FreePBX API Module Command Injection Vulnerability (CVE-2026-40520)

FreePBX, a widely used open-source PBX (Private Branch Exchange) system, is vulnerable to a command injection flaw within its API module. Specifically, versions 17.0.8 and earlier are affected by CVE-2026-40520. The vulnerability resides in the initiateGqlAPIProcess() function, where GraphQL mutation input fields are directly passed to the shell_exec() function without proper sanitization or escaping. This allows an authenticated attacker with a valid bearer token to inject and execute arbitrary commands on the underlying host operating system as the web server user. The attack vector involves sending a specially crafted GraphQL moduleOperations mutation containing backtick-wrapped commands within the module field. Successful exploitation grants the attacker the ability to compromise the FreePBX server and potentially pivot to other internal systems.

Attack Chain

1. The attacker authenticates to the FreePBX API using a valid bearer token.
2. The attacker crafts a GraphQL moduleOperations mutation request.
3. Within the module field of the mutation, the attacker injects a command using backticks (e.g., \id` `).
4. The attacker sends the malicious GraphQL request to the /api endpoint.
5. The initiateGqlAPIProcess() function processes the request without proper sanitization.
6. The injected command is passed to the shell_exec() function within Api.class.php.
7. The shell_exec() function executes the injected command on the FreePBX server as the web server user (e.g., www-data, apache).
8. The attacker gains arbitrary command execution on the server.

Impact

Successful exploitation of this command injection vulnerability (CVE-2026-40520) allows an attacker to execute arbitrary commands on the FreePBX server with the privileges of the web server user. This can lead to complete compromise of the PBX system, allowing the attacker to eavesdrop on calls, modify call routing, steal sensitive data, install malware, and potentially pivot to other systems on the network. Given the critical role of PBX systems in business communications, a successful attack can disrupt operations, damage reputation, and result in significant financial losses.

Recommendation

• Upgrade the FreePBX API module to a version greater than 17.0.8 to patch CVE-2026-40520.
• Deploy the Sigma rule Detect FreePBX GraphQL Command Injection to identify exploitation attempts by detecting backticks in GraphQL mutation requests.
• Monitor web server logs for POST requests to the /api endpoint containing GraphQL mutations with backtick-wrapped commands to detect command injection attempts.
• Implement input validation and sanitization measures for all GraphQL input fields to prevent command injection vulnerabilities.