Red Hat Security Advisory 2012-0033-01 - PHP is an HTML-embedded scripting language commonly used with the Apache HTTP Server. It was found that the hashing routine used by PHP arrays was susceptible to predictable hash collisions. If an HTTP POST request to a PHP application contained many parameters whose names map to the same hash value, a large amount of CPU time would be consumed. This flaw has been mitigated by adding a new configuration directive, max_input_vars, that limits the maximum number of parameters processed per request. By default, max_input_vars is set to 1000.
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