CVE-2021-23961Network attack vector

Internal service discovery via unrestricted ports

Published Feb 26, 2021 · Updated Aug 3, 2024

Network service probing in Mozilla Firefox before 85 allows remote attackers to discover internal hosts and local services via a malicious webpage. Firefox permitted webpage-initiated connections to ports 69, 137, 161, 1719, 6566, and 10080 instead of blocking them, extending NAT slipstreaming probes. A user must load attacker-controlled content; the bounded result is disclosure of reachable hosts and listening services, not direct access to service data.

CVSS severity7.4
High
EPSS probability1.43%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Network service probing in Mozilla Firefox before 85 allows remote attackers to discover internal hosts and local services via a malicious webpage. Firefox permitted webpage-initiated connections to ports 69, 137, 161, 1719, 6566, and 10080 instead of blocking them, extending NAT slipstreaming probes. A user must load attacker-controlled content; the bounded result is disclosure of reachable hosts and listening services, not direct access to service data.

The record

CVE
CVE-2021-23961
Published
Feb 26, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
T1046
Attack vector
network
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=< 85

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=< 85

Published CVSS scores

7.4NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N
4.3NIST NVDAV:N/AC:M/Au:N/C:P/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)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1046Network Service Discovery

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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