How the SolarWinds Hack (almost) went Undetected

My lightning talk from the SEC-T 0x0D conference has now been published on YouTube. This 13 minute talk covers tactics and techniques that the SolarWinds hackers used in order to avoid being detected.

Some of these tactics included using DNS based command-and-control (C2) that mimicked Amazon AWS DNS traffic, blending in with SolarWind’s legitimate source code and handpicking only a small number of targets.

One thing I forgot to mention in my SEC-T talk though, was the speed at which the attackers were working to analyze incoming data from the trojanized installs and selecting organizations to target for stage two operations.

For example, just during June 2020 the attackers got more than 1300 new organizations that started beaconing in using the DNS-based C2. The beaconed data only included the organizations’ Active Directory domain name and a list of installed security applications. Based on this information the attackers had to decide whether or not they wanted to target the organization. We have previously estimated that less than 1% of the organizations were targeted, while the malicious backdoor was disabled for all the other 99% who had installed the trojanized SolarWinds Orion update.

The attackers typically decided whether or not to target an organization within one week from when they started beaconing. This means that the attackers probably had several hundred organizations in queue for a targeting decision on any given week between April and August 2020. That's a significant workload!

Posted by Erik Hjelmvik on Monday, 18 October 2021 10:30:00 (UTC/GMT)

Tags: #SolarWinds​ #SEC-T​ #video​ #backdoor​ #SUNBURST​ #Solorigate​ #STAGE2​ #Stage 2​ #DNS​ #C2​ #ASCII-art​

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.

UPDATE March 1, 2021

Fixed errors in the Stage 2 beacon structure and added a CyberChef recipe link.

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.

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:

Image: Stage 2 beacon structure of the SolarWinds backdoor

The base32 encoding not only uses a custom alphabet, it also employs a reversed endianess and byte order compared to "normal" implementations. We have created a CyberChef recipe that performs this custom base32 decoding, so that the structure can be verified more easily. A list with 45 different Stage 2 avsvmcloud.com subdomains can be found in our Finding Targeted SUNBURST Victims with pDNS blog post. Feel free to replace the input to our CyberChef recipe with any of those subdomains.

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.

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​ #backdoor​ #SUNBURST​ #Solorigate​ #FireEye​ #Microsoft​ #CNAME​ #STAGE2​ #Stage 2​ #DNS​ #avsvmcloud.com​ #C2​ #CyberChef​ #ASCII-art​

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:

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:

• central.pima.gov (confirmed)
• cisco.com (confirmed)
• corp.qualys.com (confirmed)
• coxnet.cox.com (confirmed)
• ddsn.gov
• fc.gov
• fox.local
• ggsg-us.cisco.com (confirmed)
• HQ.FIDELIS (confirmed)
• jpso.gov
• lagnr.chevrontexaco.net
• logitech.local
• los.local
• mgt.srb.europa* (confirmed)
• ng.ds.army.mil
• nsanet.local (not the NSA)
• paloaltonetworks* (confirmed)
• phpds.org
• scc.state.va.us (confirmed)
• suk.sas.com
• vgn.viasatgsd.com
• wctc.msft
• WincoreWindows.local

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.

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.

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​

Finding Targeted SUNBURST Victims with pDNS

Our SunburstDomainDecoder tool can now be used to identify SUNBURST victims that have been explicitly targeted by the attackers. The only input needed is passive DNS (pDNS) data for avsvmcloud.com subdomains.

Companies and organizations that have installed trojanized a SolarWinds Orion update containing the SUBURST backdoor will send DNS queries for seemingly random subdomains of avsvmcloud.com. Some of these DNS queries actually contain the victim's internal AD domain encoded into the subdomain, as explained in our blog post Reassembling Victim Domain Fragments from SUNBURST DNS.

Three Stages of SUNBURST Backdoor Operation

Most SUNBURST victims were luckily not targeted by the attackers. This means that the backdoor never made it past "STAGE1" of the infection process. Nevertheless, the attackers did choose to proceed to "STAGE2" with some victims. As explained in FireEye's blog post SUNBURST Additional Technical Details, the "C2 coordinator" can proceed to the next stage by responding with a DNS A record pointing to an IP address within any of these three ranges:

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

According to FireEye's "Diagram of actor operations and usage of SUNBURST", the decision to proceed to the next stage is based upon whether or not the victim's internal AD domain is "interesting to attack".

Note: "STAGE2" is referred to as "associated mode" in FireEye's blog post.

SUNBURST backdoors that have entered STAGE2 will allow CNAME records in DNS responses to be used as new C2 domains.

We have discovered that the SUNBURST backdoor actually uses a single bit in the queried avsvmcloud.com subdomain in order to flag that it has entered STAGE2 and is accepting new C2 domains in CNAME records. This bit is called flag, ext or dnssec in the malicious SUNBURST implant and can be extracted from DNS queries that have an encoded timestamp, such as those indicating which security products that are installed.

