CVE-2021-23993

Name: CVE-2021-23993
Creator: NVD / NIST
Published: 2021-06-24T14:15:09.257+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.5/10EPSS 0.43%

Last modified Jun 17, 2026

CVE-2021-23993 is a medium-severity vulnerability rated 6.5/10 on the CVSS scale. An attacker may perform a DoS attack to prevent a user from sending encrypted email to a correspondent. If an attacker creates a crafted OpenPGP key with a subkey that has an invalid self signature, and the Thunderbird user imports the crafted key, then Thunderbird may try to use the invalid subkey, but the RNP library rejects it from being used, causing encryption to fail. EPSS estimates a 0.43% chance of exploitation in the next 30 days.

Description

An attacker may perform a DoS attack to prevent a user from sending encrypted email to a correspondent. If an attacker creates a crafted OpenPGP key with a subkey that has an invalid self signature, and the Thunderbird user imports the crafted key, then Thunderbird may try to use the invalid subkey, but the RNP library rejects it from being used, causing encryption to fail. This vulnerability affects Thunderbird < 78.9.1.

Metrics

CVSS 3.1

6.5/10

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

EPSS Probability

0.43%

34.2th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-347

Affected Software

Vendor	Product	Versions
Mozilla	Thunderbird	< 78.9.1

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1666360Issue Tracking, Permissions Required, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-13/Release Notes, Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1666360Issue Tracking, Permissions Required, 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-2021-23993?

How severe is CVE-2021-23993?

CVE-2021-23993 has a CVSS score of 6.5/10 (MEDIUM severity). The EPSS model estimates a 0.43% probability of exploitation in the next 30 days.

How do I fix CVE-2021-23993?

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-2021-23993?

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

Scan your code now

Source: NVD / NIST