NETRESEC Network Security Blog - Tag : CNAME


Targeting Process for the SolarWinds Backdoor

The SolarWinds Orion backdoor, known as SUNBURST or Solorigate, has been analyzed by numerous experts from Microsoft, FireEye and several anti-virus vendors. However, we have noticed that many of the published reports are either lacking or incorrect in how they describe the steps involved when a client gets targeted by the threat actors. We have therefore decided to publish this writeup, which is based on the analysis we did of the SolarWinds backdoor when creating our SunburstDomainDecoder tool.

avsvmcloud.com DNS queries are not DGA related

The DNS communication between the backdoored SolarWinds Orion clients and the authoritative name server for avsvmcloud.com is not caused by a Domain Generation Algorithm (DGA), it's actually a fully functional two-way communication C2 channel. The clients encode information, such as the internal AD domain and installed security applications into the DNS queries and the DNS responses from the name server are used to instruct the clients to continue beaconing, stop beaconing or to target a client by proceeding to what we call Stage 2 operation. Thus, the authoritative name server for avsvmcloud.com was actually the C2 server for Stage 1 and 2 operation of the SolarWinds backdoor.

SolarWinds Backdoor State Diagram

Image: SolarWinds Backdoor State Diagram

Command: Continue Beaconing

The default response from the name server is the "Continue Beaconing" command, which indicates that the threat actors have not yet decided if the SolarWinds client is of interest for further activity. Receiving a DNS A record in any of the following net ranges instructs the SolarWinds backdoor to continue beaconing:

  • 8.18.144.0/23
  • 71.152.53.0/24
  • 87.238.80.0/21
  • 199.201.117.0/24

In "Stage 1" operation the SUNBURST client starts out in the "New" mode where it exfiltrates the internal AD domain name. The AD domain data is often split into multiple DNS queries to reduce the length of each DNS query. The client later proceeds to the "Append" mode when the full AD domain has been exfiltrated. In "Append" mode the client transmits a list of installed or running security applications to the DNS C2 server, as we have described in our Extracting Security Products from SUNBURST DNS Beacons blog post. The client remains in Append mode until it gets either terminated or targeted.

Note: It is also possible to reset a client back to the "New" mode with a so-called "Ipx" command, but that is out of scope for this blog post.

Command: Stop Beaconing

The stop beaconing command terminates the DNS beaconing, so that the client no longer retrieves any commands from the C2 server. The C2 communication is stopped after receiving a DNS DNS A or AAAA record in any of the following ranges:

  • 20.140.0.0/15
  • 96.31.172.0/24
  • 131.228.12.0/22
  • 144.86.226.0/24
  • 10.0.0.0/8
  • 172.16.0.0/12
  • 192.168.0.0/16
  • 224.0.0.0/3
  • fc00:: - fe00::
  • fec0:: - ffc0::
  • ff00::

Command: Target Client

A SUNBURST client that has been "targeted" will change a flag called rec.dnssec in the source code from false to true. We call this flag the "Stage 2" flag, which must be set in order for the client to accept a CNAME record and proceed to Stage 3. Symantec refer to the Stage 2 flag as "a bit flag representing whether the previous DNS response successfully contained partial or full instructions to start the secondary HTTP communication channel".

A DNS A record in any of the following three IP ranges can be used to set the "Stage 2" flag:

  • 18.130.0.0/16
  • 99.79.0.0/16
  • 184.72.0.0/15

The state of the Stage 2 flag is actually signaled in the avsvmcloud.com DNS queries, which is how we managed to identify the AD domains of 23 targeted organizations just by analyzing SUNBURST DNS queries.

Stage 2 DNS Request Structure

The structure of the SUNBURST DNS queries in Stage 1 is pretty well described by Prevasio and Symantec, so we will not cover those in this blog post. Instead we will focus specifically on the structure of the DNS queries transmitted in Stage 2 operation, where the clients request a CNAME record from the name server.

As we have explained previously the exfiltrated data gets base32 encoded, using the custom alphabet "ph2eifo3n5utg1j8d94qrvbmk0sal76c", in order to ensure that only valid domain name characters are used in the DNS beacons.

