CVE-2026-75931Network attack vector

Policy bypass via skipped IDN canonicalization

Published Aug 24, 2026 · Updated Aug 24, 2026

Interpretation conflict in Fastify fast-uri allows remote attackers to bypass host policies through scheme-relative URI references. The resolve() path emits a non-ASCII host verbatim when a scheme-relative reference inherits its scheme, while scheme-bearing parsing converts that host to ASCII. Exploitation requires an application to resolve attacker-controlled references and enforce routing or allowlist policy on the inconsistent host, which can send the application to a different destination.

CVSS severity7.5
High
EPSS probability0.23%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Interpretation conflict in Fastify fast-uri allows remote attackers to bypass host policies through scheme-relative URI references. The resolve() path emits a non-ASCII host verbatim when a scheme-relative reference inherits its scheme, while scheme-bearing parsing converts that host to ASCII. Exploitation requires an application to resolve attacker-controlled references and enforce routing or allowlist policy on the inconsistent host, which can send the application to a different destination.

The record

CVE
CVE-2026-75931
Published
Aug 24, 2026
Updated
Aug 24, 2026
Vendor
Fastify
Product
fast-uri
Classifications
CWE-436
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 24, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 24, 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?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=2.4.2 <2.4.5
  2. Affected versionversion=3.1.3 <3.1.6
  3. Affected versionversion=4.0.1 <4.1.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Fastify · fast-uriversion=2.4.2 <2.4.5; version=3.1.3 <3.1.6; version=4.0.1 <4.1.3

Published CVSS scores

7.5OpenJS FoundationCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/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

CWE-436Interpretation Conflict

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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