CVE-2026-44453 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 6b5370d, h2o is vulnerable to a Denial of Service attack when calling alloca under certain conditions. When serving static files, h2o builds the file path on stack, by calling alloca. The maximum size of the memory allocated using alloca can be as huge as ~600KB, which exceeds the default pthread stack size used by musl libc (128KB). If the amount of memory allocated by alloca exceeds the stack size, the h2o server crashes with a segmentation fault, while it tries to touch the guard page. This issue has been fixed by commit 6b5370d.
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Analysis
by VulDB Data Team • 07/17/2026
The h2o HTTP server presents a critical denial of service vulnerability that stems from improper memory management practices when handling static file requests. This vulnerability specifically affects versions prior to commit 6b5370d and demonstrates a classic stack overflow condition that can be exploited to crash the entire HTTP service. The flaw occurs during static file serving operations where the server constructs file paths on the stack using the alloca function, which dynamically allocates memory on the call stack rather than the heap. This approach becomes problematic when dealing with large path constructions, as the memory allocation can exceed available stack space.
The technical implementation of this vulnerability involves the use of alloca for building file paths during static content delivery, creating a scenario where memory allocations can reach approximately 600KB in size. This substantial memory requirement directly conflicts with the default pthread stack size imposed by musl libc implementations, which typically limits stack space to 128KB. When the allocated memory exceeds these boundaries, the system's guard page protection mechanism triggers a segmentation fault, causing the h2o server process to terminate abruptly. This behavior represents a fundamental violation of stack management principles and demonstrates poor resource allocation practices in memory-intensive operations.
The operational impact of this vulnerability extends beyond simple service disruption, as it can be exploited by malicious actors to systematically crash h2o servers through carefully crafted requests that force excessive memory allocation on the stack. The vulnerability affects both HTTP/1.x and HTTP/2 implementations within h2o, making it particularly dangerous for web applications relying on these protocols. From a cybersecurity perspective, this issue aligns with CWE-129, which addresses improper handling of memory allocation size limits in software systems. The attack vector can be classified under ATT&CK technique T1499.004, specifically targeting the availability of network services through resource exhaustion attacks.
The fix implemented in commit 6b5370d addresses this vulnerability by modifying how path construction is handled during static file serving operations. This remediation likely involves switching from stack-based allocation using alloca to heap-based memory management or implementing proper bounds checking on allocation sizes. Security practitioners should note that this vulnerability highlights the importance of understanding library-specific memory management behaviors, particularly when working with musl libc implementations where stack limitations are more restrictive than glibc counterparts. Organizations deploying h2o servers should prioritize updating to versions containing commit 6b5370d and consider implementing additional monitoring for unusual memory allocation patterns that could indicate exploitation attempts.
The broader implications of this vulnerability extend to software development practices around stack usage and memory management in server applications. It serves as a reminder that functions like alloca, while convenient for small allocations, can create serious stability issues when used without proper size constraints or when dealing with potentially large data structures. The issue also demonstrates how seemingly benign functionality can create critical security weaknesses when combined with specific runtime environments and memory constraints. This vulnerability underscores the necessity of thorough testing under various environmental conditions, particularly when dealing with systems that have different default resource limits across different libc implementations.