CVE-2026-5735: Memory safety bugs present in Firefox 149.0.1 and Thunderbird 149.0.1

Q: How do I fix CVE-2026-5735?

firefox: upgrade to 149.0.2; thunderbird: upgrade to 149.0.2. Exact commands for each OS are on this page.

Q: Is CVE-2026-5735 being exploited?

CVE-2026-5735 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-5735 affect?

CVE-2026-5735 affects firefox, thunderbird. Severity is critical (CVSS 9.8). Memory safety bugs present in 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 vulner

Q: How urgent is CVE-2026-5735?

Priority 49/100. Critical severity; Remotely exploitable, no login, no user action

This month

CRITICAL 9.8REMOTEUPGRADEUPGRADE

CVE-2026-5735 is a critical-severity flaw (CVSS 9.8) affecting firefox, thunderbird. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Memory safety bugs present in 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 was fixed in Firefox 149.0.2 and Thunderbird 149.0.2.

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 61%)
CISA exploited list	No
Weakness	CWE-787, CWE-125
Published	2026-04-07
Last modified	2026-10-23

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Last updated 2026-10-23 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-07 13:16 UTCPublished CVE record published
2026-10-23 23:10 UTCUpdated Record last modified

mozilla firefox UPGRADE

Affected: < 149.0.2

Fixed in: 149.0.2

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy firefox || rpm -q firefox || firefox --version

2. UPGRADE to 149.0.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade firefox

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade firefox

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

mozilla thunderbird UPGRADE

Affected: < 149.0.2

Fixed in: 149.0.2

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy thunderbird || rpm -q thunderbird || thunderbird --version

2. UPGRADE to 149.0.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade thunderbird

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade thunderbird

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Exact fixes by distribution

Fedora 46 · python-fontconfig · FIXED · 0.6.3-1.fc46 · FEDORA-2026-f92ca8b01f · source

sudo dnf upgrade --advisory=FEDORA-2026-f92ca8b01f

Fedora ELN · valgrind · FIXED · 3.28.0-1.eln159 · FEDORA-2026-c593ef1d15 · source

sudo dnf upgrade --advisory=FEDORA-2026-c593ef1d15

RHEL 10 · firefox · NOT AFFECTED

RHEL 10 · rhel10/firefox-flatpak · NOT AFFECTED

RHEL 6 · firefox · NOT AFFECTED

RHEL 7 · firefox · NOT AFFECTED

RHEL 8 · firefox · NOT AFFECTED

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-5735?

firefox: upgrade to 149.0.2; thunderbird: upgrade to 149.0.2. Exact commands for each OS are on this page.

Is CVE-2026-5735 being exploited?

CVE-2026-5735 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-5735 affect?

CVE-2026-5735 affects firefox, thunderbird. Severity is critical (CVSS 9.8). Memory safety bugs present in 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 vulner

How urgent is CVE-2026-5735?

Priority 49/100. Critical severity; Remotely exploitable, no login, no user action

Software pages

Mozilla Firefox vulnerabilities

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References

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