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What ackbarr wrote is right, I think.
This hole is just a phpBB's specific bug. ( "/e" in preg_replace() )
PHP version and this hole are irrelevant.
The coding style of phpBB looks quite insecure.
Although this hole is patched in 2.0.11, attackers will crack phpBB via the other holes easily.
Of course, PHP <= 4.3.9 has a vulnerability with unseriarize() and newbb should be patched with such an old PHP.