The structure of the Stage 2 request, before it gets base32 encoded and appended as an avsvmcloud.com subdomain, looks like this:

Field Size Description
XOR Key 8 bits A value between 0x80 and 0xff used to XOR encrypt the rest of the data.
GUID 64 bits Client ID encrypted using a 16 bit rotating XOR key from Packet Type and first 12 bits of Timestamp.
Packet Type 4 bits A value of 0x1, could in theory be 0x2 but that's very unlikely.
Timestamp 19 bits Number of 30 minute periods since start of 2010 (UTC).
Stage 2 Flag 1 bit A flag set to "1" in Stage 2 operation, otherwise "0".
SolarWinds Backdoor Stage 2 DNS Beacon Structure

Image: Stage 2 beacon structure of the SolarWinds backdoor

Sleep Timers

The DNS responses from the name server not only controls how the SolarWinds backdoor should transition between the various stages, it also controls for how long the backdoor should wait before sending the next DNS beacon.

The delay is assigned by AND-ing the last octet of the received IP address with bitmask 0x54. The result from the AND operation is then used to select a sleep interval in the table below, within which the client picks a random number of minutes to sleep.

AND Result Name Sleep Interval
0x00 1 hour 30-120 minutes
0x04 4 hours 240-300 minutes
0x10 8 hours 480-600 minutes
0x14 1 day 1440-1560 minutes
0x40 3 days 4320-5760 minutes
0x44 1 week 10020-10140 minutes
0x50 2 weeks 20100-20220 minutes
0x54 1 month 43140-43260 minutes

An exception to the table above is clients that have entered Stage 2, which will only wait one to three minutes before requesting a CNAME.

Example DNS C2 for a Non-Targeted Client

Below is an example of DNS queries and responses from a SUNBURST client that wasn't targeted by the threat actors. These particular queries and responses come from a post on SolarWinds' community forum.

  • 2020-07-04 00:03 UTC
    Query: if9prvp9o36mhihw2hrs260g12eu1 ⇒ AD domain "omeros.local"
    Response: 8.18.145.139 ⇒ sleep 1h, then Continue
  • 2020-07-04 01:08 UTC
    Query: hnhb3v1b37dvv09icg0edp0 ⇒ Carbon Black is running
    Response: 8.18.145.62 ⇒ sleep 1 day, then Continue
  • 2020-07-05 01:15 UTC
    Query: ea99hr2sfen95nkjlc5g ⇒ Nothing new to report
    Response: 8.18.144.150 ⇒ sleep 1 day, then Continue
  • 2020-07-06 02:42 UTC
    Query: 707gigk9vbc923hf27fe ⇒ Nothing new to report
    Response: 8.18.145.151 ⇒ sleep 1 day, then Continue
  • 2020-07-07 03:52 UTC
    Query: 6eivqct649pcg0g16ol4 ⇒ Nothing new to report
    Response: 20.140.84.127 ⇒ Stop DNS beacon

Note: Queried domain names in this list are subdomains of appsync-api.eu-west-1.avsvmcloud.com.

Example DNS C2 for a Targeted Client

Disclaimer: We have very few DNS queries and responses for targeted victims, hence the transactions below are improvised based on data from VriesHd, Joe Słowik and FireEye. Please view these transactions as an example of what the communication might look like for a targeted victim rather than what actually happened to this particular target.

  • 2020-06-11 04:00 UTC
    Query: r8stkst71ebqgj66ervisu10bdohu0gt ⇒ AD domain, part 1 "central.pima.g"
    Response: 8.18.144.1 ⇒ Sleep 1h, then Continue
  • 2020-06-11 05:00 UTC
    Query: ulfmcf44qd58t9e82w ⇒ AD domain, part 2 "ov"
    Response: 8.18.144.2 ⇒ Sleep 1h, then Continue
  • 2020-06-11 06:00 UTC
    Query: p50jllhvhmoti8mpbf6p2di ⇒ Nothing to report
    Response: 8.18.144.16 ⇒ Sleep 8h, then Continue
  • 2020-06-11 14:00 UTC
    Query: (?) ⇒ Nothing new to report
    Response: 8.18.144.17 ⇒ Sleep 8h, then Continue
  • 2020-06-11 22:35 UTC
    Query: j5uqlssr1hfqnn8hkf172mp ⇒ Nothing to report
    Response: 184.72.181.52 ⇒ Target client for Stage 2 operation (1-3 minutes sleep)
  • 2020-06-11 22:37 UTC
    Query: 7sbvaemscs0mc925tb99 ⇒ Client in Stage 2 operation, requesting CNAME
    Response: deftsecurity.com ⇒ CNAME for Stage 3 HTTPS C2 server

Note: Queried domains in this list are subdomains of appsync-api.us-west-2.avsvmcloud.com.

