CVE-2026-16375Network attack vector

Site-isolation failure in HTTP networking

Published Jul 21, 2026 · Updated Jul 21, 2026

A site-isolation issue in Mozilla Firefox and Thunderbird before 153 and ESR before 140.13 affects the Networking: HTTP component. Mozilla has not disclosed the faulty HTTP operation, triggering input, or resulting security-boundary failure beyond identifying the component. CISA characterizes the issue as remotely reachable without authentication or user interaction, but concrete request conditions and the bounded consequence remain undisclosed.

CVSS severity9.8
Critical
EPSS probability0.21%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

A site-isolation issue in Mozilla Firefox and Thunderbird before 153 and ESR before 140.13 affects the Networking: HTTP component. Mozilla has not disclosed the faulty HTTP operation, triggering input, or resulting security-boundary failure beyond identifying the component. CISA characterizes the issue as remotely reachable without authentication or user interaction, but concrete request conditions and the bounded consequence remain undisclosed.

The record

CVE
CVE-2026-16375
Published
Jul 21, 2026
Updated
Jul 21, 2026
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-346
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 21, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 21, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.8CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/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 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-346Origin Validation Error

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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