CVE-2026-40261

Workarounds

• Avoid installing dependencies from source by using --prefer-dist or the preferred-install: dist config setting.
• Only use trusted Composer repositories.

The Perforce::syncCodeBase() method appended the $sourceReference parameter to a shell command without proper escaping, allowing an attacker to inject arbitrary commands through a crafted source reference containing shell metacharacters. Further as in GHSA-wg36-wvj6-r67p / CVE-2026-40176 the Perforce::generateP4Command() method constructed shell commands by interpolating user-supplied Perforce connection parameters (port, user, client) without proper escaping from the source url field. Composer would execute these injected commands even if Perforce is not installed.

The source reference and url are provided as part of package metadata. Any Composer package repository can serve package metadata declaring perforce as a source type with a malicious source reference or source url. This means the vulnerability can be exploited through any package served by a compromised or malicious Composer repository. An attack does not require Perforce to be installed on the client, as Composer will attempt to execute the constructed command regardless.

This vulnerability is exploitable when installing or updating dependencies from source (--prefer-source, default when installing dev prefixed versions), even if you do not use Perforce.

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2026-40261 has a CVSS score of 8.8 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.9.6, 2.2.27); upgrading removes the vulnerable code path.

Kodem intelligence

Fixed in Composer 2.2.27 (2.2 LTS) and 2.9.6 (mainline)

Note, the fix for the source url in the Perforce::generateP4Command() was addressed as part of the patches for GHSA-wg36-wvj6-r67p / CVE-2026-40176 in the same versions.

1. What is CVE-2026-40261? CVE-2026-40261 is a high-severity improper input validation vulnerability in composer/composer (composer), affecting versions >= 2.3.0, < 2.9.6. It is fixed in 2.9.6, 2.2.27. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2026-40261? CVE-2026-40261 has a CVSS score of 8.8 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of composer/composer are affected by CVE-2026-40261? composer/composer (composer) versions >= 2.3.0, < 2.9.6 is affected.
4. Is there a fix for CVE-2026-40261? Yes. CVE-2026-40261 is fixed in 2.9.6, 2.2.27. Upgrade to this version or later.
5. Is CVE-2026-40261 exploitable, and should I be worried? Whether CVE-2026-40261 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-40261 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-40261?
   • Upgrade composer/composer to 2.9.6 or later
   • Upgrade composer/composer to 2.2.27 or later