CVE-2021-23991

Name: CVE-2021-23991
Creator: NVD / NIST
Published: 2021-06-24T14:15:09.197+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.8/10EPSS 1.03%

Last modified Jun 17, 2026

CVE-2021-23991 is a medium-severity vulnerability rated 6.8/10 on the CVSS scale. If a Thunderbird user has previously imported Alice's OpenPGP key, and Alice has extended the validity period of her key, but Alice's updated key has not yet been imported, an attacker may send an email containing a crafted version of Alice's key with an invalid subkey, Thunderbird might subsequently attempt to use the invalid subkey, and will fail to send encrypted email to Alice. This vulnerability affects Thunderbird < 78.9.1.. EPSS estimates a 1.03% chance of exploitation in the next 30 days.

Description

If a Thunderbird user has previously imported Alice's OpenPGP key, and Alice has extended the validity period of her key, but Alice's updated key has not yet been imported, an attacker may send an email containing a crafted version of Alice's key with an invalid subkey, Thunderbird might subsequently attempt to use the invalid subkey, and will fail to send encrypted email to Alice. This vulnerability affects Thunderbird < 78.9.1.

Metrics

CVSS 3.1

6.8/10

CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N

EPSS Probability

1.03%

59.5th percentile

Probability of exploitation in the next 30 days. Learn more

Affected Software

Vendor	Product	Versions
Mozilla	Thunderbird	< 78.9.1

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1673240Exploit, Issue Tracking, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-13/Release Notes, Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1673240Exploit, Issue Tracking, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-13/Release Notes, Vendor Advisory

Timeline

Published: Jun 24, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-23991?

How severe is CVE-2021-23991?

CVE-2021-23991 has a CVSS score of 6.8/10 (MEDIUM severity). The EPSS model estimates a 1.03% probability of exploitation in the next 30 days.

How do I fix CVE-2021-23991?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-23991?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST