Some programming language implementations do not sufficiently randomize their hash functions or provide means to limit key collision attacks, which can be leveraged by an unauthenticated attacker to cause a denial-of-service (DoS) condition.
Many applications, including common web framework implementations, use hash tables to map key values to associated entries. If the hash table contains entries for different keys that map to the same hash value, a hash collision occurs and additional processing is required to determine which entry is appropriate for the key. If an attacker can generate many requests containing colliding key values, an application performing the hash table lookup may enter a denial of service condition.
An application can be forced into a denial-of-service condition. In the case of some web application servers, specially-crafted POST form data may result in a denial-of-service.
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The PHP Group
Thanks to Alexander Klink and Julian Wälde for reporting these vulnerabilities.