[FD] Another TCP based IDS bypass technique. CVE-2018-6794

[Kirill Shipulin <kshipulin@xxxxxxxxxxxxxx>]

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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
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About Suricata:
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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.

PoС:
----
A Working PoC server is available here: https://github.com/kirillwow/ids_bypass
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.

References:
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CVE-2018-6794
https://redmine.openinfosecfoundation.org/issues/2427

Contacts:
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Twitter: https://twitter.com/AttackDetection
Twitter: https://twitter.com/kirill_wow
Telegram: https://t.me/kirill_wow

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