CVE-2020-6820Known exploitationNetwork attack vector

Sandbox escape via ReadableStream cache race

Published Apr 24, 2020 · Updated Oct 21, 2025

Use-after-free in Firefox before 74.0.1, Firefox ESR before 68.6.1, and Thunderbird before 68.7.0 allows remote attackers to escape the sandbox. CacheStreamControlParent can be freed when StreamList::CloseAll closes the final cached read stream, after which its mId member is accessed to send CloseAll. Winning the race requires open cache read streams during close or abort; Thunderbird email reading disables scripting, limiting exposure to browser-like contexts.

CVSS severity8.1
High
EPSS probability7.06%
Next 30 days · Sep 16, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Use-after-free in Firefox before 74.0.1, Firefox ESR before 68.6.1, and Thunderbird before 68.7.0 allows remote attackers to escape the sandbox. CacheStreamControlParent can be freed when StreamList::CloseAll closes the final cached read stream, after which its mId member is accessed to send CloseAll. Winning the race requires open cache read streams during close or abort; Thunderbird email reading disables scripting, limiting exposure to browser-like contexts.

The record

CVE
CVE-2020-6820
Published
Apr 24, 2020
Updated
Oct 21, 2025
Vendor
Mozilla
Product
Firefox
Classifications
CWE-362, T1068
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 catalog entry
  3. Oct 21, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

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

8.1CISA-ADPCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P
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')
T1068Exploitation for Privilege Escalation

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

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo