Denial of Service by ReDOS in h2oai/h2o-3

The vulnerability resides in the /3/Parse endpoint of h2oai/h2o-3 version 3.46.0.1. The root cause is that the endpoint uses a user-specified string to construct a regular expression, which is then applied to another user-specified string. This design allows an attacker to trigger expensive regex operations that can consume significant processing resources, especially when multiple concurrent requests are made [1][2].

Exploitation

An attacker can exploit this vulnerability without any prior authentication by sending multiple simultaneous requests to the /3/Parse endpoint. By carefully crafting the input strings, the attacker can force the application to spend excessive time evaluating the user-controlled regex, thereby exhausting all available worker threads. The attack does not require any special network position beyond reachability of the H2O-3 instance [2][4].

Impact

Successful exploitation leads to a complete denial of service (DoS). The H2O-3 server becomes unable to process further legitimate requests as all threads are blocked by the malicious regex evaluations. This renders the machine learning platform and its services unavailable until the attack ceases or the server is restarted [2].

Mitigation

The vulnerability has been acknowledged by the project. Users are advised to update to a patched version of H2O-3 as soon as it becomes available. As of the publication date, the issue is tracked and further details can be found in the Huntr bounty report [4] and related source code [3].