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Public proof of conceptFix available

CVE-2020-26950

A public proof of concept exists; no exploitation attested.

  • Published2020-12-09
  • Fixed in82.0.3
  • WeaknessCWE-416

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

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.

Timeline

dates in UTC
  1. 2020-12-0901:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-11-03 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-2020-26950?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2020-26950?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Mozilla firefox
BranchAffectedFixed in
82.0< 82.0.382.0.3
Mozilla firefox esr
BranchAffectedFixed in
78.4< 78.4.178.4.1
Mozilla thunderbird
BranchAffectedFixed in
78.4< 78.4.278.4.2

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

  • firefoxUbuntu:20.04:LTSall versions · fixed in 82.0.3+build1-0ubuntu0.20.04.1
  • firefoxUbuntu:18.04:LTSall versions · fixed in 82.0.3+build1-0ubuntu0.18.04.1
  • firefoxUbuntu:16.04:LTSall versions · fixed in 82.0.3+build1-0ubuntu0.16.04.1
  • firefoxRed Hat:rhel_eus:8.2::appstreamall versions · fixed in 0:78.4.1-1.el8_2and 2 other packages
  • firefoxRed Hat:rhel_eus:8.1::appstreamall versions · fixed in 0:78.4.1-1.el8_1and 2 other packages
  • firefoxRed Hat:rhel_e4s:8.0::appstreamall versions · fixed in 0:78.4.1-1.el8_0and 2 other packages
  • firefoxRed Hat:enterprise_linux:8::appstreamall versions · fixed in 0:78.4.1-1.el8_3and 2 other packages
  • firefoxRed Hat:enterprise_linux:7::workstationall versions · fixed in 0:78.4.1-1.el7_9and 1 other package

106 more package ranges on OSV.dev

Vendor advisories

  • CVE-2020-26950 Red Hat Product Security211 products fixed · 2 known affected

    Mozilla: Write side effects in MCallGetProperty opcode not accounted for

    firefox as a component of Red Hat Enterprise Linux 5 · thunderbird as a component of Red Hat Enterprise Linux 5

  • RHSA-2020:5163 Red Hat Product Security11 products fixed

    Red Hat Security Advisory: thunderbird security update

  • RHSA-2020:5164 Red Hat Product Security16 products fixed

    Red Hat Security Advisory: thunderbird security update

  • RHSA-2020:5162 Red Hat Product Security10 products fixed

    Red Hat Security Advisory: thunderbird security update

  • RHSA-2020:5166 Red Hat Product Security7 products fixed

    Red Hat Security Advisory: thunderbird security update

  • RHSA-2020:5167 Red Hat Product Security7 products fixed

    Red Hat Security Advisory: thunderbird security update

7 more advisories

Full record on NVD

Fix

  1. Update Mozilla firefox to 82.0.3
  2. Vendor fix Red Hat Product Security RHSA-2020:5163 · 11 productsFor details on how to apply this update, which includes the changes described in this advisory, refer to: https://access.redhat.com/articles/11258 All running instances of Thunderbird must be restarted for the update to take effect.
  3. Vendor fix Red Hat Product Security RHSA-2020:5138 · 7 productsFor details on how to apply this update, which includes the changes described in this advisory, refer to: https://access.redhat.com/articles/11258 After installing the update, Firefox must be restarted for the changes to take effect.

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
Metasploit modules
1 moduleexploit/multi/browser/firefox_jit_use_after_freerank: manualModule metadata: Metasploit Framework, © Rapid7, Inc., BSD-3-Clause.
Metasploit Framework

Sources

  • NVDCVSS 3.18.8read 2026-10-23
  • NVDCVSS 2.09.3read 2026-10-23
  • NVDweakness (CWE)CWE-416read 2026-10-23
  • FIRST EPSSEPSS score42.3 %read 2026-11-03
  • Metasploit FrameworkMetasploit modules1known since 2026-11-01
  • OSVaffected packages—

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Published
2020-12-09
Last Modified
2026-06-17
NVD status
Modified

Frequently asked questions

What is CVE-2020-26950?

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.

Is CVE-2020-26950 actively exploited?

Active exploitation of CVE-2020-26950 has not been confirmed. Its EPSS score was 42.3% on 2026-11-03, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2020-26950?

CVE-2020-26950 has a CVSS v3 base score of 8.8 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

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Sources for this record