GHSA-5gf7-wjfm-vmvm: libass: Out-of-bounds bit clears for negative Matroska ReadOrder values

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 assprocesschunk, enabled read-order checking, and later pruning of the malicious events. Read-order checking is enabled by default unless asssetcheckreadorder is called with zero; pruning must occur explicitly through asspruneevents or through automatic pruning previously enabled with assconfigureprune. 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.