Cobalt Strike: Decrypting DNS Traffic – Part 5

Cobalt Strike beacons can communicate over DNS. We show how to decode and decrypt DNS traffic in this blog post.

This series of blog posts describes different methods to decrypt Cobalt Strike traffic. In part 1 of this series, we revealed private encryption keys found in rogue Cobalt Strike packages. In part 2, we decrypted Cobalt Strike traffic starting with a private RSA key. In part 3, we explain how to decrypt Cobalt Strike traffic if you don’t know the private RSA key but do have a process memory dump. And in part 4, we deal with traffic obfuscated with malleable C2 data transforms.

In the first 4 parts of this series, we have always looked at traffic over HTTP (or HTTPS). A beacon can also be configured to communicate over DNS, by performing DNS requests for A, AAAA and/or TXT records. Data flowing from the beacon to the team server is encoded with hexadecimal digits that make up labels of the queried name, and data flowing from the team server to the beacon is contained in the answers of A, AAAA and/or TXT records.

The data needs to be extracted from DNS queries, and then it can be decrypted (with the same cryptographic methods as for traffic over HTTP).

DNS C2 protocol

We use a challenge from the 2021 edition of the Cyber Security Rumble to illustrate how Cobalt Strike DNS traffic looks like.

First we need to take a look at the beacon configuration with tool 1768.py:

Figure 1: configuration of a DNS beacon

Field “payload type” confirms that this is a DNS beacon, and the field “server” tells us what domain is used for the DNS queries: wallet[.]thedarkestside[.]org.

And then a third block of DNS configuration parameters is highlighted in figure 1: maxdns, DNS_idle, … We will explain them when they appear in the DNS traffic we are going to analyze.

Seen in Wireshark, that DNS traffic looks like this:

Figure 2: Wireshark view of Cobalt Strike DNS traffic

We condensed this information (field Info) into this textual representation of DNS queries and replies:

Figure 3: Textual representation of Cobalt Strike DNS traffic

Let’s start with the first set of queries:

Figure 4: DNS_beacon queries and replies

At regular intervals (determined by the sleep settings), the beacon issues an A record DNS query for name 19997cf2[.]wallet[.]thedarkestside[.]org. wallet[.]thedarkestside[.]org are the root labels of every query that this beacon will issue, and this is set inside the config. 19997cf2 is the hexadecimal representation of the beacon ID (bid) of this particular beacon instance. Each running beacon generates a 32-bit number, that is used to identify the beacon with the team server. It is different for each running beacon, even when the same beacon executable is started several times. All DNS request for this particular beacon, will have root labels 19997cf2[.]wallet[.]thedarkestside[.]org.

To determine the purpose of a set of DNS queries like above, we need to consult the configuration of the beacon:

Figure 5: zooming in on the DNS settings of the configuration of this beacon (Figure 1)

The following settings define the top label per type of query:

  1. DNS_beacon
  2. DNS_A
  3. DNS_AAAA
  4. DNS_TXT
  5. DNS_metadata
  6. DNS_output

Notice that the values seen in figure 5 for these settings, are the default Cobalt Strike profile settings.

For example, if DNS queries issued by this beacon have a name starting with http://www., then we know that these are queries to send the metadata to the team server.

In the configuration of our beacon, the value of DNS_beacon is (NULL …): that’s an empty string, and it means that no label is put in front of the root labels. Thus, with this, we know that queries with name 19997cf2[.]wallet[.]thedarkestside[.]org are DNS_beacon queries. DNS_beacon queries is what a beacon uses to inquire if the team server has tasks for the beacon in its queue. The reply to this A record DNS query is an IPv4 address, and that address instructs the beacon what to do. To understand what the instruction is, we first need to XOR this replied address with the value of setting DNS_Idle. In our beacon, that DNS_Idle value is 8.8.4.4 (the default DNS_Idle value is 0.0.0.0).

Looking at figure 4, we see that the replies to the first requests are 8.8.4.4. These have to be XORed with DNS_Idle value 8.8.4.4: thus the result is 0.0.0.0. A reply equal to 0.0.0.0 means that there are no tasks inside the team server queue for this beacon, and that it should sleep and check again later. So for the first 5 queries in figure 4, the beacon has to do nothing.

