# CVE-2026-61626: libass: libass is a portable subtitle renderer for the ASS/SSA (Advanced Substation Alpha/Substation Alpha) subtitle format

CVE-2026-61626 is fixed in 2 products on 1 platform, first by libass9 1:0.17.4-2ubuntu0.1~esm1, released July 13, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-61626
- Priority: Routine
- CVSS: n/a (Medium)
- EPSS: 0.0012
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jul 13, 2026

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | libass Debian 14 | 1:0.17.5-1 | [DEBIAN-CVE-2026-61626](https://patchblox.com/security/data/fix/linux/debian-cve-2026-61626.md) | Jul 14, 2026 |
| Linux | libass9 Ubuntu 26.04.1 LTS (resolute) | 1:0.17.4-2ubuntu0.1~esm1 | [libass9 1:0.17.4-2ubuntu0.1~esm1](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libass9-1-0.17.4-2ubuntu0.1-esm1.md) | Jul 13, 2026 |

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Data as of 2026-11-08T13:06:02Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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