CVE-2021-33641

Local memory corruption via stale comment buffer

Published Jan 20, 2023 · Updated Apr 3, 2025

Use-after-free in Berkeley Yacc allows local users to corrupt process memory by supplying a crafted grammar file for processing. The file reader frees the allocation holding the current input line, but comment processing later dereferences that stale buffer. A user must invoke BYACC on attacker-controlled grammar input, after which the invalid access can crash the process or alter its memory.

CVSS severity7.8
High
EPSS probability0.27%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use-after-free in Berkeley Yacc allows local users to corrupt process memory by supplying a crafted grammar file for processing. The file reader frees the allocation holding the current input line, but comment processing later dereferences that stale buffer. A user must invoke BYACC on attacker-controlled grammar input, after which the invalid access can crash the process or alter its memory.

The record

CVE
CVE-2021-33641
Published
Jan 20, 2023
Updated
Apr 3, 2025
Vendor
Thomas E. Dickey
Product
Berkeley Yacc (BYACC)
Classifications
CWE-416, T1203
Attack vector
local
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 20, 2023CVE publishedPublication date reported by the CVE source.
  2. Apr 3, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

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  1. Affected versionversion=<1.9.20200330 and <2.0.20210808

What conditions does exploitation require?

Attack vectorlocal
Required privilegesunauthenticated

What is affected?

Thomas E. Dickey · Berkeley Yacc (BYACC)version=<1.9.20200330 and <2.0.20210808

Published CVSS scores

7.8NVDCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
7.8CISA-ADPCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-416Use After Free
T1203Exploitation for Client Execution

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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