CVE-2020-26974Public exploitNetwork attack vector

Memory corruption via flex-basis type confusion

Published Jan 7, 2021 · Updated Aug 4, 2024

Type confusion in Mozilla Firefox before 84, Firefox ESR before 78.6, and Thunderbird before 78.6 allows remote attackers to corrupt memory. The Layout: Flexbox code passes a table wrapper's StyleGenericFlexBasis value to AsSize without first rejecting the content variant, so the bad cast dereferences invalid heap state. Rendering attacker-controlled content that applies flex-basis to a table wrapper is required, and the documented result is a potentially exploitable crash.

CVSS severity8.8
High
EPSS probability1.52%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Type confusion in Mozilla Firefox before 84, Firefox ESR before 78.6, and Thunderbird before 78.6 allows remote attackers to corrupt memory. The Layout: Flexbox code passes a table wrapper's StyleGenericFlexBasis value to AsSize without first rejecting the content variant, so the bad cast dereferences invalid heap state. Rendering attacker-controlled content that applies flex-basis to a table wrapper is required, and the documented result is a potentially exploitable crash.

The record

CVE
CVE-2020-26974
Published
Jan 7, 2021
Updated
Aug 4, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-787, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 7, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <84

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <84
Mozilla · Thunderbirdversion=unspecified <78.6
Mozilla · Firefox ESRversion=unspecified <78.6

Published CVSS scores

6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P
8.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-787Out-of-bounds Write
T1499.004Application or System Exploitation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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