CVE-2026-16732

Forwarded-header spoofing via trustProxy hop-count

Published Aug 18, 2026 · Updated Aug 19, 2026

Request spoofing in Fastify 5.8.3 through 5.12.0 allows remote attackers to falsify host, protocol, and client IP data via X-Forwarded-* headers. The guard added after CVE-2026-3635 validates forwarded-header reads with proxyFn(socket.remoteAddress, 0), but numeric trustProxy values compile to predicates that ignore the connecting address, so any hop count of 1 or more always passes. Exposure requires direct network reachability to the Fastify origin despite a front-facing proxy; reachable deployments can suffer host injection, HTTPS-enforcement bypass, secure-cookie and CSRF-origin bypass, host-based routing errors, and cache poisoning.

CVSS severity6.1
Medium
EPSS probability0.14%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Request spoofing in Fastify 5.8.3 through 5.12.0 allows remote attackers to falsify host, protocol, and client IP data via X-Forwarded-* headers. The guard added after CVE-2026-3635 validates forwarded-header reads with proxyFn(socket.remoteAddress, 0), but numeric trustProxy values compile to predicates that ignore the connecting address, so any hop count of 1 or more always passes. Exposure requires direct network reachability to the Fastify origin despite a front-facing proxy; reachable deployments can suffer host injection, HTTPS-enforcement bypass, secure-cookie and CSRF-origin bypass, host-based routing errors, and cache poisoning.

The record

CVE
CVE-2026-16732
Published
Aug 18, 2026
Updated
Aug 19, 2026
Vendor
Fastify
Product
Fastify
Classifications
CWE-348, T1190
Attack vector
adjacent
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 18, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 19, 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=5.8.3 <5.12.1

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesunauthenticated

What is affected?

Fastify · Fastifyversion=5.8.3 <5.12.1

Published CVSS scores

6.1GitHubCVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/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.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectoradjacent
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-348Use of Less Trusted Source
T1190Exploit Public-Facing Application

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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