CVE-2021-43543Public exploitNetwork attack vector

Same-origin script execution via dropped sandbox flags

Published Dec 8, 2021 · Updated Aug 4, 2024

CSP sandbox bypass in Mozilla Firefox before 95, Firefox ESR before 91.4.0, and Thunderbird before 91.4.0 allows remote attackers to run script. The DOM Security OpenObject path reads sandbox flags from the loading top-level browsing context rather than the embedded subdocument, dropping restrictions for object and embed loads. A victim must load attacker-controlled embedded content; that content then executes same-origin script despite the document's CSP sandbox.

CVSS severity6.1
Medium
EPSS probability1.35%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

CSP sandbox bypass in Mozilla Firefox before 95, Firefox ESR before 91.4.0, and Thunderbird before 91.4.0 allows remote attackers to run script. The DOM Security OpenObject path reads sandbox flags from the loading top-level browsing context rather than the embedded subdocument, dropping restrictions for object and embed loads. A victim must load attacker-controlled embedded content; that content then executes same-origin script despite the document's CSP sandbox.

The record

CVE
CVE-2021-43543
Published
Dec 8, 2021
Updated
Aug 4, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-79, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 8, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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 <95

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <95
Mozilla · Thunderbirdversion=unspecified <91.4.0
Mozilla · Firefox ESRversion=unspecified <91.4.0

Published CVSS scores

6.1NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
6.1SUSECVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
4.3NVDAV:N/AC:M/Au:N/C:N/I:P/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-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1203Exploitation for Client Execution

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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