Origin validation error in Mozilla Firefox
Priority LOW 30CWE-346Published · Updated
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- Required
- Attack complexity
- Low
- Impact type
- Security bypass
- Confidentiality · integrity · availability
- None · High · None
- Fix status
- Partial Red Hat: 9 fixed, 1 without a fix
Attack profile from CVSS 3.1 · NVD. Fix status from CNA and Red Hat. Impact type from CWE-346.
Summary
The Opportunistic Encryption feature of HTTP2 (RFC 8164) allows a connection to be transparently upgraded to TLS while retaining the visual properties of an HTTP connection, including being same-origin with unencrypted connections on port 80. However, if a second encrypted port on the same IP address (e.g. port 8443) did not opt-in to opportunistic encryption; a network attacker could forward a connection from the browser to port 443 to port 8443, causing the browser to treat the content of port 8443 as same-origin with HTTP. This was resolved by disabling the Opportunistic Encryption feature, which had low usage. This vulnerability affects Firefox < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.
Why it matters now
EPSS 0.00832 57th percentileScored 2026-11-07
Fix Partial Firefox (Mozilla): 94Thunderbird (Mozilla): 91.3+1 more productRed Hat: 8 advisoriesRed Hat: 9 fixed, 1 without a fix
SSVC decision points
Not available: CISA has not published SSVC decision points for this record.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| Mozilla | Firefox | before 94 | 94 | Patch available | CNA |
| Mozilla | Thunderbird | before 91.3 | 91.3 | Patch available | CNA |
| Mozilla | Firefox ESR | before 91.3 | 91.3 | Patch available | CNA |
- Firefox VendorMozilla Affected before 94 Fixed in 94 Patch available CNA
- Thunderbird VendorMozilla Affected before 91.3 Fixed in 91.3 Patch available CNA
- Firefox ESR VendorMozilla Affected before 91.3 Fixed in 91.3 Patch available CNA
CPE configurations NVD · 6
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:* | before 94.0 | Vulnerable | Mozilla |
| cpe:2.3:a:mozilla:firefox_esr:*:*:*:*:*:*:*:* | before 91.3.0 | Vulnerable | Mozilla |
| cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:* | before 91.3.0 | Vulnerable | Mozilla |
| cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:* | 9.0 | Vulnerable | Debian |
| cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:* | 10.0 | Vulnerable | Debian |
| cpe:2.3:o:debian:debian_linux:11.0:*:*:*:*:*:*:* | 11.0 | Vulnerable | Debian |
- cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:* Versions before 94.0 Vulnerable ·Mozilla
- cpe:2.3:a:mozilla:firefox_esr:*:*:*:*:*:*:*:* Versions before 91.3.0 Vulnerable ·Mozilla
- cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:* Versions before 91.3.0 Vulnerable ·Mozilla
- cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:* Versions 9.0 Vulnerable ·Debian
- cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:* Versions 10.0 Vulnerable ·Debian
- cpe:2.3:o:debian:debian_linux:11.0:*:*:*:*:*:*:* Versions 11.0 Vulnerable ·Debian
Remediation
- Fix · CNA Firefox (Mozilla) Fixed in 94
- Fix · CNA Thunderbird (Mozilla) Fixed in 91.3
- Fix · CNA Firefox ESR (Mozilla) Fixed in 91.3
Red Hat 8 advisories
- RHSA-2021:4605 Released 2021-11-10 · Red Hat rating Importantfirefox security update
- Red Hat Enterprise Linux AppStream EUS (v. 8.2) firefox-91.3.0-1.el8_2
- RHSA-2021:4607 Released 2021-11-10 · Red Hat rating Importantfirefox security update
- Red Hat Enterprise Linux AppStream EUS (v. 8.1) firefox-91.3.0-1.el8_1
- RHSA-2021:4134 Released 2021-11-04 · Red Hat rating Importantthunderbird security update
- Red Hat Enterprise Linux Client (v. 7) thunderbird-91.3.0-2.el7_9
- Red Hat Enterprise Linux Server Optional (v. 7) thunderbird-91.3.0-2.el7_9
- Red Hat Enterprise Linux Workstation (v. 7) thunderbird-91.3.0-2.el7_9
Show 5 more Red Hat advisories
- RHSA-2021:4130 Released 2021-11-04 · Red Hat rating Importantthunderbird security update
- Red Hat Enterprise Linux AppStream (v. 8) thunderbird-91.3.0-2.el8_4
- RHSA-2021:4132 Released 2021-11-04 · Red Hat rating Importantthunderbird security update
- Red Hat Enterprise Linux AppStream EUS (v. 8.2) thunderbird-91.3.0-2.el8_2
- RHSA-2021:4133 Released 2021-11-04 · Red Hat rating Importantthunderbird security update
- Red Hat Enterprise Linux AppStream EUS (v. 8.1) thunderbird-91.3.0-2.el8_1
- RHSA-2021:4123 Released 2021-11-03 · Red Hat rating Importantfirefox security update
- Red Hat Enterprise Linux AppStream (v. 8) firefox-91.3.0-1.el8_4
- RHSA-2021:4116 Released 2021-11-03 · Red Hat rating Importantfirefox security update
- Red Hat Enterprise Linux Client (v. 7) firefox-91.3.0-1.el7_9
- Red Hat Enterprise Linux Client Optional (v. 7) firefox-91.3.0-1.el7_9
- Red Hat Enterprise Linux Server (v. 7) firefox-91.3.0-1.el7_9
- Red Hat Enterprise Linux Server Optional (v. 7) firefox-91.3.0-1.el7_9
- Red Hat Enterprise Linux Workstation (v. 7) firefox-91.3.0-1.el7_9
- Red Hat Enterprise Linux Workstation Optional (v. 7) firefox-91.3.0-1.el7_9
Fixed in the same release
Records fixed by the same vendor advisory or release, as the CNA lists them.
