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Fencer Intel
medium 5.5

CVE-2020-24240 json

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.

published 6y ago modified 4mo ago also EUVD-2020-16975

cvss
5.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.3%
EPSS 1.3% chance of exploitation in 30 days, percentile 70%
public exploits
0
none known

Summary

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.

Patch / fix

  • bison Debian 14, Debian 13, Debian 12 2:3.7.2+dfsg-1

debian advisories DEBIAN-CVE-2020-24240

  • bison Alpine 3.24, Alpine 3.23, Alpine 3.22 +9 more 3.7.2-r0

alpine advisories ALPINE-CVE-2020-24240

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

Is CVE-2020-24240 in your stack? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

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Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2020-24240 updated cvss 5.5

bison: use-after-free via crafted input file containing a NULL byte can lead to DoS

  • Red Hat Enterprise Linux 5 bison Red Hat Enterprise Linux 6 bison, bison-runtime
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 7 bison, bison-runtime Red Hat Enterprise Linux 8 bison, bison-runtime
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

1.28% chance of exploitation in the next 30 days, higher than 70% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by version score vector
NVD 3.1 5.5 CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
NVD 2.0 7.1 AV:N/AC:M/Au:N/C:N/I:N/A:C

Classification

Weakness

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.