CVE-2026-40176: Composer Command Injection RCE Vulnerability

CVE-2026-40176 Overview

CVE-2026-40176 is a command injection vulnerability affecting Composer, the widely-used dependency manager for PHP. The vulnerability exists in the Perforce::generateP4Command() method, which constructs shell commands by interpolating user-supplied Perforce connection parameters (port, user, client) without proper escaping. An attacker can inject arbitrary commands through these values in a malicious composer.json file declaring a Perforce VCS repository, leading to command execution in the context of the user running Composer—even if Perforce is not installed on the system.

Critical Impact

Attackers can achieve arbitrary command execution on systems running Composer against untrusted projects with maliciously crafted composer.json files, potentially compromising the entire development environment.

Affected Products

• Composer versions 1.0 through 2.2.26
• Composer versions 2.3 through 2.9.5

Discovery Timeline

• 2026-04-15 - CVE-2026-40176 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2026-40176

Vulnerability Analysis

The vulnerability is rooted in improper input validation (CWE-20) within Composer's Perforce VCS repository handling code. When processing a composer.json file that declares a Perforce-type VCS repository, the Perforce::generateP4Command() method builds shell commands by directly interpolating user-controlled parameters—specifically the port, user, and client fields—without adequate sanitization or escaping.

This design flaw allows an attacker to craft a malicious composer.json file where these Perforce connection parameters contain shell metacharacters and arbitrary commands. When a developer or CI/CD system executes any Composer command (such as composer install or composer update) against a project containing this malicious configuration, the injected commands execute with the privileges of the user running Composer.

Importantly, VCS repositories are only loaded from the root composer.json or the Composer config directory, which means this vulnerability cannot be exploited through composer.json files of packages installed as dependencies. The attack requires convincing a user to run Composer commands on an untrusted project directly.

Root Cause

The root cause is insufficient input sanitization in the Perforce::generateP4Command() method. The method constructs shell commands using string interpolation of user-supplied values without escaping special shell characters. This violates the principle of never trusting user input when constructing shell commands, particularly when that input originates from project configuration files that may be obtained from untrusted sources.

Attack Vector

This vulnerability requires local access and user interaction to exploit. An attacker must craft a malicious composer.json file with specially crafted Perforce repository parameters and convince a victim to run Composer commands against the project. Attack scenarios include:

The attacker creates a seemingly legitimate open-source project containing a composer.json with a malicious Perforce VCS repository definition. When a developer clones this repository and runs composer install, the injected commands execute on their system.

Alternatively, an attacker could submit a pull request to an existing project that introduces a malicious Perforce repository entry, hoping it passes code review. CI/CD pipelines that automatically run Composer on pull requests would be particularly vulnerable.

The attack works even on systems where Perforce is not installed, as the command injection occurs during the shell command construction phase before any Perforce binaries are invoked.

Detection Methods for CVE-2026-40176

Indicators of Compromise

• Presence of composer.json files with unusual Perforce VCS repository definitions containing shell metacharacters (e.g., ;, |, $(), backticks)
• Unexpected process spawning from Composer PHP processes
• Anomalous outbound network connections or file system modifications during Composer operations

Detection Strategies

• Implement static analysis scanning of composer.json files for suspicious Perforce repository definitions before executing Composer commands
• Monitor for unusual child processes spawned by PHP or Composer processes in development and CI/CD environments
• Deploy SentinelOne Singularity Platform to detect and prevent command injection attempts through behavioral analysis

Monitoring Recommendations

• Enable detailed logging for all Composer operations in CI/CD pipelines
• Monitor development workstations for unexpected command execution patterns during package management operations
• Review composer.json files in version control for unauthorized additions of VCS repository definitions

How to Mitigate CVE-2026-40176

Immediate Actions Required

• Update Composer to version 2.9.6 (mainline) or 2.2.27 (2.2 LTS) immediately
• Audit existing projects for suspicious Perforce VCS repository definitions in composer.json files
• Avoid running Composer commands on projects from untrusted sources until patches are applied

Patch Information

The Composer development team has released patched versions that properly escape user-supplied parameters when constructing Perforce shell commands. Users should update to:

• Composer 2.9.6 for users on the mainline branch
• Composer 2.2.27 for users on the 2.2 LTS branch

Detailed release notes and security advisories are available from the GitHub Composer Release 2.9.6 and the GitHub Security Advisory GHSA-wg36-wvj6-r67p.

Workarounds

• Manually review all composer.json files from untrusted sources before running any Composer commands, specifically examining VCS repository definitions
• Implement organizational policies prohibiting the use of Perforce VCS repositories in Composer configurations if not required
• Run Composer in isolated environments (containers, VMs) when working with untrusted projects to limit potential damage from exploitation

# Update Composer to patched version
composer self-update 2.9.6

# For 2.2 LTS users
composer self-update 2.2.27

# Verify installed version
composer --version