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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.

Video: Hiding in Plain Sight, How the SolarWinds Hack went Undetected

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.

SolarWinds Hack Timeline

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.

SolarWinds C2 IP addresses

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

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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.

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 0x01 and 0x7F used to XOR encrypt the rest of the data.
GUID 64 bits Client ID encrypted using 16 bit rotating XOR with the last 15 bits of Timestamp and the Stage 2 flag.
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

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.

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

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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.exe output showing innout.corp nswhealth.net cisco.com fa.lcl int.lukoil-international.uz tr.technion.ac.il bisco.int phabahamas.org banccentral.com bk.local htwanmgmt.local

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:

<SUBDOMAIN>.appsync-api.eu-west-1.avsvmcloud.com
<SUBDOMAIN>.appsync-api.us-west-2.avsvmcloud.com
<SUBDOMAIN>.appsync-api.us-east-1.avsvmcloud.com
<SUBDOMAIN>.appsync-api.us-east-2.avsvmcloud.com

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.

cat uniq-hostnames.txt | python decode.py
02m6hcopd17p6h450gt3.appsync-api.us-west-2.avsvmcloud.com .gh
039n5tnndkhrfn5cun0y0sz02hij0b12.appsync-api.us-west-2.avsvmcloud.com ad001.mtk.lo
04spiistorug1jq5o6o0.appsync-api.us-west-2.avsvmcloud.com isi
060mpkprgdk087ebcr1jov0te2h.appsync-api.us-east-1.avsvmcloud.com belkin.com
06o0865eliou4t0btvef0b12eu1.appsync-api.us-east-1.avsvmcloud.com gncu.local
07605jn8l36uranbtvef0b12eu1.appsync-api.us-east-1.avsvmcloud.com gncu.local
07q2aghbohp4bncce6vi0odsovertr2s.appsync-api.us-east-1.avsvmcloud.com csnt.princegeor
07ttndaugjrj4pcbtvef0b12eu1.appsync-api.us-east-1.avsvmcloud.com gncu.local
08amtsejd02kobtb6h07ts2fd0b12eu1.appsync-api.eu-west-1.avsvmcloud.com sm-group.local
0b0fbhp20mdsv4scwo11r0oirssrc2vv.appsync-api.us-east-2.avsvmcloud.com ville.terrebonn
[...]

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";

CreateSecureString function in SolarWinds.Orion.Core.BusinessLayer.OrionImprovementBusinessLayer.CryptoHelper
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.

SunburstDomainDecoder.exe < uniq-hostnames.txt
F18613981DEC4D1A 2020-10-02T21:00:00.0000000Z 02m6hcopd17p6h450gt3
BD6DEFBBE9FEA3A9 ad001.mtk.lo 039n5tnndkhrfn5cun0y0sz02hij0b12
2BF8DE15406EA780 2020-08-25T03:00:00.0000000Z 043o9vacvthf0v95t81l
573DEB889FC54130 2020-08-13T21:00:00.0000000Z,​WindowsDefender_RUNNING,CrowdStrike_RUNNING 04jrge684mgk4eq8m8adfg7
518092C8FD571806 2020-06-09T22:30:00.0000000Z 04r0rndp6aom5fq5g6p1
F18613981DEC4D1A 2020-07-06T08:30:00.0000000Z 04spiistorug1jq5o6o0
BC1CB013239B4B92 2020-04-25T10:00:00.0000000Z 05q2sp0v4b5ramdf71l7
3ED2E979D53B2523 belkin.com 060mpkprgdk087ebcr1jov0te2h
4225A5C345C1FC8E gncu.local 06o0865eliou4t0btvef0b12eu1
[...]

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.

[...]
F59BBAACBA3493C0 dufferincounty.on.ca
F5D6AA262381B084 glu.com
F9024D5B1E9717C6 gyldendal.local
F90BDDB47E495629 central.pima.gov
F956B5EF56BCF666 coxnet.cox.com
F9A9387F7D252842 city.kingston.on.ca
FB0B50553BC00DED gloucesterva.net
FBB6164BC2B0DFAD ARYZTA.COM
FD04AC52C95A1B0A bmrn.com
FDFCAB8E4C0AB3EE ansc.gob.pe
FE7FF8C9104A0508 thoughtspot.int
FF6760F36DB3D7DC smes.org

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

Creative Commons CC-BY

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.
Provided that you give appropriate credit, provide a link to the license, and indicate if changes were made.

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

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