libass: Out-of-bounds bit clears for negative Matroska ReadOrder values

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-61626
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-10-30 15:16:52 UTC
Updated2026-10-30 19:39:14 UTC
Descriptionlibass 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.

Risk And Classification

Primary CVSS: v4.0 7.3 HIGH from [email protected]

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

EPSS: 0.001180000 probability, percentile 0.017100000 (date 2026-11-02)

Problem Types: CWE-787 | CWE-787 CWE-787: Out-of-bounds Write

VersionSourceTypeScoreSeverityVector
4.0[email protected]Secondary7.3HIGHCVSS: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/C...
4.0CNADECLARED7.3HIGHCVSS:4.0/AV:L/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

CVSS v4.0 Breakdown

Attack Vector
Local
Attack Complexity
High
Attack Requirements
None
Privileges Required
None
User Interaction
Passive
Confidentiality
High
Integrity
High
Availability
High
Sub Conf.
None
Sub Integrity
None
Sub Availability
None

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

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Libass Libass affected >= 0.17.4, < 0.17.5 Not specified

References

ReferenceSourceLinkTags
github.com/libass/libass/commit/23da65a130bb8c5b2cf0c7df0dd7d424607f98f1 [email protected] github.com
github.com/libass/libass/releases/tag/0.17.5 [email protected] github.com
github.com/libass/libass/security/advisories/GHSA-5gf7-wjfm-vmvm [email protected] github.com
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