NETRESEC Network Security Blog - Tag : stream


Covert Man-on-the-Side Attacks

Man-on-the-Side

After Edward Snowden exposed NSA's Man-on-the-Side attack capabilities we've started to see IDS signatures that can detect such attacks being released and re-discovered. However, despite these efforts Man-on-the-Side attacks, such as QUANTUM INSERT, can still be carried out without triggering these IDS signatures.

I recently taught a network forensics class in Stockholm. One of the topics covered in this training was how to detect Man-on-the-Side attacks in full content PCAP files.

Man-on-the-Side Explained A Man-on-the-Side (MOTS) attack has the following two characteristics:
  • The attacker can read the traffic and insert new messages, but not to modify or delete messages sent by other participants.
  • The attacker relies on a timing advantage to make sure that the response he sends to the request of a victim arrives before the legitimate response.

In practice this means that the attacker relies on packet injection to insert a TCP packet with a payload to be executed by the victim, such as an HTTP redirect to a malicious web site (source The Intercept). The TCP sequence number of this injected packet will typically be the same as that in the real HTTP response coming from the legitimate web server. Thus, the end node will see two overlapping TCP segments with different application layer data.

In one of the labs, in the network forensics training, students were tasked with finding a Man-on-the-Side attack in a 2.3 GB PCAP dataset. However, the way this MOTS attack was carried out made it invisible to normal signatures designed to detect TCP stream overlaps with different data, such as the Suricata signature 2210050.

alert tcp any any -> any any (msg:"SURICATA STREAM reassembly overlap with different data"; stream-event:reassembly_overlap_different_data; classtype:protocol-command-decode; sid:2210050; rev:2;)

The reason why Suricata and other methods fail to detect this attack is because the injected packet contained both application layer data (an HTTP redirect) and a TCP FIN flag. Upon receiving this spoofed packet the client (victim) followed the redirect as well as closed down its current TCP socket to the web server, by responding with a FIN+ACK packet. Subsequent packets sent by the real web server were then ignored by the client since the TCP socket was already closed when they arrived.

Stream reassembly engines in intrusion detection systems also ignore packets sent after the TCP tear-down, since the TCP session is assumed to be closed at this point. Overlapping TCP segments with different data are therefore not detected by intrusion detection systems when an injected TCP packet carries the FIN flag. I've created an example PCAP file, which illustrate this behavior, called mots-with-fin.pcap (this is not the MOTS attack analyzed in my training). Here's what the PCAP file looks like when analyzed with Tshark:

tshark -r mots-with-fin.pcap -T fields -e ip.src -e ip.dst -e ip.ttl -e tcp.seq -e tcp.flags -e http.response.code -e http.response.phrase
10.0.1.4       91.225.248.129 64 189665416  0x0002
91.225.248.129 10.0.1.4       54 4114717473 0x0012
10.0.1.4       91.225.248.129 64 189665417  0x0010
10.0.1.4       91.225.248.129 64 189665417  0x0018
91.225.248.129 10.0.1.4       64 4114717474 0x0019 302 Found <--INJECTED
10.0.1.4       91.225.248.129 64 189665756  0x0010
91.225.248.129 10.0.1.4       54 4114717474 0x0010
10.0.1.4       91.225.248.129 64 189665756  0x0011
91.225.248.129 10.0.1.4       54 4114717474 0x0018 301 Moved Permanently

Frame number 5 is the injected “302 Found” packet spoofed by the attacker. The TCP flag value 0x19 translates to FIN+PUSH+ACK, which is the attackers attempt to tear-down the TCP connection. The client responds with a FIN+ACK (0x11) in frame 8. The final frame is the real HTTP response coming from the legitimate web server.


