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

Affected: libass

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.

USN-8815-1 — libass vulnerabilities

Affected: Ubuntu:Pro:14.04:LTS: libass, Ubuntu:Pro:18.04:LTS: libass, Ubuntu:Pro:20.04:LTS: libass, Ubuntu:Pro:24.04:LTS: libass, Ubuntu:Pro:26.04:LTS: libass

It was discovered that libass incorrectly handled parsing operations for specific nested character strings. An attacker could use this issue to cause libass to crash, resulting in a denial of service, or possibly execute arbitrary code. This issue only affected Ubuntu 14.04 LTS, Ubuntu 16.04 LTS, Ubuntu 18.04 LTS and Ubuntu 20.04 LTS. (CVE-2020-24994)

It was discovered that libass incorrectly handled certain outline processing operations. An attacker could use this issue to cause libass to crash, resulting in a denial of service. This issue only affected Ubuntu 18.04 LTS and Ubuntu 20.04 LTS. (CVE-2020-26682)

It was discovered that libass incorrectly handled certain ASS subtitle files when measuring wrapped lines. An attacker could use this issue to cause libass to crash, resulting in a denial of service, or possibly execute arbitrary code. This issue only affected Ubuntu 24.04 LTS and Ubuntu 26.04 LTS. (CVE-2026-61627)

It was discovered that libass incorrectly handled certain Matroska subtitle chunks with negative ReadOrder values. An attacker could use this issue to cause libass to crash, resulting in a denial of service, or possibly execute arbitrary code. This issue only affected Ubuntu 26.04 LTS. (CVE-2026-61626)

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

USN-8815-1 — libass vulnerabilities