CVE-2026-27739: Angular SSR is vulnerable to SSRF and Header Injection via request handling pipeline

A Server-Side Request Forgery (SSRF) vulnerability has been identified in the Angular SSR request handling pipeline. The vulnerability exists because Angular’s internal URL reconstruction logic directly trusts and consumes user-controlled HTTP headers specifically the Host and X-Forwarded- family to determine the application's base origin without any validation of the destination domain.

Specifically, the framework didn't have checks for the following: - Host Domain: The Host and X-Forwarded-Host headers were not checked to belong to a trusted origin. This allows an attacker to redefine the "base" of the application to an arbitrary external domain. - Path & Character Sanitization: The X-Forwarded-Host header was not checked for path segments or special characters, allowing manipulation of the base path for all resolved relative URLs. - Port Validation: The X-Forwarded-Port header was not verified as numeric, leading to malformed URI construction or injection attacks.

This vulnerability manifests in two primary ways:

- Implicit Relative URL Resolution: Angular's HttpClient resolves relative URLs against this unvalidated and potentially malformed base origin. An attacker can "steer" these requests to an external server or internal service. - Explicit Manual Construction: Developers injecting the REQUEST object to manually construct URLs (for fetch or third-party SDKs) directly inherit these unsanitized values. By accessing the Host / X-Forwarded- headers, the application logic may perform requests to attacker-controlled destinations or malformed endpoints.

Impact

When successfully exploited, this vulnerability allows for arbitrary internal request steering. This can lead to: - Credential Exfiltration: Stealing sensitive Authorization headers or session cookies by redirecting them to an attacker's server. - Internal Network Probing: Accessing and transmitting data from internal services, databases, or cloud metadata endpoints (e.g., 169.254.169.254) not exposed to the public internet. - Confidentiality Breach: Accessing sensitive information processed within the application's server-side context.

Attack Preconditions

- The victim application must use Angular SSR (Server-Side Rendering). - The application must perform HttpClient requests using relative URLs OR manually construct URLs using the unvalidated Host / X-Forwarded- headers using the REQUEST object. - Direct Header Access: The application server is reachable by an attacker who can influence these headers without strict validation from a front-facing proxy. - Lack of Upstream Validation: The infrastructure (Cloud, CDN, or Load Balancer) does not sanitize or validate incoming headers.

Patches

- 21.2.0-rc.1 - 21.1.5 - 20.3.17 - 19.2.21

Workarounds - Use Absolute URLs: Avoid using req.headers for URL construction. Instead, use trusted variables for your base API paths. - Implement Strict Header Validation (Middleware): If you cannot upgrade immediately, implement a middleware in your server.ts to enforce numeric ports and validated hostnames.

ts const ALLOWEDHOSTS = new Set(['your-domain.com']);

app.use((req, res, next) => { const hostHeader = (req.headers['x-forwarded-host'] ?? req.headers['host'])?.toString(); const portHeader = req.headers['x-forwarded-port']?.toString();

if (hostHeader) { const hostname = hostHeader.split(':')[0]; // Reject if hostname contains path separators or is not in allowlist if (/^[a-z0-9.:-]+$/i.test(hostname) || (!ALLOWEDHOSTS.has(hostname) && hostname !== 'localhost')) { return res.status(400).send('Invalid Hostname'); } }

// Ensure port is strictly numeric if provided if (portHeader && !/^\d+$/.test(portHeader)) { return res.status(400).send('Invalid Port'); }

next(); });

References

- Fix - Docs