Signed-content spoofing via outer MIME layer
Published Dec 22, 2022 · Updated Apr 16, 2025
Signature validation bypass in Mozilla Thunderbird before 91.4.1 allows remote attackers to spoof signed email content via crafted MIME layers. Thunderbird evaluates only the inner OpenPGP/MIME signed part while presenting outer message content, such as an injected mailing-list footer, as covered by that signature. Opening the crafted message is required; the result is a false trust indication for attacker-controlled content rather than code execution or service interruption.
Summary
What happened
Signature validation bypass in Mozilla Thunderbird before 91.4.1 allows remote attackers to spoof signed email content via crafted MIME layers. Thunderbird evaluates only the inner OpenPGP/MIME signed part while presenting outer message content, such as an injected mailing-list footer, as covered by that signature. Opening the crafted message is required; the result is a false trust indication for attacker-controlled content rather than code execution or service interruption.
The record
- CVE
- CVE-2021-4126
- Published
- Dec 22, 2022
- Updated
- Apr 16, 2025
- Vendor
- Mozilla
- Product
- Thunderbird
- Classifications
- T1203
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Dec 22, 2022CVE publishedPublication date reported by the CVE source.
- 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.
- Affected versionversion=unspecified <91.4.1
What conditions does exploitation require?
What is affected?
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.
Weakness, pattern, technique
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