CVE-2020-12387Public exploitNetwork attack vector

Client crash via Web Worker shutdown race

Published May 26, 2020 · Updated Aug 4, 2024

Use-after-free in Firefox before 76, Firefox ESR before 68.8, and Thunderbird before 68.8.0 allows remote attackers to crash the client. During Web Worker shutdown, asynchronous XMLHttpRequest and FileReader operations race teardown and leave callbacks using freed worker state. Triggering requires attacker-controlled web content; Thunderbird disables scripting in mail, limiting exposure to browser-like contexts, and success interrupts the client.

CVSS severity8.1
High
EPSS probability1.40%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
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 76, Firefox ESR before 68.8, and Thunderbird before 68.8.0 allows remote attackers to crash the client. During Web Worker shutdown, asynchronous XMLHttpRequest and FileReader operations race teardown and leave callbacks using freed worker state. Triggering requires attacker-controlled web content; Thunderbird disables scripting in mail, limiting exposure to browser-like contexts, and success interrupts the client.

The record

CVE
CVE-2020-12387
Published
May 26, 2020
Updated
Aug 4, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-416, CWE-362, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 26, 2020CVE 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

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 <76

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <76
Mozilla · Thunderbirdversion=unspecified <68.8.0
Mozilla · Firefox ESRversion=unspecified <68.8

Published CVSS scores

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 exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

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

Public exploit references

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

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