Detecting STAGE2 DNS Requests

Our SunburstDomainDecoder tool has now been updated to include a "STAGE2" tag in the output for DNS queries containing this stage 2 flag. This means that organizations like national CERTs, who perform incident response coordination and victim notification, can now use SunburstDomainDecoder in order to identify and notify targeted SUNBURST victims that have entered STAGE2.

Here's the output we get when feeding SunburstDomainDecoder with Bambenek's uniq-hostnames.txt passive DNS data and only displaying lines containing "STAGE2":

Most of these subdomains are listed in FireEye's Indicator_Release_NBIs.csv file as having CNAME pointers to other SUNBURST C2 domains like: freescanonline[.]com, deftsecurity[.]com and thedoccloud[.]com. But the first domain, with GUID 22334A7227544B1E, was actually not part of FireEye's IOC data.

Even more STAGE2 domains and GUID values can be found by analyzing other passive DNS resources, such as this passive DNS dump on pastebin by Rohit Bansal.

After removing the domains already present in FireEye's IOC we're left with the following FQDN's that have been requested by SUNBURST backdoors in STAGE2:

• 1dbecfd99ku6fi2e5fjb.appsync-api.us-east-1.avsvmcloud.com
• 4n4vte5gmor7j9lpegsf.appsync-api.eu-west-1.avsvmcloud.com
• 5qbtj04rcbp3tiq8bo6t.appsync-api.us-east-1.avsvmcloud.com

Update January 7, 2021

Paul Vixie kindly shared his SunburstDomainDecoder output on Twitter yesterday. Paul's results show that the victim with GUID FC07EB59E028D3EE, which corresponds to the "6a57jk2ba1d9keg15cbg.appsync-api.eu-west-1.avsvmcloud[.]com" CNAME entry in FireEye's IOC, was Pima County. This means that 3C327147876E6EA4 is the only GUID among the CNAME records published by FireEye that cannot yet be tied to a victim organization. Paul's data also reveals two new STAGE2 victim GUIDs (65A28A36F24D379D and 8D2267C5A00796DA).

Update January 12, 2021

With help of SunburstDomainDecoder 1.9 and passive DNS data from Dancho Danchev we've been able to verify that Palo Alto have installed the maliocous SUNBURST backdoor and that it entered into STAGE2 opreration on September 29, 2020. Palo Alto's CEO Nikesh Arora has confirmed that they were hit by SUNBURST (or "SolarStorm" as they call it).

Update January 25, 2021

On December 17 VriesHd tweeted a link to a Google Docs spreatsheet containing aggregated SUNBURST DNS request data.

One month later VriesHd made some substatial additions to the "SB2" spreadsheet, which by then contained several new STAGE2 victims. We have since then actively been trying to reach out to the targeted organizations, either directly or through CERT organizations, who perform incident response coordination and help with the victim notification process. VriesHd's passive DNS collection has now been incorporated into the SUNBURST STAGE2 Victim Table below.

Targeted SUNBURST Victims

Here's a summary of the STAGE2 beacons from SUNBURST victims that can be extracted from publicly available data:

SUNBURST STAGE2 Victim Table
Sources: John Bambenek, Joe Słowik, Rohit Bansal, Dancho Danchev , Paul Vixie, FireEye and VriesHd.

Identifying More SUNBURST STAGE2 Victims

Companies and organizations with access to more passive DNS resources will hopefully be able to use SunburstDomainDecoder to identify additional targeted SUNBURST victims that have progressed to STAGE2.

Download SunburstDomainDecoder

Our tool SunburstDomainDecoder is released under a Creative Commons CC-BY license, and can be downloaded here:

You can also read more about SunburstDomainDecoder in our blog post Reassembling Victim Domain Fragments from SUNBURST DNS.

Posted by Erik Hjelmvik on Monday, 04 January 2021 21:11:00 (UTC/GMT)

Tags: #Netresec​ #pDNS​ #SUNBURST​ #SolarWinds​ #Solorigate​ #SunburstDomainDecoder​ #SolarStorm​ #STAGE2​ #avsvmcloud​ #C2​

Extracting Security Products from SUNBURST DNS Beacons

The latest version of our SunburstDomainDecoder (v1.7) can be used to reveal which endpoint protection applications that are installed on trojanized SolarWinds Orion deployments. The security application info is extracted from DNS queries for "avsvmcloud.com" subdomains, which is used by SUNBURST as a beacon and C2 channel.

Here's an example showing that City of Kingston, Ontario, Canada were running Windows Defender on their trojanized SolarWinds deployment back in June:

The "F9A9387F7D252842" value is the victim's unique SUNBURST GUID. See our blog post Reassembling Victim Domain Fragments from SUNBURST DNS for more info about how the GUID value is encoded into the DNS traffic.

