CVE-2026-44452 in h2o
Summary
by MITRE • 07/17/2026
h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 8dc37cb, when h2o receives a ClientHello message over TLS or QUIC and it contains a zero-length SNI extension, the h2o server runs over the zero-length hostname while trying to copy the hostname, assuming that it is NULL-terminated. This is a potential denial-of-service attack vector in sense that it might trigger segmentation violation. This issue has been fixed by commit 8dc37cb.
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Analysis
by VulDB Data Team • 07/17/2026
The h2o HTTP server presents a critical vulnerability that stems from improper handling of Server Name Indication (SNI) extensions within TLS and QUIC connections. This flaw exists in the server's processing logic where it encounters ClientHello messages containing zero-length SNI extensions, creating a scenario where the system attempts to copy what it believes to be a NULL-terminated hostname string. The vulnerability manifests as a potential denial-of-service condition that could result in segmentation violations, effectively crashing the server and rendering it unavailable to legitimate clients. The issue specifically impacts versions of h2o prior to commit 8dc37cb, indicating that this represents a well-defined regression that was introduced into the codebase and subsequently addressed through targeted fixes.
This vulnerability fundamentally represents a buffer over-read condition that occurs when the server's SNI handling logic fails to properly validate the length of incoming hostname data before attempting memory operations. The flaw arises from the assumption that all hostname strings will contain proper NULL termination, which becomes invalid when processing zero-length extensions. Such improper input validation creates an execution path where unbounded memory access occurs, leading to unpredictable behavior including potential crashes or system instability. The attack vector is particularly concerning as it requires only a malformed ClientHello message to trigger the vulnerable code path, making it accessible to any attacker capable of establishing TLS or QUIC connections to the server.
The operational impact of this vulnerability extends beyond simple service disruption, as segmentation violations in core HTTP server processes can lead to complete system instability and potential information disclosure. When exploited successfully, this vulnerability allows attackers to cause unauthorized service interruption without requiring elevated privileges or complex attack chains. The fix implemented in commit 8dc37cb addresses the root cause by ensuring proper validation of SNI extension lengths before attempting any string operations, thereby preventing the buffer over-read condition that previously led to system crashes. This type of vulnerability falls under CWE-129, which specifically addresses improper validation of length of input buffers, and aligns with ATT&CK technique T1499.004 for network denial of service attacks.
The security implications of this vulnerability underscore the critical importance of proper input validation in network-facing applications, particularly those handling TLS connections where client-provided data must be carefully processed. The fix demonstrates a defensive programming approach that validates all inputs before processing, including edge cases such as zero-length strings that might not be immediately obvious during normal operation. This vulnerability serves as an example of how seemingly minor oversights in protocol implementation can lead to significant security implications, particularly in high-availability services where denial-of-service attacks can have substantial business impact. Organizations utilizing h2o servers should prioritize updating to versions containing commit 8dc37cb to mitigate this potential attack vector while maintaining service availability and system stability.