CVE-2021-29964Public exploit

Local information disclosure via unterminated WM_COPYDATA string

Published Jun 24, 2021 · Updated Aug 3, 2024

Out-of-bounds read in Mozilla Firefox before 89, Firefox ESR before 78.11, and Thunderbird before 78.11 allows local users to expose process memory. CommandLineParserWin::HandleCommandLine accepts a malformed WM_COPYDATA payload as an unterminated string and reads beyond its supplied boundary. A locally installed program, including one in a less-privileged account, must message a running instance on Windows; adjacent memory can then influence a navigation target.

CVSS severity7.1
High
EPSS probability0.82%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Out-of-bounds read in Mozilla Firefox before 89, Firefox ESR before 78.11, and Thunderbird before 78.11 allows local users to expose process memory. CommandLineParserWin::HandleCommandLine accepts a malformed WM_COPYDATA payload as an unterminated string and reads beyond its supplied boundary. A locally installed program, including one in a less-privileged account, must message a running instance on Windows; adjacent memory can then influence a navigation target.

The record

CVE
CVE-2021-29964
Published
Jun 24, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-125
Attack vector
local
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 24, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <89

What conditions does exploitation require?

Attack vectorlocal
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <89
Mozilla · Thunderbirdversion=unspecified <78.11
Mozilla · Firefox ESRversion=unspecified <78.11

Published CVSS scores

5.8NIST National Vulnerability DatabaseAV:N/AC:M/Au:N/C:P/I:N/A:P
7.1NIST National Vulnerability DatabaseCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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 exploitPublished

Weakness, pattern, technique

CWE-125Out-of-bounds Read

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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