CVE-2021-23997Network attack vector

Remote code execution via NaN font-cache mismatch

Published Jun 24, 2021 · Updated Aug 3, 2024

Use-after-free in Firefox before 88 allows remote attackers to execute arbitrary code after a user opens crafted web content. A massive SVG transform produces a NaN font-inflation factor; inconsistent numeric conversion and comparison leave a freed font-cache entry reachable through gfxFont::GetUnicodeRangeMap. Exploitation requires user interaction with attacker-controlled content and remains subject to the browser sandbox unless another escape is available.

CVSS severity8.8
High
EPSS probability0.82%
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 Firefox before 88 allows remote attackers to execute arbitrary code after a user opens crafted web content. A massive SVG transform produces a NaN font-inflation factor; inconsistent numeric conversion and comparison leave a freed font-cache entry reachable through gfxFont::GetUnicodeRangeMap. Exploitation requires user interaction with attacker-controlled content and remains subject to the browser sandbox unless another escape is available.

The record

CVE
CVE-2021-23997
Published
Jun 24, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-681, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 24, 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=unspecified <88

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <88

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 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 exploitUnconfirmed

Weakness, pattern, technique

CWE-681Incorrect Conversion between Numeric Types
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.

Technologies

Your stack

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Your stack

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