CVE-2020-26950

HIGHNVD 8.88.8—
EchelonGraph scoreMEDIUM confidence

Score 8.8 from GitHub Security Advisory (severity: HIGH) published 2022-05-24. NVD baseline CVSS 8.8; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, nvd
Weaponized
8.8EG
EchelonGraph verdictPatch this weekExploitation is likely or a public exploit exists.
  • Public exploit code is available (Metasploit, epss top5pct, public exploit)
CISA-KEV: Not listedEPSS PROB: 42%CVSS: 8.8Exploit: Metasploit · epss top5pct · public exploitExposed services: Not assessed

A fix is available — apply it.

In certain circumstances, the MCallGetProperty opcode can be emitted with unmet assumptions resulting in an exploitable use-after-free condition. This vulnerability affects Firefox < 82.0.3, Firefox ESR < 78.4.1, and Thunderbird < 78.4.2.

CVSS v3
8.8
EG Score
8.8HIGHmedium confidence
EG Risk
70
EG Risk 70/100CISA SSVC

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
Severity88% × 45%
Exploitation65% × 40%
Automatability30% × 15%
CISA SSVC: Track at low or medium mission impact; Track* or Attend at high (mission-essential systems).
Action: A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.
EPSS PROB
42%
EPSS %ILE
98th
KEV
Not listed

CISA SSVCTrack at low or medium mission impact; Track* or Attend at high (mission-essential systems).

A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.

Exploitation public PoC (EG-KEV: weaponized) · Automatable unknown (not published for this CVE) · Technical impact partial (CVSS below 9.0). Mission impact is CISA's Mission & Well-being decision point, and only you can judge it: high means the affected system is essential to your organisation's mission, or its compromise could cause irreversible harm to people. CISA's decision table

Published

December 9, 2020

Last Modified

November 21, 2024

Advisory Details (2)

Auto-updated Jul 26, 2026
Patch available.
generic

Security Vulnerabilities fixed in Firefox 82.0.3, Firefox ESR 78.4.1, and Thunderbird 78.4.2 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2020-49/
generic

1675905 - (CVE-2020-26950) Write side effects in MCallGetProperty opcode not accounted for

https://bugzilla.mozilla.org/show_bug.cgi?id=1675905

Patch Availability(14)

Vendor / EcosystemFix (by version range) / PatchReleasedSource
ubuntuxul-ext-lightning (1:78.5.0+build3-0ubuntu0.20.10.1) @ groovy2026-05-23ubuntu
ubuntufirefox-mozsymbols (82.0.3+build1-0ubuntu0.16.04.1) @ xenial2026-05-23ubuntu
redhatthunderbird-0:78.4.3-1.el6_102020-11-23redhat
redhatthunderbird-0:78.4.3-1.el8_22020-11-23redhat
redhatthunderbird-0:78.4.3-1.el8_02020-11-23redhat
redhatthunderbird-0:78.4.3-1.el8_12020-11-23redhat
redhatthunderbird-0:78.4.3-1.el7_92020-11-23redhat
redhatthunderbird-0:78.4.3-1.el8_32020-11-18redhat
redhatfirefox-0:78.4.1-1.el8_12020-11-17redhat
redhatfirefox-0:78.4.1-1.el8_02020-11-17redhat
redhatfirefox-0:78.4.1-1.el8_22020-11-17redhat
redhatfirefox-0:78.4.1-1.el6_102020-11-12redhat
redhatfirefox-0:78.4.1-1.el8_32020-11-12redhat
redhatfirefox-0:78.4.1-1.el7_92020-11-12redhat

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.

Affected Packages

(12 across 6 ecosystems)
Debian:10(2)
PackageVulnerable rangeFix by version rangeDependents
firefox-esr60.7.2esr-1 ... 78.4.1esr-1~deb9u1 (74 versions)
  • every version up to 78.4.1esr-1~deb10u1: fixed in 78.4.1esr-1~deb10u1
—
thunderbird1:60.7.2-1 ... 1:78.4.2-1~deb9u1 (92 versions)
  • every version up to 1:78.4.2-1~deb10u1: fixed in 1:78.4.2-1~deb10u1
—
Debian:11(2)
PackageVulnerable rangeFix by version rangeDependents
firefox-esr—
  • every version up to 78.4.1esr-1: fixed in 78.4.1esr-1
—
thunderbird—
  • every version up to 1:78.4.2-1: fixed in 1:78.4.2-1
—
Debian:12(2)
PackageVulnerable rangeFix by version rangeDependents
firefox-esr—
  • every version up to 78.4.1esr-1: fixed in 78.4.1esr-1
—
thunderbird—
  • every version up to 1:78.4.2-1: fixed in 1:78.4.2-1
—
Debian:13(2)
PackageVulnerable rangeFix by version rangeDependents
firefox-esr—
  • every version up to 78.4.1esr-1: fixed in 78.4.1esr-1
—
thunderbird—
  • every version up to 1:78.4.2-1: fixed in 1:78.4.2-1
—
Debian:14(2)
PackageVulnerable rangeFix by version rangeDependents
firefox-esr—
  • every version up to 78.4.1esr-1: fixed in 78.4.1esr-1
—
thunderbird—
  • every version up to 1:78.4.2-1: fixed in 1:78.4.2-1
—
Debian:9(2)
PackageVulnerable rangeFix by version rangeDependents
firefox-esr45.9.0esr-1 ... 78.4.0esr-2 (181 versions)
  • every version up to 78.4.1esr-1~deb9u1: fixed in 78.4.1esr-1~deb9u1
—
thunderbird1.5-4 ... 1:78.4.1-1 (173 versions)
  • every version up to 1:78.4.2-1~deb9u1: fixed in 1:78.4.2-1~deb9u1
—

Weakness Classification(1)

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

All Vendor Advisories

(14)

Every vendor that published an advisory referencing this CVE — pulled from our cve_vendor_advisories aggregation. Click any row for the vendor's original advisory page.

Data Freshness Timeline

(refreshed 5× in last 7d / 21× 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 134 total refreshes for this CVE.

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

Publicly available exploits

(1 reference)

Working exploit code is in the public domain (1 Metasploit module). Defenders should treat patch urgency accordingly — public PoCs typically lead to mass-exploitation within 24-72 hours.

  • Metasploitexploit/multi/browser/firefox_jit_use_after_free✓ verified
    First seen Nov 18, 2020

    Firefox MCallGetProperty Write Side Effects Use After Free Exploit

    Open source ↗

Related CVEs(same product + same vendor + same CWE)

Frequently asked(5)

What is CVE-2020-26950?
CVE-2020-26950 is a high vulnerability published on December 9, 2020. In certain circumstances, the MCallGetProperty opcode can be emitted with unmet assumptions resulting in an exploitable use-after-free condition. This vulnerability affects Firefox < 82.0.3, Firefox ESR < 78.4.1, and Thunderbird < 78.4.2.
When was CVE-2020-26950 disclosed?
CVE-2020-26950 was first published on December 9, 2020, 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-2020-26950 actively exploited?
CVE-2020-26950 is not currently on CISA's Known Exploited Vulnerabilities catalog. EchelonGraph's EG-KEV model classifies it as weaponized: a functional public exploit exists. FIRST EPSS estimates a 42% probability of exploitation in the next 30 days (98th percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2020-26950?
CVE-2020-26950 has a CVSS v3 base score of 8.8 (NVD).
How do I remediate CVE-2020-26950?
A fix for CVE-2020-26950 is available: update to the fixed version the vendor names in its advisory.

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