Detecting MOTS Attacks

Martin Bruse was one of the guys taking the network forensics class last week. After realizing that there currently doesn't seem to exist any effective method for automatically detecting TCP segment overlaps with different data, regardless of the TCP state, Martin developed a tool called qisniff. This is what it looks like when mots-with-fin.pcap is analyzed with qisniff:

go run qisniff.go -file mots_with_fin.pcap
-
91.225.248.129:80(http)->10.0.1.4:54015 4114717474
<A>
HTTP/1.1 302 Found
Location: //www.netresec.com
Content-Length: 0


</A>
<B>
HTTP/1.1 301 Moved Permanently
Date: Tue, 21 Apr 2015 00:40:01 GMT
X-
</B>

In the output above we can see the injected content <A> and the legitimate content from the real web server <B>. What qisniff does is basically reassembling streams and comparing the application layer data in new TCP segments with that in previously received segments. This is a very generic way of detecting any form of packet injection in a TCP stream, regardless if it is done as part of a Quantum Insert attack, an Airpwn injection or some brand new packet injection attack.

Martin's qisniff tool is open sourced under a GPLv2 license and is available on GitHub here: https://github.com/zond/qisniff

To run qisniff you need to have Go 1.5 installed as well as gopacket.


Credits

We would like to thank Fox-IT for publishing their great blog post Deep dive into QUANTUM INSERT, in which they shed some light on many technical details of Man-on-the-Sida attacks as well as published IDS signatures designed to detect such attacks.


UPDATE 2016-02-02

David Stainton has updated his HoneyBadger tool, which is specifically designed detect TCP injection attacks, so that it now also detects injected TCP packets with the FIN flag set. The update was released on January 31, in update 1457755.

HoneyBadger detecting an injected TCP packet with FIN flag Image: HoneyBadger detecting injected packet in the mots-with-fin.pcap file we released.

UPDATE 2016-10-25

I have now released my own tool called "findject", which is a simple python script that can detect packet injection attacks like QUANTUM INSERT. You can read more about how to detect this type of attacks with findject in my blog post "Detect TCP content injection attacks with findject".

findject logo

Posted by Erik Hjelmvik on Monday, 21 September 2015 08:23:00 (UTC/GMT)

Tags: #MOTS #PCAP #TCP #Suricata #stream

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Keyword Search in PCAP files

Sherlock Holmes and Magnifying Glass via Inside Croydon A new function in the free version of CapLoader 1.2 is the "Find Keyword" feature. This keyword search functionality makes it possible to seek large capture files for a string or byte pattern super fast!

You might say, so what? PCAP string search can already be done with tools like tcpflow, ngrep and even Wireshark; what's the benefit of adding yet another tool to this list? One benefit is that CapLoader doesn't just give you the packet or content that matched the keyword, it will instead extract the whole TCP or UDP flow that contained the match. CapLoader also supports many different encodings, which is demonstrated in this blog post.

Here are a few quick wins with CapLoader's keyword search feature:

  • Track User-Agent - Search for a specific user agent string to extract all the HTTP traffic from a particular browser or malware.
  • Track Domain Name - Search for a particular domain name to get all DNS lookups as well as web traffic relating to that domain (including HTTP "referer" field matches).
  • Extract Messages - Search for a keyword in e-mail or chat traffic to get the whole e-mail or conversation, not just the single packet that matched.
  • Extract Files - Search for a unique string or byte sequence in a file (such as a piece of malware) to enable extraction of the complete file transfer.

EXAMPLE: DigitalCorpora M57

As an example, let's search the digital corpora file net-2009-12-06-11:59.pcap (149 MB) for the keyword "immortal". Follow these steps in order to veify our analysis using the free edition of CapLoader.

  1. Start CapLoader and select File -> Open URL, enter:
    http://digitalcorpora.org/corp/nps/scenarios/2009-m57-patents/net/net-2009-12-06-11:59.pcap.gz
  2. Edit -> Find Keyword (or Ctrl+F), enter "immortal" CapLoader Find Keyword Form
  3. Click the "Find and Select All Matching Flows" button
  4. One TCP flow is now selected (Flow_ID 5469, 192.168.1.104:2592 -> 192.168.1.1:25) CapLoader with one selected flow
  5. Right click the selected flow (ID 5469) and select "Flow Transcript"
CapLoader Flow Transcript of SMTP email attachment

CapLoader transcript of SMTP email flow

Looks as if an email has been sent with an attachment named "microscope1.jpg". However, the string "immortal" cannot be seen anywhere in the transcript view. The match that CapLoader found was actually in the contents of the attachment, which has been base64 encoded in the SMTP transfer in accordance with RFC 2045 (MIME).

