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Intro (How CVE-2026-44437 works):
Angular SSR (Server-Side Rendering) trusts the `X-Forwarded-Prefix` HTTP header when the application is deployed behind a reverse proxy. This header tells the SSR engine the original base path requested by the client, allowing correct generation of absolute URLs. The affected versions (19.0.0-next.0 up to, but not including, 19.2.25, 20.3.25, 21.2.9, and 22.0.0-next.7) contain a validation flaw. The internal function that sanitizes the header value does not recursively decode or block URL-encoded path traversal sequences. Specifically, a dot encoded as `%2e` and another dot as `%2e` – forming `%2e%2e` – is treated as a harmless string during the initial check. Later, when the application uses this prefix to construct file paths or route resolutions, the server (or underlying middleware) decodes the percent-encoding. The sequence `%2e%2e` becomes .., which is the classic directory traversal token. By sending a crafted header like X-Forwarded-Prefix: /%2e%2e/evil, an attacker bypasses the path normalization filter. The SSR logic then resolves `/evil` relative to a parent directory, potentially reading sensitive files, accessing restricted routes, or causing a path confusion attack. This issue is most dangerous when the proxy does not strip or re-encode the header and the Angular SSR application uses the prefix in file operations or route matching. The vulnerability is fixed in versions 19.2.25, 20.3.25, 21.2.9, and 22.0.0-next.7 by adding recursive decoding validation and rejecting any prefix containing `..` after normalization.
DailyCVE Form:
Platform: Angular SSR
Version: 19.0.0-next.0-21.2.8
Vulnerability: Encoded path traversal
Severity: Critical
date: 05/13/2026
Prediction: Patched on 2026-05-13
Analytics under heading What Undercode Say:
Detect vulnerable header handling curl -H "X-Forwarded-Prefix: /%2e%2e/evil" http://target-angular-ssr.com/ -v Check if response accesses parent content curl -H "X-Forwarded-Prefix: /%2e%2e/%2e%2e/etc/passwd" http://target-angular-ssr.com/assets/ Log analysis for suspicious prefixes sudo grep "X-Forwarded-Prefix.%2e" /var/log/nginx/access.log
Exploit:
Send an HTTP request to the Angular SSR application with a malicious `X-Forwarded-Prefix` header containing `/%2e%2e/` followed by a target path. Example: X-Forwarded-Prefix: /%2e%2e/admin/config. The server decodes this to ../admin/config, allowing access to a parent directory resource that should be forbidden. This can lead to reading server files, bypassing authentication, or SSR template injection if the prefix is used in `fs.readFile` operations.
Protection:
Upgrade to Angular SSR version 19.2.25, 20.3.25, 21.2.9, or 22.0.0-next.7. If patching is not immediate, configure the reverse proxy to reject or sanitize any `X-Forwarded-Prefix` containing %2e, .., or encoded dot sequences. Use middleware to validate that the normalized prefix does not escape the intended base directory. Example proxy rule: `if ($http_x_forwarded_prefix ~ “%2e|\.\.”) { return 403; }`
Impact:
Successful exploitation allows an attacker to traverse outside the application root, potentially reading arbitrary files (e.g., .env, configuration secrets), accessing internal API endpoints, or triggering server-side request forgery (SSRF) if the prefix influences fetch requests. The vulnerability is rated Critical due to the ease of exploitation and the high likelihood of sensitive data exposure on publicly reachable Angular SSR deployments.
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Sources:
Reported By: nvd.nist.gov
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