CVE-2026-26203 is a use-after-free vulnerability categorized under CWE-416, found in the pjmedia-video module of the PJSIP open-source multimedia communication library. The vulnerability exists in versions prior to 2.17 and is triggered when the H.264 packetizer processes malformed bitstreams that lack NAL unit start codes. Specifically, the packetizer performs unchecked pointer arithmetic, which results in reading memory located before the allocated buffer, causing a heap buffer underflow. This memory mismanagement can lead to undefined behavior such as application crashes, memory corruption, or potentially exploitable conditions depending on the context of use. The vulnerability does not require authentication or user interaction but does require local access (AV:L). The CVSS 4.0 base score is 5.1 (medium severity), reflecting limited attack vector scope but notable impact on stability and integrity. The issue was addressed in PJSIP version 2.17 by adding proper validation and bounds checking to the packetizer code. No public exploits are known, but the flaw poses a risk to any multimedia or VoIP applications relying on vulnerable PJSIP versions for H.264 video stream processing.

The primary impact of CVE-2026-26203 is on the confidentiality, integrity, and availability of systems using vulnerable versions of PJSIP for multimedia communication. Exploitation can cause application crashes or memory corruption, potentially leading to denial of service or, in some cases, arbitrary code execution if an attacker can carefully craft malformed H.264 streams. This could disrupt VoIP services, video conferencing, or other multimedia applications, affecting business continuity and user experience. Since PJSIP is widely used in embedded devices, softphones, and communication platforms, organizations globally that rely on these technologies could face service outages or security breaches. Although the attack vector is local, compromised internal systems or malicious insiders could exploit this vulnerability. The absence of known exploits reduces immediate risk, but the vulnerability remains a concern for long-term security posture and resilience.

Organizations should upgrade all PJSIP deployments to version 2.17 or later, where the vulnerability is patched. For environments where immediate upgrade is not feasible, applying custom patches or recompiling with added bounds checking in the pjmedia-video H.264 packetizer is recommended. Additionally, implement strict input validation and filtering of H.264 streams before processing to detect and reject malformed packets lacking proper NAL unit start codes. Network segmentation and limiting local access to multimedia processing components can reduce the attack surface. Monitoring logs for crashes or unusual behavior in multimedia applications may help detect exploitation attempts. Finally, maintain an inventory of all products and devices using PJSIP to ensure comprehensive coverage of the patching process.