or sensitive data by accessing internal APIs or metadata services, potentially leading to further compromise. The vulnerability undermines confidentiality and can indirectly affect integrity if attackers leverage SSRF to pivot to other attacks. Availability impact is limited but possible if internal services are overwhelmed or manipulated. Because exploitation requires no authentication or user interaction and can be performed remotely, the attack surface is broad. Organizations relying on cloud infrastructure, CDNs, or load balancers that do not sanitize headers are particularly at risk. The vulnerability can affect a wide range of industries, including technology, finance, healthcare, and government sectors that use Angular SSR for server-side rendering of web applications.

To mitigate CVE-2026-27739, organizations should prioritize upgrading Angular CLI and Angular SSR to the patched versions 21.2.0-rc.1, 21.1.5, 20.3.17, or 19.2.21 as soon as possible. Until upgrades can be applied, developers should avoid constructing URLs using untrusted HTTP headers such as Host and X-Forwarded-*; instead, use hardcoded or environment-configured trusted base URLs for API calls. Implement middleware in the server.ts file to validate incoming Host headers and enforce numeric port validation, rejecting requests with suspicious or malformed headers. Deploy front-facing proxies, load balancers, or CDNs that strictly sanitize and validate incoming HTTP headers to prevent attacker-controlled header injection. Conduct thorough code reviews to identify any usage of relative URLs in HttpClient requests and manual URL construction from request headers, refactoring these to use safe, validated values. Monitor server logs for unusual internal request patterns indicative of SSRF attempts. Additionally, apply network segmentation and access controls to limit the impact of SSRF by restricting server access to sensitive internal resources. Regularly update dependencies and monitor security advisories for Angular and related components.