CVE-2025-54414: Anubis Web AI Firewall XSS Vulnerability

CVE-2025-54414 Overview

CVE-2025-54414 affects Anubis, a Web AI Firewall Utility that challenges incoming connections to protect upstream resources from scraper bots. Versions 1.21.2 and below allow attackers to craft malicious pass-challenge pages that execute arbitrary JavaScript or trigger nonstandard URL schemes. The flaw resides in the /.within.website/x/cmd/anubis/api/pass-challenge route, which fails to validate the ?redir= parameter. Attackers can supply URL schemes such as javascript: to run script in the victim's browser context. An initial fix tagged as 1.21.2 was incomplete and its release aborted during final testing. The maintainers resolved the issue in version 1.21.3.

Critical Impact
Attackers can execute arbitrary JavaScript in a victim's browser by tricking them into visiting a malicious pass-challenge URL, enabling session theft, credential harvesting, or client-side attacks against protected upstream resources.

Affected Products

Anubis Web AI Firewall Utility versions 1.21.2 and below
TecharoHQ Anubis prior to 1.21.3
Deployments exposing the /.within.website/x/cmd/anubis/api/pass-challenge endpoint

Discovery Timeline

2025-07-26 - CVE-2025-54414 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-54414

Vulnerability Analysis

Anubis performs proof-of-work style challenges against connecting clients before proxying requests to upstream services. After a successful challenge, the pass-challenge API redirects the browser to the URL supplied in the ?redir= query parameter. The vulnerable implementation does not restrict this parameter to safe URL schemes. Attackers can therefore supply javascript: URIs or other nonstandard schemes that browsers will execute or hand off to registered protocol handlers. Because the redirect occurs from a trusted origin protected by Anubis, script executes in that origin's context. This maps to [CWE-80] Improper Neutralization of Script-Related HTML Tags in a Web Page.

Root Cause

The root cause is missing validation on the redir query parameter of the pass-challenge handler. The handler accepted any value and used it directly as the redirect target rather than confirming that the URL scheme was http, https, or a scheme-less local path. This allowed injection of active-content schemes such as javascript: and other browser-registered handlers.

Attack Vector

Exploitation requires user interaction. An attacker constructs a URL that hits the pass-challenge endpoint on an Anubis-protected site with a crafted redir value pointing to a javascript: payload. The victim clicks the link, completes or bypasses the challenge, and the browser evaluates the attacker-supplied script in the site's origin. The vulnerability is exploitable remotely over the network without authentication, but requires the victim to follow the malicious link. See the GitHub Security Advisory GHSA-jhjj-2g64-px7c for full technical details.

Detection Methods for CVE-2025-54414

Indicators of Compromise

Requests to /.within.website/x/cmd/anubis/api/pass-challenge where the redir parameter begins with javascript:, data:, vbscript:, or other non-http(s) schemes.
Referrer chains showing external inbound links to the pass-challenge endpoint with unusual redir payloads.
Web server or reverse-proxy logs containing URL-encoded script payloads within the redir query string.

Detection Strategies

Deploy web application firewall rules that inspect the redir parameter on the pass-challenge route and block requests where the scheme is not http, https, or empty.
Alert on anomalous outbound redirects issued by Anubis containing non-standard URL schemes.
Correlate suspicious pass-challenge requests with downstream browser telemetry showing script execution or credential-form submissions to unexpected destinations.

Monitoring Recommendations

Enable verbose access logging on the Anubis endpoint and forward logs to a centralized analytics platform for query and retention.
Monitor the running Anubis version across deployments and alert when instances remain on 1.21.2 or earlier.
Track referral traffic patterns for spikes in inbound links targeting the pass-challenge route from untrusted sources.

How to Mitigate CVE-2025-54414

Immediate Actions Required

Upgrade Anubis to version 1.21.3 or later as documented in the GitHub Release v1.21.3.
Audit reverse-proxy and WAF configurations to reject requests to /.within.website/x/cmd/anubis/api/pass-challenge when redir uses a disallowed scheme.
Review recent access logs for evidence of prior exploitation attempts targeting the vulnerable endpoint.

Patch Information

The issue was fixed in Anubis version 1.21.3. The corrective change is described in the upstream GitHub Pull Request. Note that a partial fix was tagged as 1.21.2 but the release was aborted during final testing, so 1.21.2 remains vulnerable. Administrators must upgrade directly to 1.21.3.

Workarounds

Block requests to /.within.website/x/cmd/anubis/api/pass-challenge where the ?redir= parameter does not start with http, https, or a local path.
Enforce a strict Content Security Policy on upstream applications to reduce the impact of injected script execution.
Restrict access to the pass-challenge endpoint from untrusted networks where the deployment topology allows.

bash

# Example NGINX rule blocking non-http(s) redir schemes on the pass-challenge route
location = /.within.website/x/cmd/anubis/api/pass-challenge {
    if ($arg_redir ~* "^(javascript|data|vbscript|file|about):") {
        return 400;
    }
    proxy_pass http://anubis_upstream;
}