CVE-2024-39844 in ZNC
Summary
by MITRE • 07/03/2024
In ZNC before 1.9.1, remote code execution can occur in modtcl.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 07/28/2026
The vulnerability in ZNC versions prior to 1.9.1 represents a critical remote code execution flaw within the modtcl module, which serves as a bridge between ZNC and Tcl scripting capabilities. This issue arises from insufficient input validation and sanitization mechanisms within the module's handling of user-supplied data, creating an avenue for malicious actors to inject and execute arbitrary commands on the affected system. The modtcl module was designed to allow users to extend ZNC functionality through Tcl scripts, but the implementation failed to properly isolate or validate script inputs, leading to a dangerous escalation path.
The technical exploitation of this vulnerability stems from improper handling of command injection vectors within the Tcl interpreter interface. When ZNC processes user commands through modtcl, it fails to adequately sanitize or escape special characters that could be interpreted by the underlying system shell, allowing attackers to craft malicious input sequences that bypass normal security boundaries. This flaw operates at the intersection of multiple security domains including shell command execution, script interpretation, and privilege escalation mechanisms.
From an operational perspective, this vulnerability presents a severe risk to ZNC installations where modtcl is enabled, as it allows remote attackers to execute arbitrary commands with the privileges of the ZNC process. The impact extends beyond immediate system compromise to include potential data exfiltration, lateral movement within networks, and establishment of persistent access points. Attackers could leverage this vulnerability to gain full control over affected servers, potentially leading to complete infrastructure compromise.
Organizations should immediately upgrade to ZNC version 1.9.1 or later to address this vulnerability, as the fix includes comprehensive input validation and sanitization measures for all Tcl command processing. Additional mitigations include disabling the modtcl module when not actively required, implementing network-level restrictions to limit access to ZNC services, and monitoring for suspicious command execution patterns. This vulnerability aligns with CWE-78 (Improper Neutralization of Special Elements used in a Command) and maps to ATT&CK technique T1059.007 (Tcl) under the execution category, highlighting the need for proper input validation and secure coding practices in interpreted scripting environments.