CVE-2021-4140
It was possible to construct specific XSLT markup that would be able to bypass an iframe sandbox. This vulnerability affects Firefox ESR <…
- Published
- Updated
- Reserved
- Assigned by
- mozilla
Description
It was possible to construct specific XSLT markup that would be able to bypass an iframe sandbox. This vulnerability affects Firefox ESR < 91.5, Firefox < 96, and Thunderbird < 91.5.
Public exploit
An exploit or proof of concept is public: anyone can try it against an unpatched system. CISA's SSVC rates its exploitation: a proof of concept is public.
Timeline
ID reserved
By mozilla
- 12 months later
· 12 months later
Published
- 2 years later
· 2 years later
Record updated
Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| CISA-ADPADP (enrichment) | 3.1 | Critical9.8 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H |
| NVDNVD · Modified | 3.1 | Critical10.0 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H |
- EPSS (exploitation in 30 days)
- 1.4% · top 27% of all CVEs (7 Nov 2026)
- CISA SSVC
- Exploitation: poc · Automatable: yes · Technical impact: total
The list uses the assigner's score when there is one, then CISA's, then NVD's: Critical 9.8.
Weakness
- Iframe sandbox bypass with XSLT
- CWE-91 XML Injection (aka Blind XPath Injection) (opens in a new tab)All about CWE-91 and its CVEs
Affected products
- affected unspecified – < 91.5
- affected unspecified – < 96
- affected unspecified – < 91.5
References (4)
- https://www.mozilla.org/security/advisories/mfsa2022-01/ (opens in a new tab)x_transferred
- https://www.mozilla.org/security/advisories/mfsa2022-02/ (opens in a new tab)x_transferred
- https://www.mozilla.org/security/advisories/mfsa2022-03/ (opens in a new tab)x_transferred
- https://bugzilla.mozilla.org/show_bug.cgi?id=1746720 (opens in a new tab)x_transferred
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisories (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://trackcve.app/cve/CVE-2021-4140)Record last updated .