CVE-2021-4140Network attack vector

Iframe sandbox bypass via XSLT meta refresh

Published Dec 22, 2022 · Updated Apr 16, 2025

Iframe sandbox bypass in Mozilla Firefox before 96, Firefox ESR before 91.5, and Thunderbird before 91.5 allows remote attackers to escape restrictions. A special-case path for meta refresh generated by an XSLT stylesheet bypasses the sandbox-aware navigation handling used for non-XSLT loads. Exploitation requires attacker-controlled XSLT content in a sandboxed iframe, after which refresh navigation escapes the frame's restrictions.

CVSS severity10.0
Critical
EPSS probability1.34%
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

Iframe sandbox bypass in Mozilla Firefox before 96, Firefox ESR before 91.5, and Thunderbird before 91.5 allows remote attackers to escape restrictions. A special-case path for meta refresh generated by an XSLT stylesheet bypasses the sandbox-aware navigation handling used for non-XSLT loads. Exploitation requires attacker-controlled XSLT content in a sandboxed iframe, after which refresh navigation escapes the frame's restrictions.

The record

CVE
CVE-2021-4140
Published
Dec 22, 2022
Updated
Apr 16, 2025
Vendor
Mozilla
Product
Firefox
Classifications
CWE-91
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 22, 2022CVE publishedPublication date reported by the CVE source.
  2. Apr 16, 2025Record 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 <96

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <96
Mozilla · Thunderbirdversion=unspecified <91.5
Mozilla · Firefox ESRversion=unspecified <91.5

Published CVSS scores

10.0NISTCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/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)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-91XML Injection (aka Blind XPath Injection)

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