The email attachment can easily be extracted from the PCAP file using NetworkMiner. However, to keep things transparent, let's just do a simple manual verification of the matched data. The first three lines of the email attachment are:

/9j/4AAQSkZJRgABAQEAkACQAAD/2wBDAAEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEB
AQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQH/2wBDAQEBAQEBAQEBAQEBAQEB
AQEBAQEBAQEBAQEBAQEBAQEBAQFwYXNzd29yZD1pbW1vcnRhbAEBAQEBAQEBAQEBAQH/wAAR
Decoding this with base64 gives us:
0000000: ffd8 ffe0 0010 4a46 4946 0001 0101 0090 ......JFIF......
0000010: 0090 0000 ffdb 0043 0001 0101 0101 0101 .......C........
0000020: 0101 0101 0101 0101 0101 0101 0101 0101 ................
0000030: 0101 0101 0101 0101 0101 0101 0101 0101 ................
0000040: 0101 0101 0101 0101 0101 0101 0101 0101 ................
0000050: 0101 0101 0101 0101 01ff db00 4301 0101 ............C...
0000060: 0101 0101 0101 0101 0101 0101 0101 0101 ................
0000070: 0101 0101 0101 0101 0101 0101 0101 0101 ................
0000080: 7061 7373 776f 7264 3d69 6d6d 6f72 7461 password=immorta
0000090: 6c01 0101 0101 0101 0101 0101 0101 ffc0 l...............

Tools like ngrep, tcpflow and Wireshark won't find any match for the string "immortal" since they don't support searching in base64 encoded data. CapLoader, on the other hand, supports lots of encodings.

Supported Text Encodings

CapLoader currently supports fast searching of text strings in any of the following encodings:

  • ASCII
  • Base64 (used in email attachments and HTTP POST's)
  • DNS label encoding (RFC 1035)
  • HTML
  • Quoted Printable (used in body of email messages)
  • Unicode
  • URL encoding
  • UTF8

CapLoader also supports several local character sets, including the following code pages:

  • 437 MS-DOS Latin US
  • 850 MS-DOS Latin 1
  • 932 Japanese
  • 936 Simplified Chinese
  • 949 Korean
  • 1251 Windows Cyrillic (Slavic)
  • 1256 Windows Arabic

Having all these encodings also makes it possible to search network traffic for words like хакер, القراصنة, ハッカー, 黑客 or 해커.

The Art of War by Sun Tzu

Getting CapLoader

CapLoader is a commercial tool that also comes in a free trial edition. The search feature is available in both versions, so feel free to download CapLoader and try it your self!

CapLoader is available from the following URL:
http://www.netresec.com/?page=CapLoader

Posted by Erik Hjelmvik on Wednesday, 02 April 2014 13:15:00 (UTC/GMT)

Tags: #search #find #keyword #flow #stream #PCAP #SMTP #transcript #free #network

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Recommended Books

» The Practice of Network Security Monitoring, Richard Bejtlich (2013)

» Applied Network Security Monitoring, Chris Sanders and Jason Smith (2013)

» Network Forensics, Sherri Davidoff and Jonathan Ham (2012)

» The Tao of Network Security Monitoring, Richard Bejtlich (2004)

» Practical Packet Analysis, Chris Sanders (2017)

» Windows Forensic Analysis, Harlan Carvey (2009)

» TCP/IP Illustrated, Volume 1, Kevin Fall and Richard Stevens (2011)

» Industrial Network Security, Eric D. Knapp and Joel Langill (2014)