8.1

CVE-2026-100803: mozilla firefox Origin Validation Error

CVSS: 8.1v3.1HighAnalyzed
AliasesUBUNTU-CVE-2026-100803DEBIAN-CVE-2026-100803
Published 29-09-2026Last modified 01-10-2026
Open JSON

Vulnerability summary

CVE-2026-100803 is a high (CVSS 8.1) Origin Validation Error vulnerability in mozilla firefox. EPSS estimates a 0.19% chance of exploitation in the next 30 days, higher than 7.8% of scored vulnerabilities.

Overall risk
32/100
0100
Medium · default modelHow is this calculated?
CVSS score
8.1/10
010
High · v3.1 (CVE.org)
EPSS score
0.19%
0%100%
Low · Change unavailable
KEVNot listedIn no KEV catalog
RansomwareNone recordedNo known use recorded
Public exploits0No public exploits recorded

Advisory lineage

Upstream 0 · Downstream 10 · Related 8
MGASA-2026-0491MGASA-2026-0492OPENSUSE-SU-2026:11917-1OPENSUSE-SU-2026:12097-1OPENSUSE-SU-2026:12098-1OPENSUSE-SU-2026:21980-1

Timeline

  1. 29-09-2026
    PublishedVulnerability first disclosed.
  2. 01-10-2026
    Last modifiedVulnerability information updated.

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.

CVSS metrics

4 scores · 4 sources · 1 version

EPSS trend

Probability

0.19%

Chance of exploitation in the next 30 days

Percentile

8%

Versus all other CVEs

30-day change

Change unavailable

Change unavailable

30-09-202619-10-2026
EPSS score
0.19%

Known exploited

Compare KEV catalogs

Not in any of the 3 tracked catalogs.

Public exploits

0 found

No public exploits indexed.

Weaknesses

CWE · root cause

Affected systems

Ubuntu8 products
  • mozjs102All versions (No fix recorded)
  • mozjs115All versions (No fix recorded)
  • mozjs38All versions (No fix recorded)
  • mozjs52All versions (No fix recorded)
  • mozjs68All versions (No fix recorded)
Debian2 products
  • firefox-esr< 153.4.0esr-1~deb12u1 | < 153.4.0esr-1~deb13u1 | < 153.4.0esr-1
  • thunderbird< 1:140.17.0esr-1~deb12u1 | < 1:140.17.0esr-1~deb13u1 | < 1:153.4.0esr-1
Mozilla2 products
  • firefox< 115.42.0 | ≥ 116.0, < 140.17.0 | ≥ 141.0, < 153.4.0 | ≥ 154.0.0, < 157.0.0
  • thunderbird< 140.17.0 | ≥ 141.0, < 153.4.0 | ≥ 154.0, < 157.0

3 vendors · 12 products

References

13 links
Show all 13 references