Angular SSR Flaw Enables Unauthorized Server-Side Requests in Web Apps

A critical vulnerability has been discovered in Angular Server-Side Rendering (SSR) that could allow attackers to perform Server-Side Request Forgery (SSRF) and Header Injection attacks.

Tracked as CVE-2026-27739, this flaw enables unauthorized server-side requests in web applications, potentially leading to credential theft, internal network probing, and data exposure.

The vulnerability affects multiple versions of @angular/ssr, @nguniversal/common, and @nguniversal/express-engine, making it a widespread concern for developers utilizing Angular’s SSR capabilities.

Angular SSR Vulnerability

According to Angular, the core of this issue lies in how Angular’s internal URL reconstruction logic processes user-controlled HTTP headers.

Specifically, the framework trusts the Host and X-Forwarded-* family of headers to determine the application’s base origin without verifying the destination domain.

Angular failed to implement checks for three key areas:

• Host Domain: The Host and X-Forwarded-Host headers were not verified against trusted origins, allowing attackers to change the application’s base to a malicious external domain.
• Path & Character Sanitization: The X-Forwarded-Host header lacked checks for path segments or special characters, enabling manipulation of the base path for relative URLs.
• Port Validation: The X-Forwarded-Port header was not strictly validated as numeric, which could lead to malformed URI construction and injection attacks.

Impact and Attack Vectors

When exploited, this vulnerability allows attackers to steer internal requests to arbitrary destinations.

This is primarily achieved through Angular’s HttpClient resolving relative URLs against the unvalidated base origin, or when developers manually construct URLs using the unsanitized headers.

The impact of a successful attack can be severe:

• Credential Exfiltration: Attackers can steal sensitive data, such as Authorization headers or session cookies, by redirecting requests to servers they control.
• Internal Network Probing: Malicious actors can access and transmit data from internal services, databases, or cloud metadata endpoints (like 169.254.169.254) that are not publicly exposed.
• Confidentiality Breach: Sensitive information processed within the application’s server-side environment can be compromised.

Attack Preconditions

For an attack to be successful, several conditions must be met:

• The target application must be using Angular SSR.
• The application must use HttpClient for relative URLs or manually build URLs using unvalidated headers.
• The attacker must be able to influence these headers directly, without strict validation from a front-facing proxy.
• There must be a lack of upstream validation from infrastructure like cloud providers, CDNs, or load balancers.

Metadata	Details
CVE ID	CVE-2026-27739
Severity	Critical
CVSS v4 Score	10.0
Weakness	CWE-918: Server-Side Request Forgery (SSRF)
Affected Packages	@angular/ssr, @nguniversal/common, @nguniversal/express-engine
Patched Versions	21.2.0-rc.1, 21.1.5, 20.3.17, 19.2.21

Mitigation and Workarounds

Developers are strongly urged to update their @angular/ssr packages to the patched versions immediately.

If an immediate upgrade is not feasible, there are workarounds available:

1. Use Absolute URLs: Avoid using req.headers for URL construction. Instead, rely on trusted, hardcoded variables for your base API paths.
2. Implement Strict Header Validation: Add middleware to your server.ts file to enforce numeric ports and validated hostnames, ensuring only approved domains can interact with the server.

By taking these steps, organizations can protect their Angular applications from unauthorized server-side requests and maintain the security of their internal networks.

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