CVE-2021-29982Public exploitNetwork attack vector

One-bit memory disclosure via clobbered JIT register

Published Aug 17, 2021 · Updated Aug 3, 2024

Type confusion in the JIT engine in Mozilla Firefox and Thunderbird before 91 allows remote attackers to disclose one memory bit via crafted script. Ion code generation passes the same register for the object and output to MacroAssembler::isCallableOrConstructor, whose class check clobbers the object before the constructor-flag check reads it. User interaction with script-capable content is required; Thunderbird mail reading disables scripting, and Mozilla states that the one-bit behavior is not exploitable beyond disclosure.

CVSS severity6.5
Medium
EPSS probability1.12%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Type confusion in the JIT engine in Mozilla Firefox and Thunderbird before 91 allows remote attackers to disclose one memory bit via crafted script. Ion code generation passes the same register for the object and output to MacroAssembler::isCallableOrConstructor, whose class check clobbers the object before the constructor-flag check reads it. User interaction with script-capable content is required; Thunderbird mail reading disables scripting, and Mozilla states that the one-bit behavior is not exploitable beyond disclosure.

The record

CVE
CVE-2021-29982
Published
Aug 17, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-772
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 17, 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 <91

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <91
Mozilla · Thunderbirdversion=unspecified <91

Published CVSS scores

6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
4.3NIST NVDAV:N/AC:M/Au:N/C:P/I:N/A:N

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)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-772Missing Release of Resource after Effective Lifetime

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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