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CVE-2020-26950

62
FAUCET Score

CVE-2020-26950 is a high-severity use-after-free vulnerability affecting Mozilla Firefox, Firefox ESR, and Thunderbird, stemming from unmet assumptions in the MCallGetProperty opcode. This flaw carries a CVSS score of 8.8, indicating it can be exploited remotely with low complexity, potentially leading to full compromise of confidentiality, integrity, and availability. While not listed on CISA's KEV, a Metasploit module exists for this vulnerability, and it has garnered significant community discussion and media coverage, including reports of its exploitation in a Chinese hacking contest.

Impacted Technologies

VendorProductVersion(s)CPE
< 82.0.3CPE matchmatch criteria
cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:*
< 78.4.1CPE matchmatch criteria
cpe:2.3:a:mozilla:firefox_esr:*:*:*:*:*:*:*:*
< 78.4.2CPE matchmatch criteria
cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:*

CVSS Data

CVSS version used by this source: 3.1

8.8HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
NONE
User Interaction
REQUIRED
Scope
UNCHANGED
Confidentiality Impact
HIGH
Integrity Impact
HIGH
Availability Impact
HIGH
Exploitability Score
2.8
Impact Score
5.9
CvssVersion
3.1

Exploit Intelligence

EPSS Score
42.33%
Probability of exploitation in next 30 days
EPSS Percentile
98.6%
Percentile rank of EPSS score among Peer Group
As of 2026-07-25
Model: v2026.06.15
Metasploit: Firefox MCallGetProperty Write Side Effects Use After Free Exploit · Nov 18, 2020
This CVE's current EPSS score of 0.4233 is in the 99th percentile among its peer group of 14,848 CVEs.

Social Chatter

The average CVE in this peer group has 0.0 Twitter, 0.1 Reddit, 0.2 Bluesky, 0.1 Mastodon, and 0.2 GitHub mentions.

Media Mentions

The average CVE in this peer group has 0.3 InfoSec Media, 0.0 Vendor Blog, and 0.1 Security Researcher mentions.

Remediation

Patch Available

Vendor Patches (13)

redhatpatch availablevia redhat_api
Product: Red Hat Enterprise Linux 8.2 Extended Update SupportFixed in: thunderbird-0:78.4.3-1.el8_2
View patch
redhatpatch availablevia redhat_api
Product: Red Hat Enterprise Linux 6Fixed in: firefox-0:78.4.1-1.el6_10
View patch
redhatpatch availablevia redhat_api
Product: Red Hat Enterprise Linux 6Fixed in: thunderbird-0:78.4.3-1.el6_10
View patch
redhatpatch availablevia redhat_api
Product: Red Hat Enterprise Linux 7Fixed in: firefox-0:78.4.1-1.el7_9
View patch
redhatpatch availablevia redhat_api
Product: Red Hat Enterprise Linux 7Fixed in: thunderbird-0:78.4.3-1.el7_9
View patch
redhatpatch availablevia redhat_api
Product: Red Hat Enterprise Linux 8Fixed in: firefox-0:78.4.1-1.el8_3
View patch
redhatpatch availablevia redhat_api
Product: Red Hat Enterprise Linux 8Fixed in: thunderbird-0:78.4.3-1.el8_3
View patch
redhatpatch availablevia redhat_api
Product: Red Hat Enterprise Linux 8.0 Update Services for SAP SolutionsFixed in: firefox-0:78.4.1-1.el8_0
View patch
redhatpatch availablevia redhat_api
Product: Red Hat Enterprise Linux 8.0 Update Services for SAP SolutionsFixed in: thunderbird-0:78.4.3-1.el8_0
View patch
redhatpatch availablevia redhat_api
Product: Red Hat Enterprise Linux 8.1 Extended Update SupportFixed in: firefox-0:78.4.1-1.el8_1
View patch
redhatpatch availablevia redhat_api
Product: Red Hat Enterprise Linux 8.1 Extended Update SupportFixed in: thunderbird-0:78.4.3-1.el8_1
View patch
redhatpatch availablevia redhat_api
Product: Red Hat Enterprise Linux 8.2 Extended Update SupportFixed in: firefox-0:78.4.1-1.el8_2
View patch
mozillavendor investigatingvia nvd_reference
View patch

Vendor Advisories (1)

redhatCVE-2020-26950Critical

Mozilla: Write side effects in MCallGetProperty opcode not accounted for

Nov 9, 2020

References

packetstormsecurity.com / files/166175/Firefox-MCallGetProperty-Write-Side-Effects-Use-After-Free.html
ExploitThird Party AdvisoryVDB Entry
bugzilla.mozilla.org / show_bug.cgi
ExploitIssue TrackingVendor Advisory
mozilla.org / security/advisories/mfsa2020-49
Vendor Advisory