CVE-2026-27739

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.

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.

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.

const ALLOWED_HOSTS = 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) || 
       (!ALLOWED_HOSTS.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

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.

Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside. Typical impact: access to internal metadata services, internal APIs, or cloud credentials.

Kodem intelligence

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

1. What is CVE-2026-27739? CVE-2026-27739 is a critical-severity server-side request forgery (SSRF) vulnerability in @angular/ssr (npm), affecting versions >= 21.2.0-next.0, < 21.2.0-rc.0. It is fixed in 21.2.0-rc.1, 21.1.5, 20.3.17, 19.2.21. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
2. Which packages are affected by CVE-2026-27739?
   • @angular/ssr (npm) (versions >= 21.2.0-next.0, < 21.2.0-rc.0)
   • @nguniversal/common (npm) (versions <= 16.2.0)
   • @nguniversal/express-engine (npm) (versions <= 16.2.0)
3. Is there a fix for CVE-2026-27739? Yes. CVE-2026-27739 is fixed in 21.2.0-rc.1, 21.1.5, 20.3.17, 19.2.21. Upgrade to this version or later.
4. Is CVE-2026-27739 exploitable, and should I be worried? Whether CVE-2026-27739 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-27739 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-27739?
   • Upgrade @angular/ssr to 21.2.0-rc.1 or later
   • Upgrade @angular/ssr to 21.1.5 or later
   • Upgrade @angular/ssr to 20.3.17 or later
   • Upgrade @angular/ssr to 19.2.21 or later