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
Trending — 3 sources updated this week
10.0
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 4%CVSS: 10.0Exploit: NoneExposed: 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
—
EPSS
89.0%
KEV
Not listed

Published

December 8, 2021

Last Modified

November 21, 2024

Advisory Details (10)

Auto-updated Jul 18, 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 14× in last 7d / 44× 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.

  1. 2026-07-22 22:50 UTCEG score recompute
  2. 2026-07-22 14:06 UTCEPSS rescore
  3. 2026-07-22 14:06 UTCEPSS rescore
  4. 2026-07-21 15:22 UTCEPSS rescore
  5. 2026-07-21 15:22 UTCEPSS rescore
  6. 2026-07-20 17:05 UTCEPSS rescore
  7. 2026-07-19 14:29 UTCEPSS rescore
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  9. 2026-07-19 02:27 UTCEPSS rescore
  10. 2026-07-18 10:02 UTCEPSS rescore
  11. 2026-07-18 10:02 UTCEPSS rescore
  12. 2026-07-18 06:29 UTCOSV refresh
  13. 2026-07-16 17:00 UTCEPSS rescore
  14. 2026-07-16 17:00 UTCEPSS rescore
  15. 2026-07-15 16:55 UTCEPSS rescore
  16. 2026-07-15 01:58 UTCEPSS rescore
  17. 2026-07-13 22:27 UTCEPSS rescore
  18. 2026-07-13 06:10 UTCEPSS rescore
  19. 2026-07-12 05:44 UTCEPSS rescore
  20. 2026-07-12 05:44 UTCEPSS rescore
  21. 2026-07-11 08:25 UTCEPSS rescore
  22. 2026-07-11 08:25 UTCEPSS rescore
  23. 2026-07-09 19:07 UTCEPSS rescore
  24. 2026-07-08 15:12 UTCEPSS rescore
  25. 2026-07-07 13:43 UTCEPSS rescore
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  1. 2026-07-06 16:25 UTCEPSS rescore
  2. 2026-07-06 02:21 UTCEPSS rescore
  3. 2026-07-05 02:28 UTCEPSS rescore
  4. 2026-07-05 02:28 UTCEPSS rescore
  5. 2026-07-04 06:29 UTCEPSS rescore
  6. 2026-07-04 06:29 UTCEPSS rescore
  7. 2026-07-01 15:04 UTCEPSS rescore
  8. 2026-06-30 23:20 UTCEPSS rescore
  9. 2026-06-30 23:20 UTCEPSS rescore
  10. 2026-06-30 03:30 UTCOSV refresh
  11. 2026-06-28 04:54 UTCEPSS rescore
  12. 2026-06-28 04:54 UTCEPSS rescore
  13. 2026-06-27 03:06 UTCEPSS rescore
  14. 2026-06-25 13:48 UTCEPSS rescore
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  16. 2026-06-24 14:03 UTCEPSS rescore
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  18. 2026-06-23 21:31 UTCEPSS rescore
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  20. 2026-06-22 14:24 UTCEPSS rescore
  21. 2026-06-22 14:24 UTCEPSS rescore
  22. 2026-06-21 14:55 UTCEPSS rescore
  23. 2026-06-21 01:58 UTCEPSS rescore
  24. 2026-06-21 01:58 UTCEPSS rescore
  25. 2026-06-19 19:24 UTCEPSS rescore
  26. 2026-06-19 19:24 UTCEPSS rescore
  27. 2026-06-18 17:51 UTCEPSS rescore
  28. 2026-06-18 17:51 UTCEPSS rescore
  29. 2026-06-17 17:51 UTCEPSS rescore
  30. 2026-06-16 17:51 UTCEPSS rescore
  31. 2026-06-16 17:51 UTCEPSS rescore
  32. 2026-06-15 17:46 UTCEPSS rescore
  33. 2026-06-14 23:15 UTCEPSS rescore
  34. 2026-06-13 22:58 UTCEPSS rescore
  35. 2026-06-13 22:58 UTCEPSS rescore
  36. 2026-06-12 23:10 UTCEPSS rescore
  37. 2026-06-12 23:10 UTCEPSS rescore
  38. 2026-06-12 15:10 UTCOSV refresh
  39. 2026-06-11 13:58 UTCEPSS rescore
  40. 2026-06-10 22:16 UTCEPSS rescore
  41. 2026-06-10 13:20 UTCEPSS rescore
  42. 2026-06-08 14:15 UTCEPSS rescore
  43. 2026-06-08 14:15 UTCEPSS rescore
  44. 2026-06-07 15:23 UTCEPSS rescore
  45. 2026-06-07 15:23 UTCEPSS rescore
  46. 2026-06-06 13:46 UTCEPSS rescore
  47. 2026-06-06 13:46 UTCEPSS rescore
  48. 2026-06-06 13:46 UTCEPSS rescore
  49. 2026-06-05 22:45 UTCEPSS rescore
  50. 2026-06-05 06:09 UTCEPSS rescore
  51. 2026-06-05 06:09 UTCEPSS rescore
  52. 2026-06-04 13:10 UTCEPSS rescore
  53. 2026-06-04 13:10 UTCEPSS rescore
  54. 2026-06-02 20:11 UTCEPSS rescore
  55. 2026-06-02 20:11 UTCEPSS rescore
  56. 2026-06-01 13:50 UTCEPSS rescore
  57. 2026-06-01 13:50 UTCEPSS rescore
  58. 2026-05-31 22:29 UTCEPSS rescore
  59. 2026-05-31 22:29 UTCEPSS rescore
  60. 2026-05-31 00:15 UTCEPSS rescore
  61. 2026-05-31 00:15 UTCEPSS rescore
  62. 2026-05-29 13:42 UTCEPSS rescore
  63. 2026-05-29 13:42 UTCEPSS rescore
  64. 2026-05-28 13:43 UTCEPSS rescore
  65. 2026-05-28 13:43 UTCEPSS rescore
  66. 2026-05-27 13:39 UTCEPSS rescore
  67. 2026-05-27 13:39 UTCEPSS rescore
  68. 2026-05-27 13:39 UTCEPSS rescore
  69. 2026-05-26 13:42 UTCEPSS rescore
  70. 2026-05-26 07:17 UTCEPSS rescore
  71. 2026-05-26 07:17 UTCEPSS rescore
  72. 2026-05-26 07:17 UTCEPSS rescore
  73. 2026-05-26 06:40 UTCEG score recompute
  74. 2026-05-26 06:40 UTCVendor advisory
  75. 2026-05-26 06:40 UTCGHSA enrichment

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 89.0% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
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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