Host-policy bypass via failed IDN canonicalization
Published Jun 29, 2026 · Updated Aug 28, 2026
Host confusion in Fastify fast-uri 2.3.1 through 3.1.2 and 4.0.0 allows remote attackers to bypass host-based policies with Unicode URLs. The IDN path calls nonexistent URL.domainToASCII, records the resulting error, and returns the uncanonicalized host from parse, normalize, and equal. Exploitation requires an application to check that result before another WHATWG-compatible parser uses the same URL, enabling unintended destinations, unauthorized resource access, or malicious redirects.
Summary
What happened
Host confusion in Fastify fast-uri 2.3.1 through 3.1.2 and 4.0.0 allows remote attackers to bypass host-based policies with Unicode URLs. The IDN path calls nonexistent URL.domainToASCII, records the resulting error, and returns the uncanonicalized host from parse, normalize, and equal. Exploitation requires an application to check that result before another WHATWG-compatible parser uses the same URL, enabling unintended destinations, unauthorized resource access, or malicious redirects.
The record
- CVE
- CVE-2026-13676
- Published
- Jun 29, 2026
- Updated
- Aug 28, 2026
- Vendor
- 389 Directory Server
- Product
- Red Hat Quay
- Classifications
- CWE-551, CWE-436
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jun 29, 2026CVE publishedPublication date reported by the CVE source.
- Aug 28, 2026Record updatedLatest update available in the CVE record.
Exploitability
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What is affected?
Published CVSS scores
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Attacks
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Technologies
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