CVE-2026-27739: Angular SSR SSRF Vulnerability

CVE-2026-27739 Overview

CVE-2026-27739 is a Server-Side Request Forgery (SSRF) vulnerability in Angular SSR (Server-Side Rendering), a rendering tool for Angular applications. The vulnerability exists within the request handling pipeline where 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.

Critical Impact

This vulnerability allows attackers to perform arbitrary internal request steering, potentially leading to credential exfiltration, internal network probing, and confidentiality breaches through malicious header injection.

Affected Products

• Angular SSR versions prior to 21.2.0-rc.1
• Angular SSR versions prior to 21.1.5
• Angular SSR versions prior to 20.3.17
• Angular SSR versions prior to 19.2.21

Discovery Timeline

• 2026-02-25 - CVE CVE-2026-27739 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-27739

Vulnerability Analysis

The SSRF vulnerability in Angular SSR stems from improper trust placed in user-controlled HTTP headers during URL construction for server-side requests. When Angular SSR processes incoming requests, it uses the Host header and X-Forwarded-* headers (such as X-Forwarded-Host, X-Forwarded-Proto, and X-Forwarded-Port) to reconstruct the application's base URL for making subsequent HTTP client requests.

The framework fails to implement essential security checks including host domain validation, path and character sanitization, and port validation. This vulnerability manifests in two primary attack scenarios: implicit relative URL resolution where HttpClient requests using relative URLs inherit the attacker-controlled base origin, and explicit manual construction where developers directly use values from the REQUEST object's headers to build URLs.

For successful exploitation, multiple conditions must align: the application must use Angular SSR, perform HttpClient requests with relative URLs or manually construct URLs using unvalidated headers, the server must be reachable by an attacker who can manipulate headers, and the infrastructure (Cloud, CDN, or Load Balancer) must not sanitize incoming headers.

Root Cause

The root cause is classified under CWE-918 (Server-Side Request Forgery). Angular SSR's URL reconstruction logic implicitly trusts the Host and X-Forwarded-* HTTP headers provided by clients without performing any validation. The framework lacked validation mechanisms for verifying the destination domain against an allowlist, sanitizing URL path components and special characters, and validating that port numbers are within expected ranges. This design flaw allows attackers to inject malicious values into these headers, redirecting server-side HTTP requests to arbitrary internal or external destinations.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by sending crafted HTTP requests to an Angular SSR application with malicious header values. By manipulating the Host header or X-Forwarded-Host header to point to an internal service or attacker-controlled server, subsequent HttpClient requests made by the application will be directed to the attacker-specified destination.

For example, an attacker could set X-Forwarded-Host: internal-api.corp.local or Host: 169.254.169.254 (AWS metadata service), causing the application's relative URL requests to be sent to these targets instead of the legitimate API endpoints. This enables attackers to access internal services, exfiltrate credentials, probe internal network infrastructure, and steal sensitive data returned in responses.

Detection Methods for CVE-2026-27739

Indicators of Compromise

• Unusual outbound HTTP requests from Angular SSR servers to internal IP ranges (e.g., 10.x.x.x, 172.16.x.x, 192.168.x.x) or cloud metadata endpoints (169.254.169.254)
• HTTP requests containing unexpected Host or X-Forwarded-* header values that don't match legitimate origins
• Server logs showing requests to internal services that should not be accessible from the application server
• Anomalous network traffic patterns from the SSR application to unexpected internal or external destinations

Detection Strategies

• Implement network monitoring to detect server-side requests to internal IP address ranges or cloud metadata endpoints
• Configure web application firewalls (WAF) to flag or block requests with suspicious Host or X-Forwarded-* header values
• Enable verbose logging on Angular SSR applications to capture outbound HTTP request destinations for analysis
• Deploy intrusion detection systems (IDS) with rules specifically targeting SSRF patterns and internal network reconnaissance

Monitoring Recommendations

• Monitor egress traffic from Angular SSR application servers for connections to internal services or metadata endpoints
• Set up alerts for high volumes of requests to diverse internal IP addresses from a single application instance
• Review application logs regularly for URL construction errors or unexpected request destinations
• Implement canary tokens on internal services to detect unauthorized access attempts via SSRF

How to Mitigate CVE-2026-27739

Immediate Actions Required

• Upgrade Angular SSR to patched versions: 21.2.0-rc.1, 21.1.5, 20.3.17, or 19.2.21 immediately
• Review application code for usage of req.headers in URL construction and refactor to use trusted configuration variables
• Implement middleware in server.ts to enforce numeric ports and validated hostnames if immediate upgrade is not possible
• Deploy network-level controls to restrict outbound connections from SSR servers to only required destinations

Patch Information

Angular has released security patches addressing this vulnerability in multiple version branches. The fixed versions are 21.2.0-rc.1, 21.1.5, 20.3.17, and 19.2.21. Organizations should upgrade to the appropriate patched version for their Angular SSR installation. The fix implements proper validation of host domains, path sanitization, and port validation in the URL reconstruction logic. For technical details on the patch implementation, refer to the GitHub Pull Request #32516 and the GitHub Security Advisory GHSA-x288-3778-4hhx.

Workarounds

• Avoid using req.headers for URL construction; instead use trusted environment variables or configuration files for base API paths
• Implement middleware in server.ts to validate and enforce allowed hostnames against an allowlist before processing requests
• Configure front-facing proxies, load balancers, or CDNs to sanitize or strip X-Forwarded-* headers from untrusted sources
• Apply network segmentation to limit the SSR server's ability to reach sensitive internal services

# Example: Configure environment-based trusted API URL instead of using request headers
# In server.ts middleware, validate incoming headers before use

# Set trusted base URL in environment configuration
export API_BASE_URL="https://api.yourdomain.com"

# Configure nginx or reverse proxy to overwrite X-Forwarded headers
# nginx.conf example:
# proxy_set_header X-Forwarded-Host $host;
# proxy_set_header X-Forwarded-Proto $scheme;
# proxy_set_header X-Forwarded-Port $server_port;