CVE-2026-16387Network attack vector

Site isolation bypass in Networking component

Published Jul 21, 2026 · Updated Jul 21, 2026

Site isolation failure in Mozilla Firefox and Thunderbird before 153 allows remote attackers to breach browser-enforced origin boundaries. Mozilla identifies the locus only as the Networking component; the restricted bug report leaves the faulty request or isolation operation undisclosed. Firefox ESR and Thunderbird ESR before 140.13 are also affected; CISA assesses that no credentials or user interaction are required.

CVSS severity9.8
Critical
EPSS probability0.20%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Site isolation failure in Mozilla Firefox and Thunderbird before 153 allows remote attackers to breach browser-enforced origin boundaries. Mozilla identifies the locus only as the Networking component; the restricted bug report leaves the faulty request or isolation operation undisclosed. Firefox ESR and Thunderbird ESR before 140.13 are also affected; CISA assesses that no credentials or user interaction are required.

The record

CVE
CVE-2026-16387
Published
Jul 21, 2026
Updated
Jul 21, 2026
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-284, CWE-346, CWE-200
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-284Improper Access Control
CWE-346Origin Validation Error
CWE-200Exposure of Sensitive Information to an Unauthorized Actor

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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