Unauthenticated middleware bypass via Host header spoofing
Published Mar 20, 2026 · Updated Mar 20, 2026
Host header spoofing in UnJS H3 2.0.0-0 through 2.0.1-rc.14 allows remote attackers to bypass route middleware. NodeRequestUrl's _url getter builds FastURL.href from the untrusted Host header, while the router selects the handler before middleware evaluates the spoofed URL. Exploitation requires an application to access event.url properties in middleware guarding a matched route, enabling unauthenticated access or actions that the skipped middleware was intended to block.
Summary
What happened
Host header spoofing in UnJS H3 2.0.0-0 through 2.0.1-rc.14 allows remote attackers to bypass route middleware. NodeRequestUrl's _url getter builds FastURL.href from the untrusted Host header, while the router selects the handler before middleware evaluates the spoofed URL. Exploitation requires an application to access event.url properties in middleware guarding a matched route, enabling unauthenticated access or actions that the skipped middleware was intended to block.
The record
- CVE
- CVE-2026-33131
- Published
- Mar 20, 2026
- Updated
- Mar 20, 2026
- Vendor
- UnJS
- Product
- H3
- Classifications
- CWE-290, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Mar 20, 2026CVE publishedPublication date reported by the CVE source.
- Mar 20, 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=>= 2.0.0-0, < 2.0.1-rc.15
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
- Host-header middleware-bypass proof of conceptproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
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Your stack
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