GEN:SID | 1:623 |
Message | SCAN NULL |
Summary | A tcp packet with none of it's control bits set was detected.
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Impact | Information regarding firewall rulesets, open/closed ports, ACLs, and possibly even OS type is possible. This technique can also be used to bypass certain firewalls or traffic filtering/shaping devices.
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Detailed Information | A tcp packet with none of it's control bits (URG, ACK, PSH, RST, SYN, FIN) was detected. Additionally, both the sequence number and acknowledgement number were set to 0. An open port will generally not respond at all, whereas a closed port will generally respond with an ACK RST. The particular response varies between operating systems, and is also governed by any filtering that may be done between the two hosts.
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Affected Systems | |
Attack Scenarios | As part of information gathering leading up to another (more directed) attack, an attacker may attempt to figure out what ports are open/closed on a remote machine.
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Ease of Attack | Intermediate. To initiate an attack of this type, an attacker either needs a tool that can send tcp packets with no control bits set or the ability to craft their own packets. The former is easy, the later requires a more advanced skillset.
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Corrective Action | Determine if this particular port would have responded as being open or closed. If open, watch for more attacks on this particular service or from the remote machine that sent the packet. If closed, simply watch for more traffic from this host.
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Additional References | |
Rule References | arachnids: 4
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