- Advisory ·
Mozilla Firefox, Firefox ESR, Thunderbird — DSA-5026
17 records Highest priority Priority MEDIUM 6 MEDIUM, 11 LOW
Other records, highest priority first
- TI-2021-16103 Incorrect authorization in Mozilla FirefoxPriority MEDIUM 50
- TI-2021-19429 Use after free in Mozilla ThunderbirdPriority MEDIUM 42
- TI-2021-19427 Incorrect type conversion in Mozilla ThunderbirdPriority MEDIUM 42
Vendor assessments
Red Hat
Red Hat rating: Important. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| Red Hat Enterprise Linux 6 | Affected | No fix planned: Out of support scope |
| Red Hat Enterprise Linux AppStream (v. 8) | Fixed | RHSA-2021:4123, RHSA-2021:4130 |
| Red Hat Enterprise Linux AppStream EUS (v. 8.1) | Fixed | RHSA-2021:4133, RHSA-2021:4607 |
| Red Hat Enterprise Linux AppStream EUS (v. 8.2) | Fixed | RHSA-2021:4132, RHSA-2021:4605 |
| Red Hat Enterprise Linux Client (v. 7) | Fixed | RHSA-2021:4116, RHSA-2021:4134 |
| Red Hat Enterprise Linux Client Optional (v. 7) | Fixed | RHSA-2021:4116 |
| Red Hat Enterprise Linux Server (v. 7) | Fixed | RHSA-2021:4116 |
| Red Hat Enterprise Linux Server Optional (v. 7) | Fixed | RHSA-2021:4116, RHSA-2021:4134 |
| Red Hat Enterprise Linux Workstation (v. 7) | Fixed | RHSA-2021:4116, RHSA-2021:4134 |
| Red Hat Enterprise Linux Workstation Optional (v. 7) | Fixed | RHSA-2021:4116 |
Timeline
- 2026-10-02 Threatist record created from CVE-2021-38507
- 2026-10-03 Alias added: GHSA-mvf9-xj62-qhwc
References
Advisory 10
- https://www.mozilla.org/security/advisories/mfsa2021-49/
- https://www.mozilla.org/security/advisories/mfsa2021-50/
- https://www.mozilla.org/security/advisories/mfsa2021-48/
- https://bugzilla.mozilla.org/show_bug.cgi?id=1730935
- DSA-5026
- [debian-lts-announce] 20211229 [SECURITY] [DLA 2863-1] firefox-esr security update
- DSA-5034
- [debian-lts-announce] 20220104 [SECURITY] [DLA 2874-1] thunderbird security update
- GLSA-202202-03
- GLSA-202208-14
More Mozilla Firefox vulnerabilities
- TI-2024-08981 Use after free in Mozilla Firefox KEVCVSS 3.1CRITICAL 9.8
- TI-2022-09832 Use after free in Mozilla Firefox KEVCVSS 3.1CRITICAL 9.6
- TI-2019-08769 Improper input validation in Mozilla Firefox ESR KEVCVSS 3.1CRITICAL 10.0
- TI-2010-03577 Mozilla Firefox 3.5.x through 3.5.14 and 3.6.x through 3.6.11, Thunderbird 3.1.6 before… KEVCVSS 3.1CRITICAL 9.8
- TI-2023-04799 Out-of-bounds write in Google Chrome KEVCVSS 3.1HIGH 8.8
- TI-2023-04477 Out-of-bounds write in Google Chrome KEVCVSS 3.1HIGH 8.8
Sources
- CVE-2021-38507 · CVE Program
- CVE-2021-38507 · NVD
- FIRST EPSS
- GHSA-mvf9-xj62-qhwc · GitHub Advisory Database · CC BY 4.0
- CVE-2021-38507 · Red Hat security data · CC BY 4.0
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