No exploitation reportedFix available

CVE-2023-6858

Q: Is CVE-2023-6858 actively exploited?

Active exploitation of CVE-2023-6858 has not been confirmed. Its EPSS score was 1.5% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

Q: What is the CVSS score for CVE-2023-6858?

CVE-2023-6858 has a CVSS v3 base score of 8.8 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

No exploitation reported by the sources we read.

Published2023-12-19
WeaknessCWE-787

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

Firefox was susceptible to a heap buffer overflow in `nsTextFragment` due to insufficient OOM handling. This vulnerability affects Firefox ESR < 115.6, Thunderbird < 115.6, and Firefox < 121.

Timeline

dates in UTC

2023-12-1914:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 18:48 UTC

Indicators tagged with this CVE, per day0 indicators over 7 days

Block these indicators

Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2023-6858?format=txt
CSV export: https://api.ismalicious.com/edl/cve/CVE-2023-6858?format=csv
TAXII collection of exploited CVEs
The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD

Debian debian linux

Branch	Affected	Fixed in	Source
10.0	= 10.0	no fix published	NVD
11.0	= 11.0	no fix published	NVD
12.0	= 12.0	no fix published	NVD

Mozilla firefox

Branch	Affected	Fixed in	Source
121.0	< 121.0	121.0	NVD

Mozilla firefox esr

Branch	Affected	Fixed in	Source
115.6	< 115.6	115.6	NVD

1 more product in the sources.

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Fix

Advisories and patches

www.mozilla.orgVendor Advisory
www.mozilla.orgVendor Advisory
www.mozilla.orgVendor Advisory

Detection

Nuclei template: none so far
Exploit-DB: no entry
Proof-of-concept repositories: none listed

Sources

CISA-ADPSSVC decisionnone2025-05-07
OpenCVESSVC decisionnone—
CISA-ADPCVSS 3.18.82025-05-07
NVDCVSS 3.18.8read 2026-10-23
CISA-ADPweakness (CWE)CWE-7872025-05-07
NVDweakness (CWE)CWE-787read 2026-10-23
FIRST EPSSEPSS score1.5 %2026-10-22

Technical details

CVSS v3 Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Published: 2023-12-19
Last Modified: 2026-06-17
NVD status: Modified

Frequently asked questions

What is CVE-2023-6858?

Firefox was susceptible to a heap buffer overflow in `nsTextFragment` due to insufficient OOM handling. This vulnerability affects Firefox ESR < 115.6, Thunderbird < 115.6, and Firefox < 121.

Is CVE-2023-6858 actively exploited?

Active exploitation of CVE-2023-6858 has not been confirmed. Its EPSS score was 1.5% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2023-6858?

CVE-2023-6858 has a CVSS v3 base score of 8.8 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

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