That changes with the 6th query: the reply is IPv4 address 8.8.4.246, and when we XOR that value with 8.8.4.4, we end up with 0.0.0.242. Value 0.0.0.242 instructs the beacon to check for tasks using TXT record queries.

Here are the possible values that determine how a beacon should interact with the team server:

Figure 6: possible DNS_Beacon replies

If the least significant bit is set, the beacon should do a checkin (with a DNS_metadata query).

If bits 4 to 2 are cleared, communication should be done with A records.

If bit 2 is set, communication should be done with TXT records.

And if bit 3 is set, communication should be done with AAAA records.

Value 242 is 11110010, thus no checkin has to be performed but tasks should be retrieved via TXT records.

The next set of DNS queries are performed by the beacon because of the instructions (0.0.0.242) it received:

Figure 7: DNS_TXT queries

Notice that the names in these queries start with api., thus they are DNS_TXT queries, according to the configuration (see figure 5). And that is per the instruction of the team server (0.0.0.242).

Although DNS_TXT queries should use TXT records, the very first DNS query of a DNS_TXT query is an A record query. The reply, an IPv4 address, has to be XORed with the DNS_Idle value. So here in our example, 8.8.4.68 XORed with 8.8.4.4 gives 0.0.0.64. This specifies the length (64 bytes) of the encrypted data that will be transmitted over TXT records. Notice that for DNS_A and DNS_AAAA queries, the first query will be an A record query too. It also encodes the length of the encrypted data to be received.

Next the beacon issues as many TXT record queries as necessary. The value of each TXT record is a BASE64 string, that has to be concatenated together before decoding. The beacon stops issuing TXT record requests once the decoded data has reached the length specified in the A record reply (64 bytes in our example).

Since the beacon can issue these TXT record queries very quickly (depending on the sleep settings), a mechanism is introduced to avoid that cached DNS results can interfere in the communication. This is done by making each name in the DNS queries unique. This is done with an extra hexadecimal label.

Notice that there is an hexadecimal label between the top label (api in our example) and the root labels (19997cf2[.]wallet[.]thedarkestside[.]org in our example). That hexadecimal label is 07311917 for the first DNS query and 17311917 for the second DNS query. That hexadecimal label consists of a counter and a random number: COUNTER + RANDOMNUMBER.

In our example, the random number is 7311917, and the counter always starts with 0 and increments with 1. That is how each query is made unique, and it also helps to process the replies in the correct order, in case the DNS replies arrive in disorder.

Thus, when all the DNS TXT replies have been received (there is only one in our example), the base 64 string (ZUZBozZmBi10KvISBcqS0nxp32b7h6WxUBw4n70cOLP13eN7PgcnUVOWdO+tDCbeElzdrp0b0N5DIEhB7eQ9Yg== in our example) is decoded and decrypted (we will do this with a tool at the end of this blog post).

This is how DNS beacons receive their instructions (tasks) from the team server. The encrypted bytes are transmitted via DNS A, DNS AAAA or DNS TXT record replies.

When the communication has to be done over DNS A records (0.0.0.240 reply), the traffic looks like this:

Figure 8: DNS_A queries

cdn. is the top label for DNS_A requests (see config figure 5).

The first reply is 8.8.4.116, XORed with 8.8.4.4, this gives 0.0.0.112. Thus 112 bytes of encrypted data have to be received.: that’s 112 / 4 = 28 DNS A record replies.

The encrypted data is just taken from the IPv4 addresses in the DNS A record replies. In our example, that’s: 19, 64, 240, 89, 241, 225, …

And for DNS_AAAA queries, the method is exactly the same, except that the top label is www6. in our example (see config figure 5) and that each IPv6 address contains 16 bytes of encrypted data.

The encrypted data transmitted via DNS records from the team server to the beacon (e.g., the tasks) has exactly the same format as the encrypted tasks transmitted with http or https. Thus the decryption process is exactly the same.

