A critical vulnerability has been discovered in Angular Server-Side Rendering (SSR) that could allow attackers to trick applications into sending unauthorized requests.
Tracked as CVE-2026-27739, this Server-Side Request Forgery (SSRF) flaw poses a severe risk to web applications using affected versions of the Angular framework.
The vulnerability stems from Angular’s internal URL reconstruction logic when handling user-controlled HTTP headers.
Specifically, the framework trusts the Host and X-Forwarded-* family of headers without properly validating the destination domain.
How the Vulnerability Works
Angular SSR relies on HTTP headers to determine the application’s base origin. However, the framework fails to verify whether the Host and X-Forwarded-Host headers originate from a trusted origin.
This oversight allows an attacker to redirect the application’s base URL to an external, malicious domain.
Furthermore, the framework does not sanitize the X-Forwarded-Host header for path segments or special characters.
It also fails to verify that the X-Forwarded-Port header contains a numeric value, opening the door to malformed URI construction and injection attacks.
This lack of validation creates two primary attack scenarios:
Attack ScenarioDescriptionBase Origin ManipulationHttpClient may resolve URLs to attacker-controlled servers.Unsafe URL ConstructionUnvalidated REQUEST headers may redirect traffic to malicious endpoints.
If successfully exploited, CVE-2026-27739 allows for arbitrary internal request steering, which can have devastating consequences for affected applications.
Attackers can use this flaw to exfiltrate sensitive credentials, such as Authorization headers or session cookies, by redirecting them to their own servers.
The vulnerability also enables internal network probing, allowing threat actors to access and transmit data from internal services, databases, or cloud metadata endpoints that are not exposed to the public internet.
Ultimately, this can lead to severe confidentiality breaches as sensitive information processed on the server is exposed. The Angular team has released patched versions to address this critical flaw.
According to the Angular advisory on GitHub, users are strongly advised to update to the following safe versions: 21.2.0-rc.1, 21.1.5, 20.3.17, and 19.2.21.
For organizations unable to upgrade immediately, workarounds are available. Developers should avoid using req.headers for URL construction and instead rely on absolute URLs with trusted base API paths.
Additionally, implementing strict header validation middleware in the server.ts file can enforce the use of numeric ports and validated hostnames, mitigating the risk of exploitation.
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