CVE-2021-38504Public exploitNetwork attack vector

Memory corruption via stale file-picker change callback

Published Dec 8, 2021 · Updated Aug 4, 2024

Use-after-free in Mozilla Firefox before 94, Firefox ESR before 91.3, and Thunderbird before 91.3 allows attackers to corrupt memory. During a webkitdirectory file selection, a reload can destroy the input element's script global before its pending change callback is created, leaving code to dereference freed state. The user must open the crafted page, invoke the file picker, and select a file or directory during the reload sequence; successful triggering crashes the tab and exposes memory corruption.

CVSS severity8.8
High
EPSS probability1.59%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use-after-free in Mozilla Firefox before 94, Firefox ESR before 91.3, and Thunderbird before 91.3 allows attackers to corrupt memory. During a webkitdirectory file selection, a reload can destroy the input element's script global before its pending change callback is created, leaving code to dereference freed state. The user must open the crafted page, invoke the file picker, and select a file or directory during the reload sequence; successful triggering crashes the tab and exposes memory corruption.

The record

CVE
CVE-2021-38504
Published
Dec 8, 2021
Updated
Aug 4, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-416, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 8, 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 <94

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <94
Mozilla · Thunderbirdversion=unspecified <91.3
Mozilla · Firefox ESRversion=unspecified <91.3

Published CVSS scores

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

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-416Use After Free
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

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Your stack

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