CVE-2021-4140

Exp

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.

Published: 2022-12-22 Last update: 2025-04-16 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-4140 is rated High Exploit Risk (60.5/100): CVSS Critical severity, with low exploitation likelihood (EPSS 0.05%). 1 public exploit reference(s) are indexed (Exploit-DB). Public exploits are available—assess exposure, apply mitigations, and prioritize patching.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Public exploit references (Exploit-DB) for CVE-2021-4140

EDB-ID Source Kind Published Link
— nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2021-4140

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-01-03 0.04% 0.05% +0.01%
2 2025-11-21 0.90% 0.04% -0.86%
3 2025-11-18 — 0.90% —

Full EPSS history (14 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-4140

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
10.0 3.1 CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 6.0 [email protected]
9.8 3.1 CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 5.9 134c704f-9b21-4f2e-91b3-4a467353bcc0

Weakness enumeration for CVE-2021-4140

OS Trackers for CVE-2021-4140

vendor priority summary link
alpine — CVE-2021-4140: 4 source package rows (firefox, firefox-esr, librewolf, thunderbird); 106 state rows across 7 repos (3.17-community, 3.18-community, 3.19-community, 3.20-community, 3.21-community, 3.22-community, edge-community); fixed 33, open 73. https://security.alpinelinux.org/vuln/CVE-2021-4140
debian not yet assigned CVE-2021-4140 not yet assigned priority: Debian including 3 source packages (firefox, firefox-esr, thunderbird), 11 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 11. https://security-tracker.debian.org/tracker/CVE-2021-4140
gentoo high CVE-2021-4140: 2 GLSA(s) (202202-03, 202208-14), 4 atom(s) (mail-client/thunderbird, mail-client/thunderbird-bin, www-client/firefox, www-client/firefox-bin); latest impact high. https://bugs.gentoo.org/buglist.cgi?quicksearch=CVE-2021-4140
redhat high — https://access.redhat.com/security/cve/CVE-2021-4140
ubuntu medium CVE-2021-4140 medium priority: Ubuntu including 6 source packages (firefox, mozjs38, mozjs52, mozjs68, mozjs78, thunderbird), 72 status rows across 12 suites (bionic, focal, hirsute, impish, jammy, kinetic, lunar, mantic, noble, trusty, upstream, xenial): DNE 37, released 19, ignored 16. https://ubuntu.com/security/CVE-2021-4140

Affected software / configurations for CVE-2021-4140

Vendor Product Version Raw CPE
mozilla firefox < 96.0 cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:*
mozilla firefox_esr < 91.5 cpe:2.3:a:mozilla:firefox_esr:*:*:*:*:*:*:*:*
mozilla thunderbird < 91.5 cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:*

References for CVE-2021-4140

cvelogic Threat Intelligence