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computer vulnerability alert 11656

TCP: packets injection via a firewall and a malware

Synthesis of the vulnerability

When an attacker installed an unprivileged malware on a client computer, and when a firewall is located between this client and a TCP server, an attacker who is located on the internet can guess valid sequence numbers, in order to inject data in this TCP session.
Impacted software: CheckPoint Power-1 Appliance, CheckPoint Security Gateway, CheckPoint Smart-1, CheckPoint UTM-1 Appliance, VPN-1, CheckPoint VSX-1, TCP protocol.
Severity of this computer vulnerability: 1/4.
Consequences of an attack: data reading, data creation/edition.
Attacker's origin: internet client.
Creation date: 28/05/2012.
Références of this announce: FGA-2012-19, sk74640, VIGILANCE-VUL-11656.

Description of the vulnerability

When a privileged malware is installed on victim's computer, it can inject data in his TCP sessions. However, if the malware is not privileged, it cannot do it.

TCP sequence and acknowledgment numbers are used to sort data. An attacker has to guess these numbers (and also IP addresses and ports, but the malware knows them via netstat), in order to inject malicious packets in an active TCP session.

Firewalls usually block TCP packets with a sequence number outside the expected window. However, when this feature is enabled, a remote attacker can send a series of packets:
 - if one of these packets went through the firewall, the malware (which for example reads packets counters, which are not always precise) indicates it to the remote attacker
 - if none of these packets went through, the malware indicates the attacker to send another series
So, after several iterations, the remote attacker guesses which sequence numbers are currently valid.

When an attacker installed an unprivileged malware on a client computer, and when a firewall is located between this client and a TCP server, an attacker who is located on the internet can guess valid sequence numbers, in order to inject data in this TCP session. This vulnerability also works be reversing the client and the server.
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