JRuby before 1.6.5.1 computes hash values without restricting the ability to trigger hash collisions predictably, which allows context-dependent attackers to cause a denial of service (CPU consumption) via crafted input to an application that maintains a hash table.
JRuby versions prior to 1.6.5.1 are vulnerable to a denial-of-service (DoS) attack. This vulnerability allows attackers to exhaust server resources by crafting malicious input that triggers hash collisions, leading to excessive CPU consumption and application unavailability.
Step 1: Payload Delivery: The attacker crafts a malicious input, typically a series of strings or data structures designed to collide within the JRuby hash table. This input is sent to an application that uses a hash table (e.g., for storing user data, session information, or configuration settings). Step 2: Hash Collision Trigger: The application processes the malicious input, and the JRuby hash function is invoked to calculate hash values for the input elements. The crafted input is designed to generate a large number of hash collisions, meaning multiple input elements map to the same hash bucket. Step 3: Hash Table Degradation: Due to the collisions, the hash table's performance degrades. Instead of the expected O(1) lookup time, the application spends an increasing amount of time searching for the correct bucket, potentially using linear probing or chaining. Step 4: Resource Exhaustion: The CPU usage spikes as the application struggles to manage the collisions. The server becomes unresponsive, and legitimate users are denied service, leading to a DoS condition.
The vulnerability stems from a flaw in JRuby's hash function implementation. Specifically, the hash function doesn't adequately protect against hash collisions. Attackers can craft input that intentionally generates collisions, forcing the hash table to perform inefficient operations (e.g., linear probing or chaining) when storing or retrieving data. This leads to a significant increase in CPU usage as the hash table degrades to O(n) performance for lookups and insertions, effectively causing a DoS. The root cause is the lack of a robust collision-resistant hash function or mitigation strategies like rehashing or using a different hash table implementation.