Conclusions

We hope this blog post clears up any misunderstandings regarding the targeting process of the SolarWinds backdoor and highlights the significance of the Stage 2 flag.

We warmly welcome any feedback or questions you might have regarding this writeup, please feel free to contact us or reach out to us through Twitter.

Posted by Erik Hjelmvik on Wednesday, 17 February 2021 20:22:00 (UTC/GMT)

Tags: #SolarWinds #SUNBURST #Solorigate #FireEye #Microsoft #CNAME #STAGE2 #DNS #avsvmcloud.com #C2 #ASCII-art

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Twenty-three SUNBURST Targets Identified

Remember when Igor Kuznetsov and Costin Raiu announced that two of the victims in FireEye's SUNBURST IOC list were ***net.***.com and central.***.gov on Kaspersky's Securelist blog in December? Reuters later reported that these victims were Cox Communications and Pima County.

We can now reveal that the internal AD domain of all SUNBURST deployments in FireEye's IOC list can be extracted from publicly available DNS logs published by twitter user VriesHd, a.k.a. "Kira 2.0", with help of our SunburstDomainDecoder tool. The data published by VriesHd is the most complete SUNBURST DNS collection we've seen, with over 35.000 avsvmcloud.com subdomains! Here is FireEye's IOC table completed with our findings:

Leaked AD Domain Sunburst C2 FQDN Stage 2 CNAME Timestamp (UTC)
central.pima.gov 6a57jk2ba1d9keg15cbg.appsync-api.eu-west-1.avsvmcloud.com freescanonline[.]com 2020-06-13 09:00
central.pima.gov 7sbvaemscs0mc925tb99.appsync-api.us-west-2.avsvmcloud.com deftsecurity[.]com 2020-06-11 22:30
central.pima.gov gq1h856599gqh538acqn.appsync-api.us-west-2.avsvmcloud.com thedoccloud[.]com 2020-06-13 08:30
coxnet.cox.com ihvpgv9psvq02ffo77et.appsync-api.us-east-2.avsvmcloud.com freescanonline[.]com 2020-06-20 02:30
corp.qualys.com k5kcubuassl3alrf7gm3.appsync-api.eu-west-1.avsvmcloud.com thedoccloud[.]com 2020-07-22 17:00
corp.qualys.com mhdosoksaccf9sni9icp.appsync-api.eu-west-1.avsvmcloud.com thedoccloud[.]com 2020-07-23 18:30

Victims Targeted with SUNBURST Stage 2 Backdoor

It was not just the victims listed in FireEye's IOC that were specifically targeted by the SUNBURST operators. As explained in our Finding Targeted SUNBURST Victims with pDNS blog post, the "STAGE2" flag in SUNBURST's DNS beacons can be used to reveal additional organizations that were singled out as interesting targets by the threat actors.

We'd like to stress that the majority of all companies and organizations that have installed a backdoored SolarWinds Orion update were never targeted by the threat actors. This means the these SUNBURST backdoors never made it past what we call "Stage 1 operation", where the backdoor encodes the internal AD domain name and installed security products into DNS requests. SUNBURST backdoors in Stage 1 operation cannot accept any commands from the C2 server without first progressing into Stage 2 operation. We estimate that about 99.5% of the installed SUNBURST backdoors never progressed into Stage 2 operation.

Here is the full list of internal AD domain names from the SUNBURST deployments in VriesHd's DNS data that actually did enter Stage 2 operation according to our analysis: 23 SUNBURST Targets Identified

Our SUNBURST STAGE2 Victim Table has now been updated with additional details about the STAGE2 signaling from these SUNBURST implants, including timestamps, avsvmcloud.com subdomains and GUID values.

Initial Microsoft Targeting FAIL

The last two entries in the AD domain list above are interesting, since they both hint that the targeted entity might be Microsoft.

