The gen_rand_string function in phpBB 2.0.19 uses insufficiently random data (small value space) to create the activation key ("validation ID") that is sent by e-mail when establishing a password, which makes it easier for remote attackers to obtain the key and modify passwords for existing accounts or create new accounts.
Exploitability
AV:NAC:LAu:NImpact
C:PI:PA:N6.4/AV:N/AC:L/Au:N/C:P/I:P/A:N