CVE-2021-29946

Name: CVE-2021-29946
Creator: NVD / NIST
Published: 2021-06-24T14:15:09.797+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 8.8/10EPSS 1.17%

Last modified Jun 17, 2026

CVE-2021-29946 is a high-severity vulnerability rated 8.8/10 on the CVSS scale. Ports that were written as an integer overflow above the bounds of a 16-bit integer could have bypassed port blocking restrictions when used in the Alt-Svc header. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.. EPSS estimates a 1.17% chance of exploitation in the next 30 days.

Description

Ports that were written as an integer overflow above the bounds of a 16-bit integer could have bypassed port blocking restrictions when used in the Alt-Svc header. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.

Metrics

CVSS 3.1

8.8/10

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

EPSS Probability

1.17%

63.3th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-190

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 88.0
Mozilla	Firefox Esr	< 78.10
Mozilla	Thunderbird	< 78.10

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1698503Permissions Required, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-14/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-15/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-16/Release Notes, Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1698503Permissions Required, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-14/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-15/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-16/Release Notes, Vendor Advisory

Timeline

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

Frequently Asked Questions

What is CVE-2021-29946?

How severe is CVE-2021-29946?

CVE-2021-29946 has a CVSS score of 8.8/10 (HIGH severity). The EPSS model estimates a 1.17% probability of exploitation in the next 30 days.

How do I fix CVE-2021-29946?

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-2021-29946?

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

Scan your code now

Source: NVD / NIST