9.1

CVE-2026-84639: mozilla thunderbird Use of Uninitialized Variable

CVSS: 9.1v3.1CriticalAnalyzed
AliasesUBUNTU-CVE-2026-84639DEBIAN-CVE-2026-84639
Published 01-09-2026Last modified 17-09-2026
Open JSON

Vulnerability summary

CVE-2026-84639 is a critical (CVSS 9.1) Use of Uninitialized Variable vulnerability in mozilla thunderbird. EPSS estimates a 0.32% chance of exploitation in the next 30 days, higher than 23.8% of scored vulnerabilities.

Overall risk
70/100
0100
High · default modelHow is this calculated?
CVSS score
9.1/10
010
Critical · v3.1 (CVE.org)
EPSS score
0.32%
0%100%
Low · 0.00 pp in 30 days
KEVNot listedIn no KEV catalog
RansomwareNone recordedNo known use recorded
Public exploits0No public exploits recorded

Advisory lineage

Upstream 0 · Downstream 13 · Related 2
MGASA-2026-0388OPENSUSE-SU-2026:11678-1RHSA-2026:70642RHSA-2026:73130RHSA-2026:73971RHSA-2026:77627

Timeline

  1. 01-09-2026
    PublishedVulnerability first disclosed.
  2. 17-09-2026
    Last modifiedVulnerability information updated.

Description

Triggering an error condition in certain MIME bodies would cause uninitialized memory to be used. This vulnerability was fixed in Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

CVSS metrics

4 scores · 4 sources · 1 version

EPSS trend

Probability

0.32%

Chance of exploitation in the next 30 days

Percentile

24%

Versus all other CVEs

30-day change

0.00 pp

From 0.32% on 20-09-2026

20-09-202620-10-2026
EPSS score
0.32%

Known exploited

Compare KEV catalogs

Not in any of the 3 tracked catalogs.

Public exploits

0 found

No public exploits indexed.

Weaknesses

CWE · root cause

Affected systems

Debian1 product
  • thunderbird< 1:140.15.0esr-1~deb12u1 | < 1:140.15.0esr-1~deb13u1 | < 1:153.2.0esr-1
Mozilla1 product
  • thunderbird< 140.15.0 | ≥ 141.0, < 153.2.0 | ≥ 154.0, < 155.0
Ubuntu1 product
  • thunderbirdAll versions (No fix recorded)

3 vendors · 3 products

References

8 links