CVE-2021-38503

CRITICALNVD 10.010.0—
EchelonGraph scoreMEDIUM confidence

Score 10.0 from GitHub Security Advisory (severity: CRITICAL) published 2021-12-09. NVD baseline CVSS 10.0; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, nvd
10.0EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 4%CVSS: 10.0Exploit: None knownExposed: 0

A fix is available — apply it.

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.

CVSS v3
10.0
EG Score
10.0(medium)
EG Risk
51(Track)
EG Risk 51/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity100% × 45%
Exploitation4% × 40%
Automatability30% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
4%
EPSS %ILE
89%
KEV
Not listed

Published

December 8, 2021

Last Modified

November 21, 2024

Advisory Details (10)

Auto-updated Jul 25, 2026
🔬 Proof of concept available. Patch available.
generic

Security Vulnerabilities fixed in Thunderbird 91.3 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2021-50/
generic

Security Vulnerabilities fixed in Firefox ESR 91.3 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2021-49/
generic

Security Vulnerabilities fixed in Firefox 94 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2021-48/
generic

[SECURITY] [DSA 5034-1] thunderbird security update

https://www.debian.org/security/2022/dsa-5034
generic

[SECURITY] [DSA 5026-1] firefox-esr security update

https://www.debian.org/security/2021/dsa-5026
generic

Mozilla Thunderbird: Multiple Vulnerabilities (GLSA 202208-14) — Gentoo security

https://security.gentoo.org/glsa/202208-14
generic

Mozilla Firefox: Multiple vulnerabilities (GLSA 202202-03) — Gentoo security

https://security.gentoo.org/glsa/202202-03
generic Patch Available

[SECURITY] [DLA 2874-1] thunderbird security update

https://lists.debian.org/debian-lts-announce/2022/01/msg00001.html
generic Patch Available

[SECURITY] [DLA 2863-1] firefox-esr security update

https://lists.debian.org/debian-lts-announce/2021/12/msg00030.html
generic🟡 PoC Available

1729517 - (CVE-2021-38503) Script via XSLT stylesheet bypasses <iframe sandbox> attribute restrictions

https://bugzilla.mozilla.org/show_bug.cgi?id=1729517

Vendor Advisories for CVE-2021-38503(2)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Patch Availability(11)

Vendor / EcosystemFixed in / PatchReleasedSource
ubuntufirefox-locale-ast (94.0+build3-0ubuntu0.21.10.1) @ impish2026-05-26ubuntu
ubuntuthunderbird-locale-id (1:91.5.0+build1-0ubuntu0.20.04.1) @ focal2026-05-26ubuntu
ubuntuthunderbird-locale-id (1:91.3.1+build1-0ubuntu0.21.10.1) @ impish2026-05-26ubuntu
redhatfirefox-0:91.3.0-1.el8_12021-11-10redhat
redhatfirefox-0:91.3.0-1.el8_22021-11-10redhat
redhatthunderbird-0:91.3.0-2.el8_12021-11-04redhat
redhatthunderbird-0:91.3.0-2.el8_22021-11-04redhat
redhatthunderbird-0:91.3.0-2.el7_92021-11-04redhat
redhatthunderbird-0:91.3.0-2.el8_42021-11-04redhat
redhatfirefox-0:91.3.0-1.el8_42021-11-03redhat
redhatfirefox-0:91.3.0-1.el7_92021-11-03redhat

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Additional Vendor Advisories

(9)

Vendors that published advisories for this CVE beyond the curated set above. Broader coverage but minimal per-row detail — click through for the original advisory.

Data Freshness Timeline

(refreshed 10× in last 7d / 42× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 105 total refreshes for this CVE.

  1. 2026-07-25 14:15 UTCEPSS rescore
  2. 2026-07-24 14:15 UTCEPSS rescore
  3. 2026-07-23 14:16 UTCEPSS rescore
  4. 2026-07-23 14:16 UTCEPSS rescore
  5. 2026-07-23 02:03 UTCEG score recompute
  6. 2026-07-22 22:50 UTCEG score recompute
  7. 2026-07-22 14:06 UTCEPSS rescore
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  17. 2026-07-18 06:29 UTCOSV refresh
  18. 2026-07-16 17:00 UTCEPSS rescore
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  1. 2026-07-11 08:25 UTCEPSS rescore
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  3. 2026-07-09 19:07 UTCEPSS rescore
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  13. 2026-06-30 23:20 UTCEPSS rescore
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  15. 2026-06-30 03:30 UTCOSV refresh
  16. 2026-06-28 04:54 UTCEPSS rescore
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  18. 2026-06-27 03:06 UTCEPSS rescore
  19. 2026-06-25 13:48 UTCEPSS rescore
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  23. 2026-06-23 21:31 UTCEPSS rescore
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  25. 2026-06-22 14:24 UTCEPSS rescore
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  27. 2026-06-21 14:55 UTCEPSS rescore
  28. 2026-06-21 01:58 UTCEPSS rescore
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  30. 2026-06-19 19:24 UTCEPSS rescore
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  39. 2026-06-13 22:58 UTCEPSS rescore
  40. 2026-06-13 22:58 UTCEPSS rescore
  41. 2026-06-12 23:10 UTCEPSS rescore
  42. 2026-06-12 23:10 UTCEPSS rescore
  43. 2026-06-12 15:10 UTCOSV refresh
  44. 2026-06-11 13:58 UTCEPSS rescore
  45. 2026-06-10 22:16 UTCEPSS rescore
  46. 2026-06-10 13:20 UTCEPSS rescore
  47. 2026-06-08 14:15 UTCEPSS rescore
  48. 2026-06-08 14:15 UTCEPSS rescore
  49. 2026-06-07 15:23 UTCEPSS rescore
  50. 2026-06-07 15:23 UTCEPSS rescore
  51. 2026-06-06 13:46 UTCEPSS rescore
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  63. 2026-05-31 22:29 UTCEPSS rescore
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  67. 2026-05-29 13:42 UTCEPSS rescore
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  69. 2026-05-28 13:43 UTCEPSS rescore
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  71. 2026-05-27 13:39 UTCEPSS rescore
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  74. 2026-05-26 13:42 UTCEPSS rescore
  75. 2026-05-26 07:17 UTCEPSS rescore

Related CVEs(same vendor + same CWE)

Frequently asked(5)

What is CVE-2021-38503?
CVE-2021-38503 is a critical vulnerability published on December 8, 2021. 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.
When was CVE-2021-38503 disclosed?
CVE-2021-38503 was first published in the National Vulnerability Database on December 8, 2021, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-38503 actively exploited?
CVE-2021-38503 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 4% probability of exploitation in the next 30 days, which ranks it in the top 11.0% of all scored CVEs.
What is the CVSS score of CVE-2021-38503?
CVE-2021-38503 has a CVSS v3 base score of 10.0 (NVD).
How do I remediate CVE-2021-38503?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-38503, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

Explore the affected products and dependency analysis for CVE-2021-38503

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