Remote Code Execution in LibreNMS Signal Alert Transport Module

LibreNMS versions 21.6.0 through 26.4.x are vulnerable to a remote code execution (RCE) vulnerability (CVE-2026-55182) within the Signal Alert Transport module. The vulnerability stems from insufficient sanitization of user-provided input in the deliverAlert function located in LibreNMS/Alert/Transport/Signal.php. An authenticated administrative user can manipulate the 'Path' and 'Recipient' fields in the Alert Transport configuration to perform command injection. These inputs are passed to an unsafe exec call, which is further exacerbated by the scripts/composer_wrapper.php script that accepts and executes these malicious arguments. By chaining these weaknesses, an attacker with existing administrative access can execute arbitrary commands on the underlying host server, leading to potential full system compromise.

Attack Chain

1. Attacker authenticates to the LibreNMS web interface with administrative privileges.
2. Attacker navigates to the 'Alert Transports' configuration menu.
3. Attacker creates a new 'Signal' type alert transport.
4. Attacker modifies the 'Path' configuration field to target ../scripts/composer_wrapper.php.
5. Attacker injects a command sequence starting and ending with a semicolon (;) into the 'Recipient' field.
6. Attacker saves the transport configuration.
7. Attacker triggers the vulnerability by clicking the 'Test Transport' button associated with the malicious entry.
8. The application executes the injected command via the vulnerable exec call within the backend script, resulting in RCE.

Impact

Successful exploitation of CVE-2026-55182 allows an authenticated attacker to achieve full remote code execution on the server hosting the LibreNMS application. This grants the attacker the ability to execute arbitrary commands with the privileges of the web service user, which can lead to data exfiltration, modification of system configurations, or lateral movement into the wider internal network. Given LibreNMS is typically deployed to monitor network infrastructure, this exposure could provide an attacker with significant visibility and control over managed network devices.

Recommendation

Prioritized, concrete actions for detection engineering teams:

• Upgrade all instances of LibreNMS to version 26.5.0 or later to include the vendor patch for CVE-2026-55182.
• Review administrative user accounts for unauthorized activity or creation of new alert transports.
• Deploy server-side auditing to monitor for unexpected child processes spawned by the web service user (e.g., apache, www-data, nginx).
• Restrict the ability of the web service user to execute system-level binaries via sudo or system policies.
• Monitor logs for the execution of composer_wrapper.php containing unusual characters in arguments, such as semicolons or shell operators.