You can also run SunburstDomainDecoder in Linux, with help of Mono, like this:

The file "uniq-hostnames.txt" is a publicly available SUNBURST passive DNS repository created by Bambenek Consulting.

Security Product Statistics

It is also possible to use the passive DNS data shared by Bambenek, Joe Słowik and others to compute statistics of which security products that are popular among SolarWinds' customers.

It is worth mentioning that SUNBURST does not report status for several other major endpoint protection vendors, such as Kaspersky, McAfee, Symantec, Sophos or Trend Micro.

Download SunburstDomainDecoder

Our tool SunburstDomainDecoder is released under a Creative Commons CC-BY license, and can be downloaded here:
https://www.netresec.com/files/SunburstDomainDecoder.zip

You can also read more about SunburstDomainDecoder in our blog post Reassembling Victim Domain Fragments from SUNBURST DNS.

Posted by Erik Hjelmvik on Tuesday, 29 December 2020 09:38:00 (UTC/GMT)

Tags: #SunburstDomainDecoder​ #SUNBURST​ #SolarWinds​ #Solorigate​ #DNS​ #Windows Defender​ #Carbon Black​ #FireEye​ #ESET​ #F-Secure​ #C2​ #beacon​

Reassembling Victim Domain Fragments from SUNBURST DNS

We are releasing a free tool called SunburstDomainDecoder today, which is created in order to help CERT organizations identify victims of the trojanized SolarWinds software update, known as SUNBURST or Solorigate.

SunburstDomainDecoder can be fed with DNS queries to avsvmcloud.com in order to reveal the full internal domain names of infected companies and organizations.

UPDATE December 18, 2020 (v1.1)

SunburstDomainDecoder has now been updated to automatically reassemble fragmented domain name segments in order to show the full domain in the output.

UPDATE December 19, 2020 (v1.2)

Domain names that have been base32 encoded, such as domain names with uppercase letters, can now be extracted with SunburstDomainDecoder. The queried SUNBURST subdomains are now also included in the output.

UPDATE December 21, 2020 (v1.6)

Improved parsing of base32 encoded domain names. SUNBURST victim domains like "LKDataCenter.com", "Sunkistgrowers.com" and "BrokenArrow.Local" can now be extracted.

UPDATE December 27, 2020 (v1.7)

Improved reassembly of long domain names, like "CIMBMY.CIMBDomain.com" and "BE.AJINOMOTO-OMNICHEM.AD", that get segmented into multiple parts. Extraction of time stamps and security applications, including "Windows Defender", "Carbon Black", "CrowdStrike", "FireEye", "ESET" and "F-Secure". See Sergei Shevchenko's blog post Sunburst Backdoor, Part III: DGA & Security Software for more details.

UPDATE January 4, 2021 (v1.8)

Security products (WinDefend, ESET etc.) are now included in the summary output at the end. SUNBURST stage2 victims, which accept C2 domains in CNAME responses, are indicated with a "STAGE2" tag. The previous release marked stage2 queries with a "DNSSEC" tag. Improved extraction of truncated base32 domains, such as "*TED.com".

UPDATE January 12, 2021 (v1.9)

DNS queries with encoded timestamps are tagged with either "AVProducts" or "Ping", depending on if they include an update of the installed/running security products and services or not. The summary data at the end has been modified to also show partial domain names, such as "paloaltonetworks*".

UPDATE February 16, 2021 (v2.0)

Slightly faster and even more accurate than previous versions.

Download SunburstDomainDecoder.zip

SUNBURST DNS Traffic

SUNBURST victims, who have installed one of the trojanized SolarWinds Orion software updates, will query for domain names formatted like this:

The "SUBDOMAIN" string has different values for each victim and the second half of this string actually contains an encoded domain name (encrypted with a simple substitution cipher).

RedDrip's decode.py

The RedDrip Team published a SUNBURST DGA decoding script yesterday, which can be used to identify SUNBURST victim organizations like CISCO and Belkin by decoding the domain names encoded in the outgoing DNS queries for subdomains of avsvmcloud.com.

This is what it looks like when RedDrip's decode.py script is fed with domain names from John Bambenek's uniq-hostnames.txt file.

The beauty of this approach is that passive DNS data can be used in order to reliably identify the victims. This is great news for national CERTs, because they typically have readily access to passive DNS data and can use the decoded domain names in order to identify and reach out to victims in their country.

After using the python script provided by ReadDrip Team I noticed two things:

1. The leaked domain names were internal domain names used on the victim organizations' corporate networks. Many of the domains were using the ".local" suffix.
2. Most of the extracted domains were truncated to around 15 bytes, which make it difficult to identify the victim organization.

