CVE-2026-44437

EUVD-2026-30191 · Published May 6, 2026

Angular SSR has Open Redirect and Request Steering via Encoded X-Forwarded-Prefix

Description

A vulnerability exists in the X-Forwarded-Prefix header processing logic within Angular SSR. The internal validation mechanism fails to properly account for URL-encoded characters, specifically dots (%2e%2e). This allows an attacker to bypass security filters by injecting encoded path traversal sequences that are later decoded and utilized by the application logic.
When an Angular SSR application is configured to trust proxy headers and is deployed behind a proxy that forwards the X-Forwarded-Prefix header without prior sanitization, an attacker can provide a payload such as /%2e%2e/evil.

The vulnerability manifests in two ways:

Open Redirect: The application processes a redirect (e.g., router redirectTo). The decoded traversal payload manipulates the Location header, forcing the browser to an unintended path or external domain.
Server-Side Request Steering: The manipulated prefix is used as the base path for server-side HttpClient requests. This causes the server to make requests to unintended internal paths or external endpoints.

Attack Preconditions

The application must use Angular SSR.
The application must perform internal redirects or use relative URLs in server-side HttpClient requests.
The upstream infrastructure (Reverse Proxy/CDN) must pass the X-Forwarded-Prefix header to the SSR process without stripping or sanitizing it.

Workarounds

Until the patch is applied, developers should manually sanitize the X-Forwarded-Prefix header in their server.ts. The workaround involves decoding the component to catch encoded traversal attempts before normalization:

app.use((req, res, next) => {
  let prefix = req.headers['x-forwarded-prefix'];
  if (Array.isArray(prefix)) prefix = prefix[0];

  if (prefix) {
    try {
      // Decode the prefix to catch encoded characters like %2e (dots)
      const decodedPrefix = decodeURIComponent(prefix);
      
      // Sanitize: remove traversal attempts and ensure a safe leading slash
      req.headers['x-forwarded-prefix'] = decodedPrefix
        .replace(/\.\./g, '')       // Remove any dot-dot sequences
        .replace(/^[/\\]+/, '/');   // Ensure it starts with exactly one slash
    } catch (e) {
      // If decoding fails, remove the potentially malicious header entirely
      delete req.headers['x-forwarded-prefix'];
    }
  }
  next();
});

Configuring Trusted Proxy Headers

By default, Angular ignores all X-Forwarded-* headers. If your application is behind a trusted reverse proxy (like a load balancer) that sets these headers, you can configure Angular to trust them.
You can configure trustProxyHeaders when initializing the application engine:

const appEngine = new AngularAppEngine({
  // Trust specific headers
  trustProxyHeaders: ['x-forwarded-host', 'x-forwarded-proto', 'x-forwarded-prefix'], 
});

const nodeAppEngine = new AngularNodeAppEngine({
  // Trust all X-Forwarded-* headers
  trustProxyHeaders: true, 
});

Patches

22.0.0-next.7
21.2.9
20.3.25
19.2.25

Resources

Technologies

Angular Angular CliAngular Ssr

Weaknesses

CWE-601CWE-22

MITRE ATT&CK

Inferred

Where this vulnerability fits in an attack

ReconImpact

Initial Access

T1190Exploit Public-Facing Application

References

Potentially impacted assets

See if this affects your attack surface

Latest trending attack

Criticalyesterday

Google Brotli decoder - Remote Code Execution via compressed requests (Exploitation Check)

This control is the second step of the Patrowl response to CVE-2026-111497 (Node2Shell). It complements the first control of 22 October 2026, which focused on the Node.js runtime, and does not replace it. Why a second control CVE-2026-111497 is an out-of-bounds write in the Google Brotli decoder, the library that decompresses Brotli ("br") data. It was first published as a Node.js vulnerability, because Node.js bundles this decoder. Later publications showed that the flaw is in the library itself. Every product that embeds an affected version of the decoder is concerned, whatever its language or vendor. A public proof of concept demonstrates code execution. Any server that decompresses request bodies sent with "Content-Encoding: br" exposes the decoder to unauthenticated attackers. Known cases include Express 5 and NestJS 11 on Node.js, Koa and Fastify setups, Bun, Envoy and Istio with request decompression enabled, and any application or reverse proxy that decodes Brotli request bodies with an affected library. The flaw also works in the other direction. A malicious server can compromise an unpatched client that decodes a Brotli response, for example Node.js fetch(), Python clients (requests, httpx, aiohttp, Scrapy) with the Brotli or brotlicffi package, or libcurl builds with Brotli support. What this control checks Patrowl sends small, valid Brotli-compressed requests to each asset and observes whether the server decodes them. The check does not depend on the technology: it detects the behaviour, whatever the stack behind it. These requests are what any legitimate client could send. They never reach the vulnerable code path and cannot affect the availability of the service. How to read the result Potentially impacted means the asset decodes Brotli request bodies: the attack surface is exposed. It does not confirm that the asset is vulnerable. The version of the Brotli decoder in use is not visible from the outside. This check covers applications that parse request bodies on their root path. A Not impacted result does not rule out exposure on other routes, nor on the client side. Checking the versions on the servers remains the only exhaustive verification. Affected and fixed versions Google Brotli 1.0.3 to 1.2.0 are affected, fixed in 1.0.10, 1.1.1 and 1.2.1. Fixed releases of the main products embedding it: Node.js 22.23.4, 24.22.1 and 26.12.1 (end-of-life lines 20.x and older and 25.x receive no fix), Bun 1.4.4, Envoy 1.37.8, 1.38.6, 1.39.3 and 1.40.0, Istio 1.29.9, 1.30.6 and 1.31.2, Python Brotli 1.2.1 and brotlicffi 1.2.1.0. Products that link against the system library depend on the Brotli package of the Linux distribution. Recommendations Update every component that embeds the Brotli decoder (only real fix). Start with the assets flagged by this control. Restart the processes after the update. Rebuild containers, single-executable applications and appliances that bundle an affected runtime or library. Reduce the exposure until the update is deployed. Reject requests with "Content-Encoding: br" at the reverse proxy, load balancer or WAF, case-insensitively and including lists such as "gzip, br". On Envoy and Istio, disable request decompression for Brotli. Review outbound calls. List the features that fetch content from servers you do not control, and patch the services that host them first.