libass up to 0.17.4 Event Pruning ass_prune_events ReadOrder null pointer dereference

Summaryinfo

A vulnerability was found in libass up to 0.17.4. It has been classified as problematic. The impacted element is the function ass_prune_events of the component Event Pruning. The manipulation of the argument ReadOrder leads to null pointer dereference. This vulnerability is documented as CVE-2026-61626. The attack can be initiated remotely. There is not any exploit available. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability was found in libass up to 0.17.4 and classified as problematic. This issue affects the function ass_prune_events of the component Event Pruning. The manipulation of the argument ReadOrder with an unknown input leads to a null pointer dereference vulnerability. Using CWE to declare the problem leads to CWE-476. A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit. Impacted is integrity, and availability. The summary by CVE is:

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.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-61626 since 07/10/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. It demands that the victim is doing some kind of user interaction. Technical details are known, but no exploit is available.

Upgrading to version 0.17.5 eliminates this vulnerability.

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Productinfo

Name

• libass

Version

• 0.17.0
• 0.17.1
• 0.17.2
• 0.17.3
• 0.17.4

Website

• Product: https://github.com/libass/libass/

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