SSRF policy bypass via unclosed authority bracket
Published Sep 2, 2026 · Updated Sep 3, 2026
Host confusion in parse() in Fastify fast-uri 2.4.5, 3.1.6, and 4.1.3 allows remote attackers to bypass host policies via malformed authorities. parse() accepts a bracketed authority that is not a valid IPv6 literal, returns the malformed host without an error, and leaves it uncanonicalized. Exposure requires an application to authorize parse().host and then give the original URL to a Node-based HTTP client, which can resolve a different destination and issue an unintended network request.
Summary
What happened
Host confusion in parse() in Fastify fast-uri 2.4.5, 3.1.6, and 4.1.3 allows remote attackers to bypass host policies via malformed authorities. parse() accepts a bracketed authority that is not a valid IPv6 literal, returns the malformed host without an error, and leaves it uncanonicalized. Exposure requires an application to authorize parse().host and then give the original URL to a Node-based HTTP client, which can resolve a different destination and issue an unintended network request.
The record
- CVE
- CVE-2026-84394
- Published
- Sep 2, 2026
- Updated
- Sep 3, 2026
- Vendor
- Fastify
- Product
- fast-uri
- Classifications
- CWE-436, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Sep 2, 2026CVE publishedPublication date reported by the CVE source.
- Sep 3, 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=2.4.5 <2.4.6
- Affected versionversion=3.1.6 <3.1.7
- Affected versionversion=4.1.3 <4.1.4
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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