NETRESEC Network Security Blog - Tag : periodicity

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Detecting Cobalt Strike and Hancitor traffic in PCAP

This video shows how Cobalt Strike and Hancitor C2 traffic can be detected using CapLoader.

I bet you’re going:

😱 OMG he’s analyzing Windows malware on a Windows PC!!!

Relax, I know what I’m doing. I have also taken the precaution of analyzing the PCAP file in a Windows Sandbox, which just takes a couple of seconds to deploy and run.

The capture file I’m looking at is called “2021-05-13-Hancitor-traffic-with-Ficker-Stealer-and-Cobalt-Strike.pcap” and can be downloaded from here:

CapLoader’s Services tab shows us that the connections to TCP 80 and 443 on are very periodic, with a detected period of exactly 1 minute. CapLoader successfully identifies the protocols for these two services as Cobalt Strike over HTTP and Cobalt Strike over SSL, respectively. The third service in this list is also very periodic, that’s the Hancitor trojan beaconing to its C2 server every two minutes.

Services tab in CapLoader

CapLoader uses machine learning to identify the application layer protocol based on the behavior of the traffic, not the port number. This means that there can be false positives, i.e. the protocol classification that CapLoader gives a flow or service might be wrong. It is more common, however, for CapLoader to yield false negatives, which means that it can't identify the protocol. The detection of Cobalt Strike inside of HTTP and SSL traffic was recently introduced in the latest 1.9 release of CapLoader. I expected this feature to detect Cobalt Strike traffic in HTTP, but I was delighted to see that CapLoader often detects even TLS encrypted Cobalt Strike beaconing with really good precision!

As shown in the video, the Cobalt Strike beacon config can easily be extracted from the network traffic using NetworkMiner and Didier Stevens’ 1768 K python script.

The output from Didier’s tool looks something like this:

0x0001 payload type 0 windows-beacon_http-reverse_http
0x0002 port 80
0x0003 sleeptime 60000
0x0004 maxgetsize 1048576
0x0005 jitter 0
0x0007 publickey 30819f30[...]
0x0008 server,get-uri ',/ca'

As you can see, it uses HTTP for transport with a “sleeptime” of 1 minute (60000 ms) and 0% jitter. This means that a new connection will be made to the Cobalt Strike C2 server every minute. The fact that there was no jitter is what gives this service such a high value in CapLoader’s “Periodicity” column.

Network Forensics Training

Are you interested in learning more about how to analyze network traffic from Cobalt Strike and other backdoors, malware and hacker tools? Then take a look at the live online network forensics classes I will be teaching in September and October!

Posted by Erik Hjelmvik on Monday, 31 May 2021 08:30:00 (UTC/GMT)

Tags: #Netresec #Cobalt Strike #CobaltStrike #periodicity #Protocol Identification #PIPI #CapLoader #Windows Sandbox #PCAP #NSM #video #videotutorial

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CapLoader 1.9 Released

CapLoader 1.9 Logo

A new version of the PCAP filtering tool CapLoader has been released today. The new CapLoader version 1.9 is now even better at identifying protocols and periodic beacons than before. The user interface has also been improved to make it easier to filter and drill down in network traffic to extract interesting, malicious or unusual traffic.

More Protocols Identified

We’ve added port-independent protocol detection for over 20 new protocols since the last release. The newly added protocols include some that are used by malicious tools and backdoors such as hTran, RevengeRAT, Tofsee and Winsecsrv, as well as legitimate protocols like WireGuard (VPN) and RemoteFX (UDP based remote desktop). We’ve also improved our support for ICS traffic analysis by adding protocol identification of SCADA protocols DNP3 and IEC 60870-5-104.

CapLoader also detects what we call “sub-protocols”, which are communication protocols that use other L7 protocols as transport. We have extended the sub-protocol detection in CapLoader 1.9 to include traffic like Anchor_DNS and dnscat traffic, which both run on top of DNS. We have also added detection of Cobalt Strike beacons over HTTP and HTTPS, even though the latter is quite difficult to detect due to the application data being encrypted.

Improved Usability

CapLoader 1.9 comes with several user interface improvements that help you solve the “needle in the haystack” problem even more efficiently than before.

