CVE-2019-18792

An issue was discovered in Suricata 5.0.0. It is possible to bypass/evade any tcp based signature by overlapping a TCP segment with a fake FIN packet. The fake FIN packet is injected just before the PUSH ACK packet we want to bypass. The PUSH ACK packet (containing the data) will be ignored by Suricata because it overlaps the FIN packet (the sequence and ack number are identical in the two packets). The client will ignore the fake FIN packet because the ACK flag is not set. Both linux and windows clients are ignoring the injected packet.

An issue was discovered in Suricata 5.0.0. It is possible to bypass/evade any tcp based signature by overlapping a TCP segment with a fake FIN packet. The fake FIN packet is injected just before the PUSH ACK packet we want to bypass. The PUSH ACK packet (containing the data) will be ignored by Suricata because it overlaps the FIN packet (the sequence and ack number are identical in the two packets). The client will ignore the fake FIN packet because the ACK flag is not set. Both linux and windows clients are ignoring the injected packet.

Se descubrió un problema en Suricata versión 5.0.0. Es posible omitir y evadir cualquier firma basada en TCP mediante la superposición de un segmento TCP con un paquete FIN falso. El paquete FIN falso es inyectado justo antes del paquete PUSH ACK que queremos omitir. Suricata ignorará el paquete PUSH ACK (que contiene los datos) porque se superpone al paquete FIN (la secuencia y el número ack son idénticos en los dos paquetes). El cliente ignorará el paquete FIN falso porque el flag ACK no está configurado. Tanto los clientes de Linux y Windows ignoran el paquete inyectado.