CVE-2026-3949 is an out-of-bounds read vulnerability found in the strukturag libheif library, versions 1.21.0 through 1.21.2. The flaw resides in the vvdec_push_data2 function within the HEIF file parser plugin (libheif/plugins/decoder_vvdec.cc). By manipulating the size parameter passed to this function, a local attacker with limited privileges can cause the program to read memory beyond the intended buffer boundaries. This can lead to unintended information disclosure or cause the application to crash, potentially affecting the stability and security of software relying on libheif for HEIF image decoding. The vulnerability requires local access and does not need user interaction or elevated privileges beyond limited local rights. The vulnerability has been publicly disclosed, increasing the risk of exploitation, although no active exploitation has been observed in the wild. The issue is addressed by a patch (commit b97c8b5f198b27f375127cd597a35f2113544d03), which corrects the bounds checking in the affected function. Given libheif's use in various media applications and operating systems, unpatched systems remain vulnerable to potential attacks that could compromise confidentiality or cause denial of service.

The primary impact of CVE-2026-3949 is an out-of-bounds read that can lead to information disclosure or application crashes. For organizations, this could mean leakage of sensitive memory contents or instability in applications processing HEIF images, potentially disrupting workflows or services that rely on media processing. Since exploitation requires local access, the threat is mainly to environments where untrusted users have some level of system access, such as multi-user systems, shared workstations, or developer environments. The vulnerability could be leveraged as part of a larger attack chain to gather information or cause denial of service. Although the CVSS score is medium (4.8), the risk increases in environments where libheif is widely used and local user access is common. Failure to patch could expose organizations to data leakage or service interruptions, impacting confidentiality and availability.

Organizations should promptly apply the official patch identified by commit b97c8b5f198b27f375127cd597a35f2113544d03 to all affected libheif versions (1.21.0 through 1.21.2). Additionally, limit local access to trusted users only, especially on systems processing HEIF images. Employ strict access controls and user privilege management to reduce the risk of local exploitation. Monitor systems for unusual crashes or memory access errors related to media processing applications. Where possible, use updated versions of libheif beyond 1.21.2 that include the fix. For environments where patching is delayed, consider sandboxing or isolating applications that use libheif to minimize potential impact. Regularly audit software dependencies and maintain an inventory of media libraries to ensure timely updates.