CVE-2026-56390 Arbitrary Output Location Change in GNU Bison

Medium 4.6 Published 29 Jul 2026 Updated 29 Jul 2026

Attackers can overwrite arbitrary files by supplying malicious grammar directives to GNU Bison, potentially compromising system integrity.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Local or same network only
Victim action needed: A user has to click or open something
Fix: Available

What is CVE-2026-56390?

CVE-2026-56390 is a medium-severity path traversal vulnerability in GNU Bison, published on 29 Jul 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

Official description

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.

Maintainers of this project were notified about this vulnerability, and fixed the issue in commit 8d101c19d4d9aaedf83a448c925513742d4efcf0. However, 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.

Weakness: CWE-73

Which versions are affected by CVE-2026-56390?

It affects 1 product.

Products include Gnu Bison.

How do I fix CVE-2026-56390?

Apply the vendor's patch.

Upgrade, and resolve every user-supplied path against an allowed base folder before opening it. More about Path traversal.

Is CVE-2026-56390 being exploited?

There is no known exploitation of CVE-2026-56390 in the wild and no public exploit code.

Sources

Also known as AZL-94136, DEBIAN-CVE-2026-56390, ECHO-c721-9c49-ec13, GHSA-3pcj-vqw8-47jm, JLSEC-2026-1346, UBUNTU-CVE-2026-56390.