The data that gets exfiltrated in DNS beacons during SUNBURST's initial stage is the internal domain the SolarWinds Orion PC is connected to and a list of installed security products on that PC. These domain names, security products and possibly also the victims' public IP addresses, was the data available to the attackers when they decided which ones they wanted to proceed to Stage 2 with and thereby activate the HTTPS backdoor built into SUNBURST.

The threat actors were probably surprised when they realized that "WincoreWindows.local" was in fact a company in West Virginia that manufactures high quality windows and doors.

Wincore Windows and Doors

The threat actors later found another backdoored SolarWinds Orion machine connected to a domain called "wctc.msft", which also sounds like it could be Microsoft. Below is a table outlining relevant events for these two SUNBURST deployments that can be extracted from VriesHd's SB2 spreadsheet with SunburstDomainDecoder.

Target ID Beaconed Data Date
A887B592B7E5B550 AD domain part 1: "WincoreW"
A887B592B7E5B550 AD domain part 2: "indows.local"
A887B592B7E5B550 AV Products: [none] 2020-05-22
🤔 Threat actor decision: Target victim A887B592B7E5B550
A887B592B7E5B550 STAGE2 request for new C2 server in CNAME 2020-05-26
🤔 Threat actor decision: These aren't the droids we're looking for
59956D687A42F160 AD domain: "wctc.msft"
59956D687A42F160 AV Products: [none] 2020-06-20
59956D687A42F160 Ping 2020-06-21
59956D687A42F160 Ping 2020-06-22
🤔 Threat actor decision: Target victim 59956D687A42F160
59956D687A42F160 STAGE2 request for new C2 server in CNAME 2020-06-23

Microsoft have been public about being hit by SUNBURST (or "Solorigate" as they call it), so we can assume that the threat actors eventually located a backdoored SolarWinds Orion installation in their networks.

Victim Notification

We spent the previous week reaching out to targeted companies and organizations, either directly or through CERT organizations. From what we understand many of these organizations were already aware that they had been targeted victims of SUNBURST, even though they might not have gone public about the breach.

The Ethical Dilemma

We have no intentions to shame the organizations that have installed a backdoored SolarWinds Orion update, regardless if they were targeted by the threat actor or not. In fact, the supply chain security problem is an extremely difficult one to tackle, even for companies and organizations with very high security standards. This could have happened to anyone!

However, since multiple passive DNS logs and SUNBURST victim lists have been circulating through publicly available channels for over a month, we felt that it was now acceptable to publicly write about the analysis we've been doing based on all this data. We'd also like to thank everyone who has helped collect and share passive DNS data, including John Bambenek, Joe Słowik, Rohit Bansal, Dancho Danchev , Paul Vixie and VriesHd. This open data has been crucial in order to develop and verify our SunburstDomainDecoder tool, which has been leveraged by numerous incident response teams to perform forensic analysis of DNS traffic from their SolarWinds Orion deployments.

More Credits

We'd like to thank CERT-SE and all other computer emergency response organizations that have helped us with the task of notifying organizations that were identified as targeted. We would also like to applaud companies and organizations like FireEye, Palo Alto Networks, Fidelis Cybersecurity, Microsoft, the U.S. Department of Energy and the U.S. Federal Courts for being transparent and publicly announcing that the SUNBURST backdoor had been used in an attempt to compromise their networks.

Posted by Erik Hjelmvik on Monday, 25 January 2021 08:25:00 (UTC/GMT)

Tags: #SUNBURST #FireEye #Solorigate #Microsoft #SolarWinds #FireEye #CNAME #STAGE2 #DNS #Passive DNS #avsvmcloud.com #pDNS #Microsoft

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Robust Indicators of Compromise for SUNBURST

Were you targeted by SUNBURST? Image credit: NASA

There has been a great deal of confusion regarding what network based Indicators of Compromise (IOC) SolarWinds Orion customers can use to self assess whether or not they have been targeted after having installed a software update with the SUNBURST backdoor. Many of the published IOCs only indicate that a backdoored SolarWinds Orion update has been installed, but the question that many security teams are trying to answer is whether or not the installed backdoor has been been used by the threat actor.

Dont trust everything you read!

