Host-policy bypass via backslash authority confusion
Published Jul 31, 2026 · Updated Jul 31, 2026
URL parser confusion in Fastify fast-uri before 4.1.2, 3.1.5, and 2.4.4 allows remote attackers to steer requests to unintended hosts. The parser requires a literal double slash for an authority and folds backslash-based introducers into the path, while Node treats those backslashes as slashes. Exploitation requires an application to validate attacker-controlled URLs with fast-uri before a Node URL or fetch consumer uses the same input.
Summary
What happened
URL parser confusion in Fastify fast-uri before 4.1.2, 3.1.5, and 2.4.4 allows remote attackers to steer requests to unintended hosts. The parser requires a literal double slash for an authority and folds backslash-based introducers into the path, while Node treats those backslashes as slashes. Exploitation requires an application to validate attacker-controlled URLs with fast-uri before a Node URL or fetch consumer uses the same input.
The record
- CVE
- CVE-2026-18446
- Published
- Jul 31, 2026
- Updated
- Jul 31, 2026
- Vendor
- Fastify
- Product
- fast-uri
- Classifications
- CWE-436, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jul 31, 2026CVE publishedPublication date reported by the CVE source.
- Jul 31, 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 <2.4.4
- Affected versionversion=3.0.0 <3.1.5
- Affected versionversion=4.0.0 <4.1.2
What conditions does exploitation require?
What is affected?
Published CVSS scores
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Attacks
What attackers are doing with it
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Public exploit references
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Technologies
Your stack
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Your stack
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