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Date: Fri, 2 Mar 2018 17:22:56 +0000
From: Kirill Shipulin <>
To: "" <>
Subject: [FD] Another TCP based IDS bypass technique. CVE-2018-6794

Vulnerability Type: Detection Bypass
Affected Product: Suricata
Vulnerable version: <4.0.4
CVE number: CVE-2018-6794
Found: 25.01.2018
By: Kirill Shipulin (@kirill_wow), Positive Technologies
Severity: Medium

About Suricata:
Suricata is a high performance Network Threat Detection, IDS, IPS and Network Security Monitoring engine. Open Source and owned by a community run non-profit foundation, the Open Information Security Foundation (OISF). Suricata is developed by the OISF, its supporting vendors and the community

Attack Description:
If as a server side you break a normal TCP 3 way handshake packets order and inject some response data before 3whs is complete then data still will be received by the a client but some IDS engines may skip content checks on that.

Attack scenario TCP flow scheme:
Client    ->  [SYN] [Seq=0 Ack= 0]           ->  Evil Server
Client    <-  [SYN, ACK] [Seq=0 Ack= 1]      <-  Evil Server
Client    <-  [PSH, ACK] [Seq=1 Ack= 1]      <-  Evil Server  # Injection before the 3whs is completed
Client    <-  [FIN, ACK] [Seq=83 Ack= 1]     <-  Evil Server
Client    ->  [ACK] [Seq=1 Ack= 84]          ->  Evil Server
Client    ->  [PSH, ACK] [Seq=1 Ack= 84]     ->  Evil Server

IDS signature checks for tcp stream or http response body will be skipped in the case of data injection. This attack technique requires all three packets from a malicious server to be received by a client side together before it completes 3whs. Proof of concept server was written in C to reproduce this and it works reliably in local networks. Since some network devices may affect packets transmission exploitation is not so reliable for the internet scenario.

This attack possibly may impact other network monitoring or intrusion detection systems because is not limited to Suricata IDS: an old Snort IDS version 2.9.4 is also affected.

Successful exploitation leads to a complete TCP-Stream response or HTTP response signatures bypass and may be used to prevent malicious payloads from network detection.

A Working PoC server is available here:
There is also a traffic capture of this data injection technique.

Timeline Summary:
2018-01-25: Issue submitted to the bug tracker.
2018-01-30: Patch ready.
2018-02-14: Suricata 4.0.4 containing the fix has been released.



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