The vulnerability CVE-2026-3949 resides in the strukturag libheif library, a widely used open-source HEIF (High Efficiency Image File Format) parser and decoder. The issue is located in the vvdec_push_data2 function within the decoder_vvdec.cc source file, part of the HEIF file parser plugin. The vulnerability is triggered by manipulating the size argument passed to this function, which leads to an out-of-bounds read condition. This means the function reads memory beyond the allocated buffer, potentially exposing sensitive data or causing application crashes. The attack vector is local, requiring the attacker to have local access and limited privileges (low privileges). No user interaction or authentication is necessary, and the vulnerability does not affect confidentiality, integrity, or availability directly but can lead to information disclosure or denial of service through application instability. The CVSS v4.0 score is 4.8 (medium), reflecting the limited attack vector and impact scope. The vulnerability has been publicly disclosed, increasing the risk of exploitation, although no active exploits have been observed in the wild. A patch has been released (commit b97c8b5f198b27f375127cd597a35f2113544d03) to correct the improper bounds checking and prevent out-of-bounds reads. Users of libheif versions 1.21.0 through 1.21.2 should apply this patch promptly to mitigate risk.

The primary impact of this vulnerability is the potential for local attackers to read memory beyond intended boundaries, which may lead to unauthorized disclosure of sensitive information residing in adjacent memory areas. While the vulnerability does not allow remote exploitation or privilege escalation, it can be leveraged by malicious local users or compromised accounts to gain insights into memory contents, potentially aiding further attacks. Additionally, out-of-bounds reads can cause application crashes or instability, leading to denial of service conditions for applications relying on libheif for HEIF image processing. Organizations that process HEIF images locally, such as media editing software, image viewers, or content management systems, may be affected. The impact is limited by the requirement for local access and low privileges, but in multi-user or shared environments, this could still pose a risk. Since libheif is used in various operating systems and applications, unpatched systems may face increased risk of information leakage or service disruption.

To mitigate CVE-2026-3949, organizations should apply the official patch identified by commit b97c8b5f198b27f375127cd597a35f2113544d03 to update libheif to a fixed version beyond 1.21.2. It is critical to verify that all software components and applications that bundle or depend on libheif are updated accordingly, including media players, image editors, and content management systems. Additionally, restrict local access to trusted users only, minimizing the risk of exploitation by untrusted or low-privilege users. Employ application sandboxing and memory protection mechanisms to limit the impact of potential out-of-bounds reads. Monitor local system logs for unusual activity related to HEIF file processing. If patching is delayed, consider disabling or restricting HEIF file support temporarily in affected applications. Finally, maintain an inventory of software using libheif to ensure comprehensive coverage of the patch deployment.