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CVE-2026-12291: Use-after-free in the Networking: HTTP component

39
Routine
HIGH 8.8UPGRADEUPGRADE

CVE-2026-12291 is a high-severity flaw (CVSS 8.8) affecting firefox, thunderbird. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Use-after-free in the Networking: HTTP component. This vulnerability was fixed in Firefox 152, Firefox ESR 140.12, Firefox ESR 115.37, Thunderbird 152, and Thunderbird 140.12.

Why this priority

  • High severity

A fixed release or distro patch exists.

Risk facts

CVSS score8.8 (High)
Attack vectorNetwork (remote)
Attack complexityLow
Privileges neededNone
User interactionRequired
EPSS exploit chance0.4% (top 68%)
CISA exploited listNo
WeaknessCWE-416, CWE-825
Published2026-06-16
Last modified2026-10-26
Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Share: LinkedIn · X · Reddit

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

mozilla firefox UPGRADE
Affected: < 115.37.0 · < 152.0.0 · >= 128.0 and < 140.12.0
Fixed in: 115.37.0, 140.12.0, 152.0.0
Safe to downgrade to: a release before 128.0
1. Check installed version
apt-cache policy firefox || rpm -q firefox || firefox --version
2. UPGRADE to 115.37.0 (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.
3. If no update is available yet, DOWNGRADE to a release before 128.0 (Debian / Ubuntu)
apt-cache madison firefox sudo apt install firefox=<VERSION_BEFORE_128.0> sudo apt-mark hold firefox
3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list firefox sudo dnf downgrade firefox-<VERSION_BEFORE_128.0> sudo dnf versionlock add firefox
Downgrade risk
Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.
After downgrade
Restart the service, and keep the package held until a fixed release ships.
mozilla thunderbird UPGRADE
Affected: < 140.12.0 · < 152.0.0
Fixed in: 140.12.0, 152.0.0
Safe to downgrade to: no unaffected older release
1. Check installed version
apt-cache policy thunderbird || rpm -q thunderbird || thunderbird --version
2. UPGRADE to 140.12.0 (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.

Advisories and coverage

Official advisories

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-2026-12291?

firefox: upgrade to 115.37.0, 140.12.0, 152.0.0; thunderbird: upgrade to 140.12.0, 152.0.0. Exact commands for each OS are on this page.

Is CVE-2026-12291 being exploited?

CVE-2026-12291 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-2026-12291 affect?

CVE-2026-12291 affects firefox, thunderbird. Severity is high (CVSS 8.8). Use-after-free in the Networking: HTTP component. This vulnerability was fixed in Firefox 152, Firefox ESR 140.12, Firefox ESR 115.37, Thunderbird 152, and Thunderbird 140.12.

How urgent is CVE-2026-12291?

Priority 39/100. High severity

Software pages

Mozilla Firefox vulnerabilities · Mozilla Thunderbird vulnerabilities

Related CVEs

References

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