CVE-2026-8391: Other issue in the JavaScript Engine component

Q: How do I fix CVE-2026-8391?

firefox: upgrade to 150.0.3. Exact commands for each OS are on this page.

Q: Is CVE-2026-8391 being exploited?

CVE-2026-8391 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-2026-8391 affect?

CVE-2026-8391 affects firefox. Severity is medium (CVSS 5.3). Other issue in the JavaScript Engine component. This vulnerability was fixed in Firefox 150.0.3, Firefox ESR 115.36, Firefox ESR 140.11, and Thunderbird 140.11.

Q: How urgent is CVE-2026-8391?

Priority 29/100. Remotely exploitable, no login, no user action

Routine

MEDIUM 5.3REMOTEUPGRADE

CVE-2026-8391 is a medium-severity flaw (CVSS 5.3) affecting firefox. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Other issue in the JavaScript Engine component. This vulnerability was fixed in Firefox 150.0.3, Firefox ESR 115.36, Firefox ESR 140.11, and Thunderbird 140.11.

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 78%)
CISA exploited list	No
Weakness	CWE-20, CWE-79, CWE-119, CWE-475
Published	2026-05-12
Last modified	2026-10-25

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Last updated 2026-10-25 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-05-12 14:17 UTCPublished CVE record published
2026-10-25 23:10 UTCUpdated Record last modified

mozilla firefox UPGRADE

Affected: < 150.0.3

Fixed in: 150.0.3

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 150.0.3 (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.

Advisories and coverage

Official advisories

AlmaLinux Errata: [ALSA-2026:21381] Important: thunderbird security update 2026-05-29
AlmaLinux Errata: [ALSA-2026:21378] Important: firefox security update 2026-05-29

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-2026-8391?

firefox: upgrade to 150.0.3. Exact commands for each OS are on this page.

Is CVE-2026-8391 being exploited?

CVE-2026-8391 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-2026-8391 affect?

CVE-2026-8391 affects firefox. Severity is medium (CVSS 5.3). Other issue in the JavaScript Engine component. This vulnerability was fixed in Firefox 150.0.3, Firefox ESR 115.36, Firefox ESR 140.11, and Thunderbird 140.11.

How urgent is CVE-2026-8391?

Priority 29/100. Remotely exploitable, no login, no user action

Software pages

Mozilla Firefox vulnerabilities · Mozilla Thunderbird vulnerabilities

Related CVEs

CVE-2026-6770 MEDIUM 6.5 UPGRADE UPGRADE Other issue in the Storage: IndexedDB component
CVE-2026-8091 CRITICAL 9.8 UPGRADE UPGRADE Incorrect boundary conditions in the Audio/Video: Playback component
CVE-2026-6760 CRITICAL 9.8 UPGRADE UPGRADE Mitigation bypass in the Networking: Cookies component
CVE-2026-6768 CRITICAL 9.8 UPGRADE UPGRADE Mitigation bypass in the Networking: Cookies component
CVE-2026-6759 HIGH 7.5 UPGRADE UPGRADE Use-after-free in the Widget: Cocoa component

References

← Back to latest CVEs