There is a widespread misunderstanding that receiving a so-called “NetBios” DNS A record (for example an address in 8.18.144.0/23) in response to a *.avsvmcloud.com DNS query would mean that you’ve been targeted. Our analysis of the decompiled SUNBURST code and passive DNS data show that that receiving a “NetBios” response does not necessarily mean that the client has been targeted. Unfortunately this misunderstanding has lead to various sensationalist stories being published with long lists of companies and organizations that are claimed to be “singled out by the hacking group for the second stage of the attack”, “explicitly selected by the SolarWinds hackers for further activities” or “breached via SolarWinds and then specifically targeted by the hackers for additional internal compromise”.

Another common misunderstanding is that clients sending *.avsvmcloud.com DNS queries with encoded timestamps, and optionally a list of installed/running AV products, have been actively targeted. Our analysis of the decompiled SUNBURST code show that the timestamped “Pings” or AV service status reports get exfiltrated in DNS traffic after the client’s internal AD domain has been sent, but before the perpetrators decide whether or not they want to activate the backdoor.

Indicators of a Targeted Attack

So what network based IOC’s can incident responders, blue teams and SOC analysts use in order to see if they have been targeted by the SUNBURST operators?

The following network based events indicate that a client has been actively targeted and the SUNBURST backdoor has progressed beyond the initial mode of operation:

  • Received a DNS A record for an *.avsvmcloud.com query, that points to an IP address in any of the following three networks: 18.130.0.0/16, 99.79.0.0/16 or 184.72.0.0/15
  • Sent an *.avsvmcloud.com DNS query with the STAGE2 flag encoded in the subdomain.
  • Received a CNAME record for a query to *.avsvmcloud.com
These three indicators are DNS based, so organizations will need to have a full historical backlog of DNS transactions ranging back to April 2020 in order to use them reliably.

Another network based IOC is HTTPS communication to one of the known STAGE3 C2 domains. However, please note that the C2 domain list might not be complete. It is even possible that a unique C2 domain is used for each victim. Nevertheless, here’s a list of the SUNBURST STAGE3 C2 domains we are currently aware of:

  • avsvmcloud[.]com
  • databasegalore[.]com
  • deftsecurity[.]com
  • digitalcollege[.]org
  • freescanonline[.]com
  • globalnetworkissues[.]com
  • highdatabase[.]com
  • incomeupdate[.]com
  • kubecloud[.]com
  • lcomputers[.]com
  • mobilnweb[.]com
  • panhardware[.]com
  • seobundlekit[.]com
  • solartrackingsystem[.]net
  • thedoccloud[.]com
  • virtualwebdata[.]com
  • webcodez[.]com
  • websitetheme[.]com
  • zupertech[.]com

Palo Alto was a Targeted SUNBURST Victim

We can now verify that Palo Alto was among the targeted SUNBURST victims, because their DNS request for "5qbtj04rcbp3tiq8bo6t.appsync.api.us.east.1.avsvmcloud.com" contains an encoded STAGE2 flag. The attack took place on September 29 at around 04:00 UTC, according to the timestamp that was also encoded into the avsvmcloud subdomain.

paloaltonetworks SUNBURST STAGE2 detected by SunburstDomainDecoder

Image: Parsing passive DNS data from Dancho Danchev with SunburstDomainDecoder v1.9 and filtering on GUID “22334A7227544B1E”.

Palo Alto's CEO Nikesh Arora has confirmed that they were hit by SUNBURST (or "SolarStorm" as they call it), but they don’t provide much details. Here’s what Nikesh wrote on December 17:

Recently, we experienced an attempt to download Cobalt Strike on one of our IT SolarWinds servers. [...]

We thought this was an isolated incident, however, on Dec. 13, we became aware that the SolarWinds software supply chain was compromised and it became clear that the incident we prevented was an attempted SolarStorm attack.

Our SUNBURST STAGE2 Victim Table has now been updated to include Palo Alto along side the other targeted victims.

Posted by Erik Hjelmvik on Monday, 11 January 2021 10:30:00 (UTC/GMT)

Tags: #SUNBURST #SolarWinds #SolarStorm #avsvmcloud #STAGE2 #DNS #CNAME #avsvmcloud.com #DNS #FireEye

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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)