The context menus in the Flows, Services and Hosts tabs can now be used to select rows based on values in any column, such as “Select all flows where Duration > 10 minutes” (when right-clicking a 10 minute flow).

The “Keyword Filter” is now called “Row Filter” in order to avoid getting it mixed up with the “Find Keyword” feature. The Row Filter has also been enhanced with a new filtering mode, to complement the Contains / All Words / Any Words / RegEx options, which is called “Column Criteria”. The Column Criteria can be used to filter the displayed rows based on the values in a user-specified column. The Column Criteria “Duration > 00:10:00” will, for example, only show flows that are 10 minutes or longer, while “ASN = 3301” shows the flows going to Telia’s AS3301.

CapLoader 1.9 with Column Criteria Row Filter Duration > 00:10:00

Image: CapLoader with Row Filter Column Criteria "Duration > 00:10:00"

We have also extended CapLoader's BPF implementation to support VLAN id’s, so that you can use expressions like “vlan 100” as input filter as well as display filter. The BPF implementation also supports logic operators, so that more advanced filters like “(tcp port 80 or port 443) and not net” can be used.

CapLoader has a method for detecting periodic connection patterns, which was introduced in CapLoader 1.4. This feature can be used to detect clients that connect to a service at regular intervals, such as a beacon used for command-and-control or email client connecting to a mail server. We have improved the periodicity detection in CapLoader 1.9 so that it now detects periodic services more accurately.

The Initial Round Trip Time (iRRT) in the Flows and Services tabs is now measured in milliseconds instead of seconds in order to avoid “bulky numbers” (h/t Eddi).

There was previously a significant delay when selecting many flows at once (like 100.000). We’ve improved the performance of this feature in CapLoader 1.9, so that you can now select several hundred thousands flows at once without having to wait for an unresponsive GUI to update.

More OSINT Lookup Services

A feature in CapLoader that often comes in handy is the ability to right-click a flow, service or host and open a website with OSINT information about the clicked IP address or domain name. We have now replaced some of the OSINT services with new better ones.

The new services we’ve added to CapLaoder 1.9 for performing online OSINT lookups of IP addresses, network services and domain names are:

Bug fixes and Credits

Several bugs have been fixed in this new release of CapLoader, much thanks to feedback we’ve received from our users. We’d like to thank Anders Regert and Mandy van Oosterhout for reporting bugs in CapLoaders “Save As” feature. We’d also like to thank Hyun Dowon for reporting a snap length corruption bug that previously appeared when exporting flows from Pcap-NG files We have also fixed an issue where capture files were previously not always merged in chronological order when being aggregated.

Updating to the Latest Release

Users who have purchased a license for CapLoader can download a free update to version 1.9 from our customer portal. All others can download a free 30 day trial from the CapLoader product page (no registration required).

Posted by Erik Hjelmvik on Tuesday, 25 May 2021 12:20:00 (UTC/GMT)

Tags: #Netresec #CapLoader #PCAP #Pcap-NG #IEC-104 #CobaltStrike #BPF #periodicity #OSINT

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Detecting Periodic Flows with CapLoader 1.4

CapLoader 1.4 logo

I am happy to announce a new release of our super-fast PCAP handling tool CapLoader! One of the new features in CapLoader makes it even easier to detect malicious network traffic without having to rely on blacklists, such as IDS signatures.

The new version of CapLoader includes new features such as:

  • Services Tab (more details below)
  • Input filter to limit number of parsed frames
  • Flow Transcript in Hosts and Services tabs
  • Keyword filtering
  • Full filtering capability for all tabs
  • Wireshark style coloring of flows, services and hosts

Services Tab

The biggest addition to version 1.4 of CapLoader is the Services tab, which presents a somewhat new way of aggregating the flows found in a PCAP file. Each row (or “service”) in the services tab represents a unique combination of <Client-IP, Server-IP, Server-port and Transport-protocol>. This means that if a single host makes multiple DNS requests to, then all those flows will be merged together as one row in the services tab.

CapLoader Services tab showing DNS requests to

This view makes it easy to see if a host is frequently accessing a particular network service. CapLoader even shows if the requests are made with regular intervals, in which case we measure the regularity and determine the most likely period between connections. The idea for measuring regularity comes from Sebastian Garcia's Stratosphere IPS, which can identify botnets by analyzing the periodicity of flows going to a C2 server.