Truncated Domains Fragmented Domains

I later learned that what seemed to be truncated domains were actually fragmented domains, where long domain names would be split into multiple queries. This revelation turns the output from RedDrip's python tool into an interesting domain name puzzle. At this point I decided to take a closer look at the malicious SolarWinds update I had downloaded from SolarWind's website a few days ago -- yes, that's right the malicious software update "SolarWinds-Core-v2019.4.5220-Hotfix5.msp" (MD5: 02af7cec58b9a5da1c542b5a32151ba1) was actually available for download from SolarWinds' website long after they had been notified about their software being backdoored!

As an example, lets' take a closer look at this DNS query from John Bambenek's passive DNS data:
r1qshoj05ji05ac6eoip02jovt6i2v0c.appsync-api.us-west-2.avsvmcloud.com

This query can be broken down into three parts:

1. r1qshoj05ji05ac6 : What is encoded here???
2. eoip02jovt6i2v0c : Base32 encoded string "city.kingston."
3. .appsync-api.us-west-2.avsvmcloud.com : DNS trailer without encoded data

So, which "City of Kingston", or "Kingston City", should we contact to let them know that they have installed a trojanized SolarWinds update? Is it Kingston Jamaica, City of Kingston NY USA, City of Kingston Ontario Canada, Kingston City Tennessee USA or City of Kingston Australia?

After analyzing the "SolarWinds.Orion.Core.BusinessLayer.dll" file (MD5: b91ce2fa41029f6955bff20079468448) from the "SolarWinds-Core-v2019.4.5220-Hotfix5.msp" I learned that the initial "r1qshoj05ji05ac6" string is representing a unique "GUID" value for the infected machine. This GUID is generated by calculating an MD5 hash of the MAC address of the first active non-Loopback network interface, the domain name and the "MachineGuid" registry key value in "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Cryptography".

This MD5 hash is then squeezed into a tiny 8 byte array by XOR'ing overlapping bytes. The "CreateSecureString" function in the trojanized SolarWinds update then "encrypts" this hash using XOR with a random key, which is prepended to the data. The XOR key and the XOR'ed data is then finally base32 encoded into what makes up the first part of the subdomain to query for. Don't let the SUNBURST source code below fool you, it is actually using base32 encoding with a custom alphabet even though the function is called "Base64Encode";

Image: SUNBURST source code generates a random value between 1 and 127 as XOR key

Each DNS lookup from an infected machine will query for a unique subdomain because a new XOR key will be generated for each request. Luckily for us, this XOR key is provided in each request, so we can use it in order to "decrypt" the subdomain and get the original 8 bytes derived from the MAC+domain+MachineGuid MD5 hash.

The output from my "SunburstDomainDecoder.exe" tool will print the "decrypted" 8 byte GUID in the first column, the decoded victim domain segment or timestamp in the second column and the queried SUNBURST subdomain in the last column. Each DNS query line read from standard input will generate a "GUID DecodedHostname SunburstSubdomain" line on standard output.

The tool then finishes off by outputting the domains that are complete or at least have the last part of their domain intact. Some of these domains are complete because they were short enough to fit in one single SUNBURST DNS query, while others have been pieced together by SunburstDomainDecoder from domain fragments arriving in separate SUNBURST DNS queries.

We can now see that it was "city.kingston.on.ca", (City of Kingston, Ontario, Canada) who had installed a trojanized SolarWinds update.

Download SunburstDomainDecoder

The C# source code and a compiled Windows binary for SunburstDomainDecoder is available here:
https://www.netresec.com/files/SunburstDomainDecoder.zip

The source code and Windows binary is shared under a Creative Commons CC-BY license, which means that you are free to:

• Share : copy and redistribute the material in any medium or format
• Adapt : remix, transform, and build upon the material for any purpose, even commercially.

Running SunburstDomainDecoder on Linux/MacOS

Wanna run SunburstDomainDecoder.exe but not in Windows? No problems, the tool runs perfectly fine in Mono. Another option is to build SunburstDomainDecoder.cs as a .NET core project in Linux.

.NET Reversing

Would you like to verify my findings or learn more about .NET reverse engineering? Cool, then I'd recommend that you download dnSpy in order to reverse engineer the SUNBURST .NET DLL (which can be extracted from the msp installer with 7zip). Or you can have a look at the already extracted OrionImprovementBusinessLayer.cs on GitHub.

Posted by Erik Hjelmvik on Thursday, 17 December 2020 22:30:00 (UTC/GMT)

Tags: #SunburstDomainDecoder​ #SUNBURST​ #SolarWinds​ #Solorigate​ #domain​ #DNS​ #pDNS​ #Windows Defender​ #Carbon Black​ #FireEye​ #ESET​ #F-Secure​ #Trojan​ #avsvmcloud​