How severe is CVE-2020-24240?

CVE-2020-24240 has a CVSS 3.x base score of 5.5, rated medium severity. It is exploitable over local access with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2020-24240 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (66th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2020-24240?

CVE-2020-24240 affects Gnu Bison. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2020-24240?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

When was CVE-2020-24240 published?

CVE-2020-24240 was published on 2020-08-25 and last updated on 2026-06-17.

CVE-2020-24240

CVE-2020-24240 is a medium-severity vulnerability in Gnu Bison with a CVSS 3.x base score of 5.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-416.

Key facts

Severity: Medium (CVSS 3.x base score 5.5)
CVSS v2: 7.1
EPSS exploit prediction: 1% (66th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-416
Affected product: Gnu Bison
Published: 2020-08-25
Last modified: 2026-06-17

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.

Frequently asked questions

What is 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. 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.
How severe is CVE-2020-24240?: CVE-2020-24240 has a CVSS 3.x base score of 5.5, rated medium severity. It is exploitable over local access with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2020-24240 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (66th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2020-24240?: CVE-2020-24240 affects Gnu Bison. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2020-24240?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
When was CVE-2020-24240 published?: CVE-2020-24240 was published on 2020-08-25 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:gnu:bison:3.7:*:*:*:*:*:*:*

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