Malware Example: Kovter.B

Here's what the Services tab looks like when loading 500 MB of PCAP files from a network where one of the hosts has been infected with malware (Win32/Kovter.B).

CapLoader service ordered on regularity

The services in the screenshot are sorted on the “Regularity” column, so that the most periodic ones are shown at the top. Services with a regularity value greater than 20 can be treated as periodic. In our case we see the top two services having a regularity of 36.9 with an estimated period of roughly 6h 2min. We can visualize the periodic behavior by opening the flows for those two services in a new instance if CapLoader. To do this, simply select the two services' rows, right-click the PCAP icon (in the top-right corner) and select “Open With > CapLoader”

CapLoader Flows tab with periodicly accessed service

As you can see in the flows tab, these services are accessed by the client on a regular interval of about 6h 2min. Doing a flow transcript of one such flow additionally reveals that the payload seems suspicious (not HTTP on TCP 80).

CapLoader transcript of Kovter.B C2 attempt (hex)
Image: Kovter.B malware trying to communicate with a C2 server

The Kovter malware failed to reach the C2 server in the attempt above, but there is a successful connection going to a C2 server at every 3'rd hour (see service number 8 in the list of the most periodically accessed services). Here's a flow transcript of one such beacon:

CapLoader Transcript of Kovter.B C2 traffic
Image: Kovter.B malware talking to C2 server at

Legitimate Periodic Services

Seven out of the 10 most periodically accessed services are actually caused by the Kovter malware trying to reach various C2 servers. The three most periodically accessed services that aren't malicious are:

  • Service #3 is a legitimate Microsoft service (SeaPort connecting to
  • Service #5 is a mail client connecting to the local POP3 server every 30 minutes.
  • Service #6 is Microsoft-CryptoAPI updating its Certificate Revocation List from every 5 hours.

Signature-Free Intrusion Detection

As shown in this blog post, analyzing the regularity of services is an efficient way of detecting C2 beacons without having to rely on IDS signatures. This method goes hand-in-hand with our Rinse-Repeat Intrusion Detection approach, which can be used to find malicous network traffic simply by ignoring traffic that seems “normal”.


Several bugs have been fixed in CapLoader 1.4, such as:

  • Support for frames with Captured Length > Real Lenght (Thanks to Dietrich Hasselhorn for finding this bug)
  • Delete key is no longer hijacked by the “Hide Selected Flows” button (Thanks to Dominik Andreansky for finding this bug).
  • CapLoader GUI now looks okay even with graphics are scaled through "custom sizing". Thanks to Roland Wagner for finding this.

Downloading CapLoader 1.4

The regularity and period detection is available in our free trial version of CapLoader. To try it out simply grab a copy here: (no registration needed)

All paying customers with an older version of CapLoader can grab a free update to version 1.4 at our customer portal.

UPDATE June 2, 2016

We're happy to announce that it is now possible to detect Kovter's C2 communication with help of an IDS signature thanks to Edward Fjellskål. Edward shared his IDS signature "NT TROJAN Downloader/Malware/ClickFraud.Win32.Kovter Client CnC Traffic" on the Emerging-Sigs mailing list yesterday. We have worked with Edward on this and the signature has been verified on our Kovter C2 dataset.

UPDATE June 8, 2016

Edward Fjellskål's IDS signature "ET TROJAN Win32.Kovter Client CnC Traffic" has now been published as an Emerging Threats open rule with SID 2022861.

#alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"ET TROJAN Win32.Kovter Client CnC? Traffic"; flow:established,to_server; dsize:4<>256; content:!"HTTP"; content:"|00 00 00|"; offset:1; depth:3; pcre:"/^[\x11\x21-\x26\x41\x45\x70-\x79]/R"; content:!"|00 00|"; distance:0; byte_jump:1,0,from_beginning,post_offset 3; isdataat:!2,relative; pcre:!"/\x00$/"; reference:url,; classtype:trojan-activity; sid:2022861; rev:1;)

Posted by Erik Hjelmvik on Monday, 23 May 2016 11:55:00 (UTC/GMT)

Tags: #CapLoader #PCAP #C2 #beacon #Intrusion Detection

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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)