---
id: "CVE-2026-61626"
kind: "vulnerability"
aliases: ["CVE-2026-61626"]
title: "libass: negative Matroska ReadOrder lets a subtitle file corrupt memory"
severity: "high"
cvss: 7.3
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["libass", "matroska", "package", "subtitle-rendering", "video-players"]
not_affected_if: ["no-event-pruning", "no-untrusted-mkv", "not-0.17.4"]
published_at: "2026-10-30T15:16:52+00:00"
first_seen_at: "2026-10-30T15:31:11+00:00"
updated_at: "2026-10-30T15:33:24+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-61626", "first_seen_at": "2026-10-30T15:31:11+00:00"}]
---

# libass: negative Matroska ReadOrder lets a subtitle file corrupt memory

libass, a subtitle rendering library, does not check negative Matroska ReadOrder values during event pruning, so a malicious Matroska stream can clear bits at attacker-chosen positions near a NULL pointer. On common systems the expected result is a denial of service (crash), and more extensive exploitation is described as theoretical and tightly constrained.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the libass version in use, for example with `pkg-config --modversion libass`, and see whether it falls in 0.17.4 through 0.17.4 before 0.17.5.
- Look for applications that load Matroska files through `ass_process_chunk` with pruning enabled via `ass_prune_events` or `ass_configure_prune`.

## Fix

- libass: move to `0.17.5` or later

## Timeline

- 2026-10-30T15:31:11+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-61626

## From the sources

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.

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