CVE-2026-39864 Overview
CVE-2026-39864 is an out-of-bounds read vulnerability in the auth module of Kamailio, an open source implementation of a SIP (Session Initiation Protocol) Signaling Server. This vulnerability allows remote attackers to cause a denial of service by crashing the Kamailio process through a specially crafted SIP packet. The attack is possible when a successful user authentication without a database backend is followed by additional user identity checks.
Kamailio, formerly known as OpenSER and SER, is widely deployed in VoIP infrastructures and telecommunications environments. Exploitation of this vulnerability could disrupt critical voice and messaging communications services.
Critical Impact
Remote attackers can crash the Kamailio SIP server process, causing denial of service to VoIP and telecommunications services that rely on this signaling infrastructure.
Affected Products
- Kamailio versions prior to 6.0.5
- Kamailio versions prior to 5.8.7
Discovery Timeline
- 2026-04-08 - CVE-2026-39864 published to NVD
- 2026-04-08 - Last updated in NVD database
Technical Details for CVE-2026-39864
Vulnerability Analysis
This vulnerability is classified as CWE-125 (Out-of-Bounds Read), a memory corruption issue that occurs when the application reads data from a memory location outside the bounds of the intended buffer. In the context of Kamailio's auth module, this manifests when processing specially crafted SIP packets under specific authentication conditions.
The attack requires network access and depends on a specific authentication flow: a successful user authentication without a database backend must occur, followed by additional user identity checks. This sequence of operations triggers the out-of-bounds read condition, leading to process crash and service disruption.
The vulnerability affects VoIP and telecommunications infrastructures that use Kamailio for SIP signaling, potentially impacting call routing, registration, and other critical telephony functions.
Root Cause
The root cause of this vulnerability lies in improper bounds checking within the auth module when handling user identity verification after authentication. When Kamailio processes authentication requests without a database backend and subsequently performs additional identity checks, the code fails to properly validate memory boundaries before reading data. This allows an attacker to trigger a read operation beyond the allocated buffer, causing memory access violations that crash the process.
Attack Vector
The attack is network-based and requires the attacker to:
- Craft a malicious SIP packet designed to exploit the vulnerable authentication flow
- Successfully complete user authentication without a database backend
- Trigger additional user identity checks that cause the out-of-bounds read
The vulnerability requires high privileges and specific conditions to exploit, as indicated by the authentication requirements. However, once these conditions are met, the attacker can reliably crash the Kamailio process, disrupting SIP services for all users.
The attack mechanism involves sending a specially crafted SIP packet that manipulates the authentication and identity verification sequence. When the auth module processes this packet, it attempts to read memory beyond the allocated buffer boundaries, resulting in a crash. For detailed technical information, see the GitHub Security Advisory.
Detection Methods for CVE-2026-39864
Indicators of Compromise
- Unexpected Kamailio process crashes or restarts, particularly during authentication operations
- Abnormal SIP traffic patterns targeting the authentication module
- Core dumps or crash logs showing memory access violations in the auth module
- Increased frequency of SIP authentication requests from suspicious IP addresses
Detection Strategies
- Monitor Kamailio process stability and implement alerting on unexpected crashes or restarts
- Implement SIP traffic analysis to identify malformed or suspicious authentication packets
- Review Kamailio logs for authentication errors followed by process termination
- Deploy network intrusion detection signatures for anomalous SIP REGISTER and authentication messages
Monitoring Recommendations
- Configure process monitoring to automatically restart Kamailio and alert on crashes
- Implement SIP deep packet inspection at network boundaries to detect malformed packets
- Enable verbose logging for the auth module during investigation periods
- Monitor system resources for signs of repeated crash and restart cycles
How to Mitigate CVE-2026-39864
Immediate Actions Required
- Upgrade Kamailio to version 6.0.5 or later for the 6.x branch
- Upgrade Kamailio to version 5.8.7 or later for the 5.8.x branch
- Implement network-level filtering to restrict SIP traffic to trusted sources
- Enable rate limiting on SIP authentication requests to reduce impact of exploitation attempts
Patch Information
The Kamailio project has released fixed versions that address this out-of-bounds read vulnerability. Users should upgrade to the following patched versions:
- Kamailio 6.0.5 - Contains the fix for the 6.x release branch
- Kamailio 5.8.7 - Contains the fix for the 5.8.x release branch
For complete patch details and release notes, refer to the GitHub Security Advisory.
Workarounds
- If upgrading is not immediately possible, consider restricting access to SIP authentication endpoints to trusted IP ranges only
- Implement a reverse proxy or SIP-aware firewall to filter malformed authentication requests
- Configure authentication to use a database backend, as the vulnerability specifically affects non-database authentication flows
- Deploy Kamailio behind a Session Border Controller (SBC) with DoS protection capabilities
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
