SSRF bypass via backslash host confusion
Published Jul 19, 2026 · Updated Jul 19, 2026
Interpretation conflict in affected Fastify fast-uri 2.x, 3.x, and 4.x releases allows remote attackers to bypass host-based URL policies. Its authority parser accepts a literal backslash within userinfo instead of delimiting the authority, while Node's WHATWG parser normalizes it to a slash and selects a different host. Exploitation requires an application to validate an untrusted URL with fast-uri and then send it through a WHATWG consumer, enabling requests to unintended internal, loopback, or metadata destinations.
Summary
What happened
Interpretation conflict in affected Fastify fast-uri 2.x, 3.x, and 4.x releases allows remote attackers to bypass host-based URL policies. Its authority parser accepts a literal backslash within userinfo instead of delimiting the authority, while Node's WHATWG parser normalizes it to a slash and selects a different host. Exploitation requires an application to validate an untrusted URL with fast-uri and then send it through a WHATWG consumer, enabling requests to unintended internal, loopback, or metadata destinations.
The record
- CVE
- CVE-2026-16221
- Published
- Jul 19, 2026
- Updated
- Jul 19, 2026
- Vendor
- Fastify
- Product
- fast-uri
- Classifications
- CWE-436
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jul 19, 2026CVE publishedPublication date reported by the CVE source.
- Jul 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.
- Affected versionversion=2.3.1 <2.4.3
- Affected versionversion=3.0.0 <3.1.4
- Affected versionversion=4.0.0 <4.1.1
What conditions does exploitation require?
What is affected?
Published CVSS scores
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Attacks
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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