Vulnerability · Published Sep 29, 2026 · Assigned by mozilla
CVE-2026-100803Same-origin policy bypass in the WebExtensions component
High 8.1
In plain English
A high-severity security flaw in Firefox from Mozilla. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 6% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Severity (CVSS 3.1)8.1High
Exploit odds (30 days)0.2%top 94.0% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known
Official description
Same-origin policy bypass in the WebExtensions component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 140.17, Thunderbird 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| AlmaLinux:10 | firefox | 140.17.0-1.el10_2 | ALSA-2026:76764 |
| AlmaLinux:8 | firefox | 140.17.0-1.el8_10.alma.1 | ALSA-2026:76850 |
| AlmaLinux:9 | firefox | 140.17.0-1.el9_8.alma.1 | ALSA-2026:76761 |
| AlmaLinux:9 | firefox-x11 | 140.17.0-1.el9_8.alma.1 | ALSA-2026:76761 |
| AlmaLinux:10 | thunderbird | 140.17.0-1.el10_2.alma.1 | ALSA-2026:80674 |
| AlmaLinux:8 | thunderbird | 140.17.0-1.el8_10.alma.1 | ALSA-2026:90913 |
| AlmaLinux:9 | thunderbird | 140.17.0-1.el9_8.alma.1 | ALSA-2026:80694 |
| Debian:12 | firefox-esr | 153.4.0esr-1~deb12u1 | DEBIAN-CVE-2026-100803 |
| Debian:13 | firefox-esr | 153.4.0esr-1~deb13u1 | DEBIAN-CVE-2026-100803 |
| Debian:14 | firefox-esr | 153.4.0esr-1 | DEBIAN-CVE-2026-100803 |
| Debian:12 | thunderbird | 1:140.17.0esr-1~deb12u1 | DEBIAN-CVE-2026-100803 |
| Debian:13 | thunderbird | 1:140.17.0esr-1~deb13u1 | DEBIAN-CVE-2026-100803 |
| Debian:14 | thunderbird | 1:153.4.0esr-1 | DEBIAN-CVE-2026-100803 |
| Rocky Linux:10 | firefox | 0:140.17.0-1.el10_2 | RLSA-2026:76764 |
| Rocky Linux:8 | firefox | 0:140.17.0-1.el8_10 | RLSA-2026:76850 |
| Rocky Linux:9 | firefox | 0:140.17.0-1.el9_8 | RLSA-2026:76761 |
| Rocky Linux:10 | thunderbird | 0:140.17.0-1.el10_2 | RLSA-2026:80674 |
| Rocky Linux:8 | thunderbird | 0:140.17.0-1.el8_10 | RLSA-2026:90913 |
| Rocky Linux:9 | thunderbird | 0:140.17.0-1.el9_8 | RLSA-2026:80694 |
| Ubuntu:22.04:LTS | mozjs102 | no fix listed | UBUNTU-CVE-2026-100803 |
| Ubuntu:24.04:LTS | mozjs102 | no fix listed | UBUNTU-CVE-2026-100803 |
| Ubuntu:24.04:LTS | mozjs115 | no fix listed | UBUNTU-CVE-2026-100803 |
| Ubuntu:22.04:LTS | mozjs78 | no fix listed | UBUNTU-CVE-2026-100803 |
| Ubuntu:22.04:LTS | mozjs91 | no fix listed | UBUNTU-CVE-2026-100803 |
| Ubuntu:22.04:LTS | thunderbird | no fix listed | UBUNTU-CVE-2026-100803 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mozilla | Firefox | see references |
| Mozilla | Thunderbird | see references |
Weakness: CWE-346 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
References
- https://bugzilla.mozilla.org/show_bug.cgi?id=2057988
- https://www.mozilla.org/security/advisories/mfsa2026-100/
- https://www.mozilla.org/security/advisories/mfsa2026-101/
- https://www.mozilla.org/security/advisories/mfsa2026-102/
- https://www.mozilla.org/security/advisories/mfsa2026-103/
- https://www.mozilla.org/security/advisories/mfsa2026-97/
- https://www.mozilla.org/security/advisories/mfsa2026-98/
- https://www.mozilla.org/security/advisories/mfsa2026-99/