CVE-2021-29981Network attack vector

Application crash via conflicting JIT register assignments

Published Aug 17, 2021 · Updated Aug 3, 2024

JIT register confusion in Mozilla Firefox and Thunderbird before 91 allows remote attackers to crash the application via crafted content. Live-range splitting can disconnect a reused input from its output, allowing unrelated values to receive the same register and causing the compiler to use the wrong virtual-register assignment. Opening attacker-controlled content is required; the documented outcome is a deterministic crash, while code-execution exploitability remains unknown.

CVSS severity8.8
High
EPSS probability1.00%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

JIT register confusion in Mozilla Firefox and Thunderbird before 91 allows remote attackers to crash the application via crafted content. Live-range splitting can disconnect a reused input from its output, allowing unrelated values to receive the same register and causing the compiler to use the wrong virtual-register assignment. Opening attacker-controlled content is required; the documented outcome is a deterministic crash, while code-execution exploitability remains unknown.

The record

CVE
CVE-2021-29981
Published
Aug 17, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
T1203
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

Compare your product and version with the public record. A matching version still requires validation against your environment.

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

8.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P

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 exploitUnconfirmed

Weakness, pattern, technique

T1203Exploitation for Client Execution

Public exploit references

No public exploit references are available in this record.

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

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo