CVE-2020-6819Known exploitationNetwork attack vector

Memory corruption via nsDocShell destructor race

Published Apr 24, 2020 · Updated Oct 21, 2025

Use-after-free in Mozilla Firefox before 74.0.1, Firefox ESR before 68.6.1, and Thunderbird before 68.7.0 allows remote attackers to corrupt memory. A race during the nsDocShell destructor lets SynchronizeLayoutHistoryState read a session-history object after it has been freed. In Thunderbird, scripting is disabled while reading email, so exploitation is confined to browser or browser-like contexts; targeted attacks have abused the flaw.

CVSS severity8.1
High
EPSS probability3.04%
Next 30 days · Sep 16, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use-after-free in Mozilla Firefox before 74.0.1, Firefox ESR before 68.6.1, and Thunderbird before 68.7.0 allows remote attackers to corrupt memory. A race during the nsDocShell destructor lets SynchronizeLayoutHistoryState read a session-history object after it has been freed. In Thunderbird, scripting is disabled while reading email, so exploitation is confined to browser or browser-like contexts; targeted attacks have abused the flaw.

The record

CVE
CVE-2020-6819
Published
Apr 24, 2020
Updated
Oct 21, 2025
Vendor
Mozilla
Product
Firefox
Classifications
CWE-362, CWE-416, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 24, 2020CVE publishedPublication date reported by the CVE source.
  2. Nov 3, 2021Exploitation reportedCISA Known Exploited Vulnerabilities entry
  3. Oct 21, 2025Record 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 <74.0.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <74.0.1
Mozilla · Thunderbirdversion=unspecified <68.7.0
Mozilla · Firefox ESRversion=unspecified <68.6.1

Published CVSS scores

6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P
8.1CISA-ADPCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
8.1NIST NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/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 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationReported
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-362Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
CWE-416Use After Free
T1203Exploitation for Client Execution

Public exploit references

No public exploit references are available in this record.

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

Reported exploitation

Technologies

Your stack

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Your stack

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