CVE-2026-61626libass: Out-of-bounds bit clears for negative Matroska ReadOrder values

Official description

libass is a portable subtitle renderer for the ASS/SSA (Advanced Substation Alpha/Substation Alpha) subtitle format. From 0.17.4 until 0.17.5, libass event pruning does not guard negative Matroska `ReadOrder` values, allowing an attacker to clear arbitrarily many bits to zero at attacker-controlled positions near the NULL pointer. Exploitation requires a malicious Matroska stream loaded through `ass_process_chunk`, enabled read-order checking, and later pruning of the malicious events. Read-order checking is enabled by default unless `ass_set_check_readorder` is called with zero; pruning must occur explicitly through `ass_prune_events` or through automatic pruning previously enabled with `ass_configure_prune`. On common systems with arithmetic right shift, 32-bit int values, the upper address region reserved to the kernel, and matching binary/kernel architecture, the expected consequence is denial of service. More extensive exploitation on uncommon systems is theoretical and strongly constrained by the target addresses, bit-clearing-only mutation, and ASLR. This issue is fixed in version 0.17.5.

Affected packages & fixed versions

Affected products (as reported)

References

• https://github.com/libass/libass/security/advisories/GHSA-5gf7-wjfm-vmvm x_refsource_CONFIRM
• https://github.com/libass/libass/commit/23da65a130bb8c5b2cf0c7df0dd7d424607f98f1 x_refsource_MISC
• https://github.com/libass/libass/releases/tag/0.17.5 x_refsource_MISC