CVE-2021-23977Public exploit

Sensitive-data disclosure via raced symlink resolution

Published Feb 26, 2021 · Updated Aug 3, 2024

TOCTOU in Firefox for Android before 86 allows attackers with a malicious installed app to read sensitive Firefox application data. An external file intent can race Firefox into following an attacker-controlled symlink, so a checked path resolves to a private file when loaded. Exploitation requires installing a malicious Android application and can expose cookies, preferences, and profile-location data without altering them.

CVSS severity5.3
Medium
EPSS probability0.87%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

TOCTOU in Firefox for Android before 86 allows attackers with a malicious installed app to read sensitive Firefox application data. An external file intent can race Firefox into following an attacker-controlled symlink, so a checked path resolves to a private file when loaded. Exploitation requires installing a malicious Android application and can expose cookies, preferences, and profile-location data without altering them.

The record

CVE
CVE-2021-23977
Published
Feb 26, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-367, T1533
Attack vector
local
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 26, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 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=< 86

What conditions does exploitation require?

Attack vectorlocal
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=< 86

Published CVSS scores

5.3NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:N/A:N
2.6NVDAV:N/AC:H/Au:N/C:P/I:N/A:N
5.3SUSECVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:N/A:N

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)
Vectorlocal
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-367Time-of-check Time-of-use (TOCTOU) Race Condition
T1533Data from Local System

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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