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No exploitation reportedFix available

CVE-2021-38503

No exploitation reported by the sources we read.

  • Published2021-12-08
  • WeaknessCWE-863

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

The iframe sandbox rules were not correctly applied to XSLT stylesheets, allowing an iframe to bypass restrictions such as executing scripts or navigating the top-level frame. This vulnerability affects Firefox < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.

Timeline

dates in UTC
  1. 2021-12-0822:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-38503?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-38503?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Debian debian linux
BranchAffectedFixed in
9.0= 9.0no fix published
10.0= 10.0no fix published
11.0= 11.0no fix published
Mozilla firefox
BranchAffectedFixed in
94.0< 94.094.0
Mozilla firefox esr
BranchAffectedFixed in
91.3< 91.391.3

1 more product in the sources.

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • firefoxAlmaLinux:8all versions · fixed in 91.3.0-1.el8_4.alma
  • thunderbirdAlmaLinux:8all versions · fixed in 91.3.0-2.el8_4.alma.plus
  • thunderbirdAlmaLinux:8all versions · fixed in 91.3.0-2.el8_4.alma

Full record on NVD

Fix

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub

Sources

  • NVDCVSS 3.110read 2026-10-23
  • NVDCVSS 2.07.5read 2026-10-23
  • NVDweakness (CWE)CWE-863read 2026-10-23
  • FIRST EPSSEPSS score4.1 %2026-10-23
  • OSVaffected packages—

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
Published
2021-12-08
Last Modified
2026-06-17
NVD status
Modified

Frequently asked questions

What is CVE-2021-38503?

The iframe sandbox rules were not correctly applied to XSLT stylesheets, allowing an iframe to bypass restrictions such as executing scripts or navigating the top-level frame. This vulnerability affects Firefox < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.

Is CVE-2021-38503 actively exploited?

Active exploitation of CVE-2021-38503 has not been confirmed. Its EPSS score was 4.1% on 2026-10-23, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-38503?

CVE-2021-38503 has a CVSS v3 base score of 10 (CRITICAL severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H.

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