Description

CERT Polska has received a report about vulnerabilities in GNU Bison software and participated in coordination of their disclosure.

The vulnerability CVE-2026-56389: GNU Bison allows for an execution of an arbitrary program during HTML report generation due to improper handling of grammar-defined configuration variables. A grammar file can override the executable used for the XML‑to‑HTML transformation step via %define tool.xsltproc, which is accepted without restriction and passed directly to execvp(). When running bison --html on a attacker-provided grammar, this behavior allows execution of an arbitrary program with the privileges of the Bison process.

This issue was fixed in commit 3169c1e7a2c6acc4c59dfcf8b089896d6881925b.

The vulnerability CVE-2026-56390: GNU Bison improperly handles grammar‑defined output paths. Grammar directives such as %output and %header allow specifying file paths, which are accepted without restriction and override caller‑supplied output options. When processing attacker-supplied grammar, this behavior allows directing generated files to arbitrary writable locations on the filesystem, potentially overwriting existing files accessible to the Bison process.

This issue was fixed in commit 8d101c19d4d9aaedf83a448c925513742d4efcf0.

Maintainers of this project were notified about both vulnerabilities, but they did not provide vulnerable version range. Version 3.8.2 was tested and confirmed as vulnerable, other versions were not tested but might also be vulnerable.

Credits

We thank Michał Majchrowicz and Marcin Wyczechowski from AFINE Team for the responsible vulnerability report.