GNU Bison improperly accepts grammar directives such as %output and %header that specify file paths without validation. When an attacker supplies a grammar, Bison writes generated files to the paths provided, overriding any caller‑supplied output options. The flaw enables overwriting any writable file in the Bison process’ environment, effectively allowing destructive file replacements or the persistence of malicious content. The weakness is reflected in CWE‑22 (Path Traversal) and CWE‑73 (Improper Restriction of Operations within the Bounds of a Resource).

The affected product is GNU Bison. Version 3.8.2 has been confirmed vulnerable; other versions that have not been tested may also be affected, so any deployment of Bison prior to the fix commit should be treated as potentially vulnerable.

The CVSS score of 4.6 indicates moderate severity, and the EPSS score of less than 1% suggests a very low likelihood of exploitation at this time. The vulnerability is not listed in the CISA KEV catalog. The likely attack vector is a local attacker capable of providing a crafted grammar file to the Bison process, or a remote attacker if Bison is exposed to untrusted input. If the attacker can supply custom grammars, they may overwrite important files that Bison can write to address, such as configuration or source files.