A method for implementing indirect GTP firewall filtering includes using a signaling message routing node to dynamically populate an indirect GTP-C firewall filtering database with IMSIs and VPLMN IDs extracted from mobility management signaling messages for updating the locations of outbound roaming subscribers. The method further includes receiving a CCR-I message generated in response to a GTP-C message. The method further includes extracting an IMSI and a VPLMN ID from the CCR-I message. The method further includes accessing the indirect GTP-C firewall filtering database using the IMSI extracted from the CCR-I message. The method further includes determining that a record corresponding to the IMSI is present in the indirect GTP-C firewall filtering database. The method further includes determining that a VPLMN ID in the record does not match the VPLMN ID extracted from the CCR-I message. The method further includes, in response to determining that the VPLMN ID in the record does not match the VPLMN ID extracted from the CCR-I message, rejecting the CCR-I message.