Medium [CVE-2026-56389] Arbitrary Code Execution via malicious grammar file
This medium-severity Red Hat Linux advisory covers CVE-2026-56389 affecting Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7.
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Summary
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. Maintainers of this project were notified about this vulnerability, and fixed the issue in commit 3169c1e7a2c6acc4c59dfcf8b089896d6881925b.
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.
Exploitation requires user interaction to process a malicious grammar file, which limits the attack surface to scenarios where untrusted input is explicitly processed. Red Hat severity: Moderate — CVSS 5.5 (CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N).
Weakness: CWE-78. Affected Red Hat products: Red Hat Enterprise Linux 10; Red Hat Enterprise Linux 6; Red Hat Enterprise Linux 7; Red Hat Enterprise Linux 8; Red Hat Enterprise Linux 9; Red Hat OpenShift Container Platform 4.
Will not fix / out of support: Red Hat Enterprise Linux 6.
- 3.8.2
Official advisory · high-confidence parse· fetched 23 days ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists — confirm against the vendor advisory.
Official advisory · high-confidence parse· fetched 23 days ago·verify at source
Mitigation checklist
- To mitigate this vulnerability, avoid processing untrusted or unverified grammar files with the `bison --html` command. Restrict the use of `bison --html` to trusted input sources only. This operational control reduces the risk of arbitrary code execution by preventing the vulnerable code path from being triggered with malicious input.
Official advisory · high-confidence parse· fetched 23 days ago·verify at source
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