Server crash via zero-length SNI hostname
Published Jul 16, 2026 · Updated Jul 16, 2026
Out-of-bounds read in H2O before commit 8dc37cb allows remote attackers to crash the server with a zero-length TLS or QUIC SNI. The ClientHello SNI handler treats the zero-length hostname as null-terminated and copies beyond its boundary until a segmentation violation occurs. No authentication or user interaction is required, but a specially formed ClientHello and reachability of an H2O TLS or QUIC listener are required.
Summary
What happened
Out-of-bounds read in H2O before commit 8dc37cb allows remote attackers to crash the server with a zero-length TLS or QUIC SNI. The ClientHello SNI handler treats the zero-length hostname as null-terminated and copies beyond its boundary until a segmentation violation occurs. No authentication or user interaction is required, but a specially formed ClientHello and reachability of an H2O TLS or QUIC listener are required.
The record
- CVE
- CVE-2026-44452
- Published
- Jul 16, 2026
- Updated
- Jul 16, 2026
- Vendor
- H2O Project
- Product
- H2O
- Classifications
- CWE-125, CWE-170, T1499.004
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jul 16, 2026CVE publishedPublication date reported by the CVE source.
- Jul 16, 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?
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- Affected versionversion=< 8dc37cb
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What is affected?
Published CVSS scores
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Attacks
What attackers are doing with it
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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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