CVE-2026-5734

CWE-787 — Out-of-bounds WriteCWE-120 — Classic Buffer Overflow12 documents9 sources
Severity
9.8CRITICAL
EPSS
0.1%
top 81.40%
CISA KEV
Not in KEV
Exploit
No known exploits
Affected products
3
Mozilla FirefoxMozilla Firefox ESRMozilla Thunderbird
Timeline
PublishedApr 7

Description

Memory safety bugs present in Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1 and Thunderbird 149.0.1. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability affects Firefox < 149.0.2, Firefox ESR < 140.9.1, Thunderbird < 149.0.2, and Thunderbird < 140.9.1.

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:HExploitability: 3.9 | Impact: 5.9

Affected Packages6 packages

▶CVEListV5mozilla/firefoxunspecified — 149.0.2
▶NVDmozilla/firefox< 140.9.1+1
▶CVEListV5mozilla/firefox_esrunspecified — 140.9.1
▶CVEListV5mozilla/thunderbirdunspecified — 149.0.2+1
▶NVDmozilla/thunderbird< 140.9.1+1

🔴Vulnerability Details

3
OSV
CVE-2026-5734: Memory safety bugs present in Firefox ESR 140↗2026-04-07
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GHSA
GHSA-qf82-86x2-7q23: Memory safety bugs present in Firefox ESR 140↗2026-04-07
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CVEList
Memory safety bugs fixed in Firefox ESR 140.9.1, Thunderbird ESR 140.9.1, Firefox 149.0.2 and Thunderbird 149.0.2↗2026-04-07
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📋Vendor Advisories

6
Red Hat
thunderbird: firefox: Memory safety bugs fixed in Firefox ESR 140.9.1, Thunderbird ESR 140.9.1, Firefox 149.0.2 and Thunderbird 149.0.2↗2026-04-07
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Debian
CVE-2026-5734: firefox - Memory safety bugs present in Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Fire...↗2026
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Mozilla
Mozilla Foundation Security Advisory 2026-29: CVE-2026-5734↗
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Mozilla
Mozilla Foundation Security Advisory 2026-28: CVE-2026-5734↗
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Mozilla
Mozilla Foundation Security Advisory 2026-25: CVE-2026-5734↗
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🕵️Threat Intelligence

1
Wiz
CVE-2026-5734 Impact, Exploitability, and Mitigation Steps | Wiz↗
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💬Community

1
Bugzilla
CVE-2026-5734 thunderbird: firefox: Memory safety bugs fixed in Firefox ESR 140.9.1, Thunderbird ESR 140.9.1, Firefox 149.0.2 and Thunderbird 149.0.2↗2026-04-07
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CVE-2026-5734 (CRITICAL CVSS 9.8) | Memory safety bugs present in Firef | cvebase.io