CVE-2026-86250 in h3js
Summary
by MITRE • 09/06/2026
h3 versions before 2.0.1-rc.18 fail to validate the chunk count parsed from user-controlled cookie values in setChunkedCookie() and deleteChunkedCookie() functions. Attackers can send a crafted cookie header with an extremely large chunk count to trigger an O(n²) cleanup loop that hangs the server process.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 09/06/2026
The vulnerability identified involves a critical input validation failure within h3 versions prior to 2.0.1-rc.18, specifically affecting the setChunkedCookie and deleteChunkedCookie functions. These components are responsible for handling HTTP cookies that utilize chunking mechanisms to store data exceeding standard size limits by splitting them into multiple smaller segments. The core technical flaw lies in the absence of rigorous validation on the chunk count extracted from user-controlled cookie values. When a server processes these headers, it relies on the numeric value indicating how many chunks comprise the cookie without verifying if this number is reasonable or within expected bounds. This lack of sanitization allows an attacker to inject a crafted HTTP header containing an excessively large integer for the chunk count parameter.
From a technical perspective, the exploitation mechanism triggers an inefficient computational loop during the cleanup phase of request processing. Instead of validating that the specified chunk count corresponds to actual data segments or falls within a predefined maximum threshold, the server proceeds with a cleanup routine that iterates based on this untrusted input. This results in an O(n squared) time complexity operation where n represents the maliciously large chunk count provided by the attacker. As the loop executes for each of these fabricated chunks, it consumes significant CPU resources and memory allocation cycles without performing meaningful work related to actual data retrieval or deletion. The operational impact is a severe denial of service condition, as the server process becomes unresponsive due to being trapped in this infinite or near-infinite computational cycle. This effectively hangs the specific worker thread handling the request and can potentially lead to broader system instability if such requests are sent at scale against multiple endpoints.
This vulnerability aligns with CWE-20 Improper Input Validation, as the application fails to verify that user-supplied data conforms to expected formats or ranges before processing it. Furthermore, from a threat modeling perspective using the MITRE ATT&CK framework, this behavior is characteristic of T1499 Endpoint Denial of Service: Resource Exhaustion via Compute Loop. The attacker leverages algorithmic complexity issues to exhaust system resources, specifically CPU cycles and memory management overheads associated with loop execution. Unlike simple buffer overflow exploits that aim for code execution, this vulnerability targets availability by causing the service to become unavailable through resource starvation.
Mitigation strategies must focus on implementing strict input validation at the entry points of cookie parsing logic. Developers should enforce a maximum allowable limit for chunk counts based on practical constraints such as HTTP header size limits and reasonable storage expectations. Additionally, introducing timeout mechanisms or circuit breakers within the request processing pipeline can prevent any single operation from monopolizing server resources indefinitely. Updating to h3 version 2.0.1-rc.18 or later is essential, as these versions include patches that properly validate chunk counts before initiating cleanup routines. Security teams should also monitor for anomalous spikes in CPU usage associated with cookie processing endpoints and consider deploying web application firewalls configured to detect unusually large numeric values in HTTP headers indicative of this specific attack vector.