libass is a portable subtitle renderer for the `ASS`/SSA (Advanced Substation Alpha/Substation…

Why it is Yellow

• CVSS 7.3 high

References

• https://github.com/libass/libass/commit/f2ef59755292bc4bb950ef22710e18a5487c399d
• https://github.com/libass/libass/releases/tag/0.17.5
• https://github.com/libass/libass/security/advisories/GHSA-pjjp-65r7-ppgm
• https://lists.debian.org/debian-lts-announce/2026/09/msg00004.html

Original description

libass is a portable subtitle renderer for the `ASS`/SSA (Advanced Substation Alpha/Substation Alpha) subtitle format. Prior to 0.17.5, libass can overread and overwrite the glyph array in `wrap_lines_measure` while rendering malicious `ASS` content. In `wrap modes other than two`, a final line ending with a literal backslash-n sequence and otherwise consisting only of such sequences or spaces can remain after early trimming and make the line-measurement loops overshoot by one glyph. When the total text size exactly matches the allocated glyph-array capacity, the first loop reads an uninitialized 32-bit fixed-point `pos.x` value, although that value is not subsequently used. The second loop writes `pos.x` and `pos.y` beyond the array, zeroing the first and adding an attacker-controllable offset to the second. No actual exploitation beyond possible denial of service is known; broader exploitation is theoretical and constrained by the addresses, limited writes, and ASLR. This issue is fixed in version 0.17.5.