CVE-2023-6858Public exploitNetwork attack vector

Client crash via stale text-fragment bounds

Published Dec 19, 2023 · Updated May 7, 2025

Heap buffer overflow in Mozilla Firefox before 121, Firefox ESR before 115.6, and Thunderbird before 115.6 allows remote attackers to crash clients. After a text-buffer allocation failure, nsTextFrame::GetContentEnd trusts a continuation offset beyond the shortened nsTextFragment and CharAt reads outside the fragment. A user must render attacker-controlled text under memory pressure; the demonstrated outcome is a crash, while data disclosure remained unverified.

CVSS severity8.8
High
EPSS probability1.54%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Heap buffer overflow in Mozilla Firefox before 121, Firefox ESR before 115.6, and Thunderbird before 115.6 allows remote attackers to crash clients. After a text-buffer allocation failure, nsTextFrame::GetContentEnd trusts a continuation offset beyond the shortened nsTextFragment and CharAt reads outside the fragment. A user must render attacker-controlled text under memory pressure; the demonstrated outcome is a crash, while data disclosure remained unverified.

The record

CVE
CVE-2023-6858
Published
Dec 19, 2023
Updated
May 7, 2025
Vendor
Mozilla
Product
Firefox
Classifications
CWE-787, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 19, 2023CVE publishedPublication date reported by the CVE source.
  2. May 7, 2025Record 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 <121

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <121
Mozilla · Thunderbirdversion=unspecified <115.6
Mozilla · Firefox ESRversion=unspecified <115.6

Published CVSS scores

8.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
8.8CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:R/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 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-787Out-of-bounds Write
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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