CVE-2026-40332: Masa CMS Open Redirect Vulnerability

CVE-2026-40332 Overview

CVE-2026-40332 is an Open Redirect vulnerability [CWE-601] in Masa CMS caused by improper handling of scheme-relative URLs. The application interprets paths beginning with double slashes (//) as internal paths and processes them without confirming the redirect target stays on the local site. Attackers can craft a URL on a trusted Masa CMS domain that redirects victims to an external attacker-controlled site. This behavior enables phishing campaigns and, in some authentication flows, may expose tokens or other sensitive data to the external site. Fixed releases are available in versions 7.2.10, 7.3.15, 7.4.10, and 7.5.3.

Critical Impact

Attackers can abuse the trusted Masa CMS domain to redirect users to attacker-controlled sites, enabling phishing and potential leakage of authentication tokens.

Affected Products

• Masa CMS versions prior to 7.2.10 in the 7.2.x branch
• Masa CMS versions prior to 7.3.15 in the 7.3.x branch
• Masa CMS versions prior to 7.4.10 in the 7.4.x branch and prior to 7.5.3 in the 7.5.x branch

Discovery Timeline

• 2026-05-06 - CVE-2026-40332 published to NVD
• 2026-05-06 - Last updated in NVD database

Technical Details for CVE-2026-40332

Vulnerability Analysis

The vulnerability stems from how Masa CMS validates redirect targets supplied as user-controlled input. The application treats URL paths beginning with // as relative internal paths. Browsers, however, interpret scheme-relative URLs such as //attacker.example/path as absolute URLs that inherit the current page's scheme. This mismatch between server-side parsing and browser interpretation allows external redirects through parameters intended for internal navigation. The flaw aligns with CWE-601: URL Redirection to Untrusted Site and impacts confidentiality when authentication flows propagate tokens via redirect.

Root Cause

The redirect handler fails to canonicalize redirect targets before processing. It checks for a leading / to classify a value as internal but does not reject the // prefix used by scheme-relative URLs. The forceDirectoryStructure feature contributes to the unsafe path interpretation. As a result, the server returns an HTTP redirect response that browsers resolve to an external host.

Attack Vector

The attack requires user interaction. An attacker delivers a crafted link pointing at the legitimate Masa CMS host, with a redirect parameter such as ?returnURL=//attacker.example/login. The victim trusts the visible domain and follows the link. The CMS issues a redirect that the browser resolves to https://attacker.example/login. The attacker can host a cloned login page to harvest credentials or capture tokens passed via query parameters or fragments during single sign-on flows.

No verified exploit code is published. See the GitHub Security Advisory for vendor technical details.

Detection Methods for CVE-2026-40332

Indicators of Compromise

• HTTP requests to Masa CMS endpoints containing redirect parameters with values beginning with // or /\ followed by an external hostname.
• Web server access logs showing 302 or 301 responses with Location headers pointing to non-local domains originating from CMS redirect handlers.
• Spikes in outbound referrals from the Masa CMS domain to unfamiliar external hosts in proxy or DNS telemetry.

Detection Strategies

• Inspect query strings on redirect endpoints for the regex pattern ^(//|/\\) in parameter values such as returnURL, next, redirect, or url.
• Correlate user-agent navigation patterns where a request to the Masa CMS domain is immediately followed by a redirect to an external host within the same session.
• Compare the Referer header of inbound login attempts on suspected phishing infrastructure against the known Masa CMS domain.

Monitoring Recommendations

• Enable verbose logging on Masa CMS redirect handlers and forward logs to a centralized SIEM or data lake for analysis.
• Alert on outbound redirects whose destination host does not match an allowlist of approved domains.
• Track phishing report inboxes for URLs that contain the Masa CMS domain combined with suspicious redirect parameters.

How to Mitigate CVE-2026-40332

Immediate Actions Required

• Upgrade Masa CMS to 7.2.10, 7.3.15, 7.4.10, or 7.5.3 depending on the deployed branch.
• Audit all custom templates and integrations that consume redirect parameters from user input and confirm they use the patched validation logic.
• Review authentication flows that pass tokens through redirect parameters and rotate any credentials suspected of exposure.

Patch Information

The maintainers fixed CVE-2026-40332 in Masa CMS versions 7.2.10, 7.3.15, 7.4.10, and 7.5.3. Patch details and remediation guidance are documented in the GitHub Security Advisory GHSA-xw99-h3mw-wj47.

Workarounds

• Reject or rewrite redirect parameter values that begin with // before they reach the redirect handler, using a web application firewall or reverse proxy rule.
• Disable the forceDirectoryStructure setting where deployment compatibility allows, as recommended by the vendor advisory.
• Enforce a strict allowlist of internal hosts in any custom redirect logic and drop requests that resolve to external domains.

# Example NGINX rule to block scheme-relative redirect parameters
if ($args ~* "(returnURL|redirect|next|url)=(//|/\\)") {
    return 400;
}