Client-IP control bypass via unstripped header tabs
Published Jul 29, 2026 · Updated Jul 29, 2026
Improper input validation in Fastify @fastify/forwarded before 3.0.2 allows remote attackers to bypass exact client-IP checks. The X-Forwarded-For parser trims only spaces between multiple addresses, leaving a leading horizontal tab in the resolved address string. Exploitation requires a consuming application to compare that unvalidated string exactly for an allowlist, blocklist, rate-limit key, or audit correlation; the malformed value cannot impersonate a trusted proxy address.
Summary
What happened
Improper input validation in Fastify @fastify/forwarded before 3.0.2 allows remote attackers to bypass exact client-IP checks. The X-Forwarded-For parser trims only spaces between multiple addresses, leaving a leading horizontal tab in the resolved address string. Exploitation requires a consuming application to compare that unvalidated string exactly for an allowlist, blocklist, rate-limit key, or audit correlation; the malformed value cannot impersonate a trusted proxy address.
The record
- CVE
- CVE-2026-18174
- Published
- Jul 29, 2026
- Updated
- Jul 29, 2026
- Vendor
- Fastify
- Product
- @fastify/forwarded
- Classifications
- CWE-184, CWE-20, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jul 29, 2026CVE publishedPublication date reported by the CVE source.
- Jul 29, 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.
- Affected versionversion=0 <3.0.2
What conditions does exploitation require?
What is affected?
Published CVSS scores
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.
Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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