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CVE-2021-23959

21
FAUCET Score

CVE-2021-23959 describes a cross-site scripting (XSS) vulnerability found in internal error pages of Firefox for Android versions prior to 85. This flaw could enable various spoofing attacks, including manipulation of error pages and the address bar. Rated as Medium severity (CVSS 6.1), it requires user interaction and could lead to low impact on confidentiality and integrity. There is no evidence of active exploitation, and public exploit code or Metasploit modules are unavailable, though it has received some community discussion and media coverage.

Impacted Technologies

VendorProductVersion(s)CPE
< 85.0CPE matchmatch criteria
cpe:2.3:a:mozilla:firefox:*:*:*:*:*:android:*:*

CVSS Data

CVSS version used by this source: 3.1

6.1MEDIUM

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
NONE
User Interaction
REQUIRED
Scope
CHANGED
Confidentiality Impact
LOW
Integrity Impact
LOW
Availability Impact
NONE
Exploitability Score
2.8
Impact Score
2.7
CvssVersion
3.1

Exploit Intelligence

EPSS Score
0.57%
Probability of exploitation in next 30 days
EPSS Percentile
43.6%
Percentile rank of EPSS score among Peer Group
As of 2026-07-27
Model: v2026.06.15
This CVE's current EPSS score of 0.0057 is in the 46th percentile among its peer group of 26,236 CVEs.

Social Chatter

The average CVE in this peer group has 0.0 Twitter, 0.0 Reddit, 0.1 Bluesky, 0.0 Mastodon, and 0.1 GitHub mentions.

Media Mentions

The average CVE in this peer group has 0.1 InfoSec Media, 0.0 Vendor Blog, and 0.0 Security Researcher mentions.

Remediation

Vendor Patches (1)

mozillavendor investigatingvia nvd_reference
View patch

References

bugzilla.mozilla.org / show_bug.cgi
Issue TrackingPermissions RequiredVendor Advisory
mozilla.org / security/advisories/mfsa2021-03
Release NotesVendor Advisory