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

Status: published. Published: 2026-10-30. Updated: 2026-11-04.

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.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- libass / libass: >= 0.17.4, < 0.17.5 (affected). Source: CNA: GitHub_M

## Severity and prioritization

- CVSS 4.0: 7.3 (high); CVSS:4.0/AV:L/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N; source: CNA: GitHub_M

- CVSS 4.0: 7.3 (high); CVSS:4.0/AV:L/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X; source: NVD: security-advisories@github.com

- CISA KEV: No listing in collected sources

- EPSS: 0.12% as of 2026-11-04; percentile 1.74%

- SSVC: exploitation none; automatable yes; technical impact total; source ADP: CISA-ADP

- Weaknesses: CWE-787

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/libass/libass/security/advisories/GHSA-5gf7-wjfm-vmvm (CNA: GitHub_M; x_refsource_CONFIRM)

- https://github.com/libass/libass/commit/23da65a130bb8c5b2cf0c7df0dd7d424607f98f1 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/libass/libass/releases/tag/0.17.5 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/libass/libass/commit/23da65a130bb8c5b2cf0c7df0dd7d424607f98f1 (security-advisories@github.com)

- https://github.com/libass/libass/releases/tag/0.17.5 (security-advisories@github.com)

- https://github.com/libass/libass/security/advisories/GHSA-5gf7-wjfm-vmvm (security-advisories@github.com)

## Provenance

- CNA: GitHub_M: https://www.cve.org/CVERecord?id=CVE-2026-61626; retrieved 2026-11-04; source updated 2026-11-04

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-61626; retrieved 2026-11-05; source updated 2026-11-04

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-61626; retrieved 2026-11-04; source updated Not reported

Revision: b925b2b068198dab5ff3493299364edda5756a192aa2af9a1a3f1f2d34f8baa7

Canonical: https://y2.dev/cve/CVE-2026-61626

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