When the beacon has to transmit its results (output of the tasks) to the team server, is uses DNS_output queries. In our example, these queries start with top label post. Here is an example:

Figure 9: beacon sending results to the team server with DNS_output queries

Each name of a DNS query for a DNS_output query, has a unique hexadecimal counter, just like DNS_A, DNS_AAAA and DNS_TXT queries. The data to be transmitted, is encoded with hexadecimal digits in labels that are added to the name.

Let’s take the first DNS query (figure 9): post.140.09842910.19997cf2[.]wallet[.]thedarkestside.org.

This name breaks down into the following labels:

  • post: DNS_output query
  • 140: transmitted data
  • 09842910: counter + random number
  • 19997cf2: beacon ID
  • wallet[.]thedarkestside.org: domain chosen by the operator

The transmitted data of the first query is actually the length of the encrypted data to be transmitted. It has to be decoded as follows: 140 -> 1 40.

The first hexadecimal digit (1 in our example) is a counter that specifies the number of labels that are used to contain the hexadecimal data. Since a DNS label is limited to 63 characters, more than one label needs to be used when 32 bytes or more need to be encoded. That explains the use of a counter. 40 is the hexadecimal data, thus the length of the encrypted data is 64 bytes long.

The second DNS query (figure 9) is: post.2942880f933a45cf2d048b0c14917493df0cd10a0de26ea103d0eb1b3.4adf28c63a97deb5cbe4e20b26902d1ef427957323967835f7d18a42.19842910.19997cf2[.]wallet[.]thedarkestside[.]org.

The name in this query contains the encrypted data (partially) encoded with hexadecimal digits inside labels.

These are the transmitted data labels: 2942880f933a45cf2d048b0c14917493df0cd10a0de26ea103d0eb1b3.4adf28c63a97deb5cbe4e20b26902d1ef427957323967835f7d18a42

The first digit, 2, indicates that 2 labels were used to encode the encrypted data: 942880f933a45cf2d048b0c14917493df0cd10a0de26ea103d0eb1b3 and 4adf28c63a97deb5cbe4e20b26902d1ef427957323967835f7d18a42.

The third DNS query (figure 9) is: post.1debfa06ab4786477.29842910.19997cf2[.]wallet[.]thedarkestside[.]org.

The counter for the labels is 1, and the transmitted data is debfa06ab4786477.

Putting all these labels together in the right order, gives the following hexadecimal data:

942880f933a45cf2d048b0c14917493df0cd10a0de26ea103d0eb1b34adf28c63a97deb5cbe4e20b26902d1ef427957323967835f7d18a42debfa06ab4786477. That’s 128 hexadecimal digits long, or 64 bytes, exactly like specified by the length (40 hexadecimal) in the first query.

The hexadecimal data above, is the encrypted data transmitted via DNS records from the beacon to the team server (e.g., the task results or output) and it has almost the same format as the encrypted output transmitted with http or https. The difference is the following: with http or https traffic, the format starts with an unencrypted size field (size of the encrypted data). That size field is not present in the format of the DNS_output data.

Decryption

We have developed a tool, cs-parse-traffic, that can decrypt and parse DNS traffic and HTTP(S). Similar to what we did with encrypted HTTP traffic, we will decode encrypted data from DNS queries, use it to find cryptographic keys inside the beacon’s process memory, and then decrypt the DNS traffic.

First we run the tool with an unknown key (-k unknown) to extract the encrypted data from the DNS queries and replies in the capture file:

Figure 10: extracting encrypted data from DNS queries

Option -f dns is required to process DNS traffic, and option -i 8.8.4.4. is used to provided the DNS_Idle value. This value is needed to properly decode DNS replies (it is not needed for DNS queries).

The encrypted data (red rectangle) can then be used to find the AES and HMAC keys inside the process memory dump of the running beacon:

Figure 11: extracting cryptographic keys from process memory

That key can then be used to decrypt the DNS traffic:

Figure 12: decrypting DNS traffic

This traffic was used in a CTF challenge of the Cyber Security Rumble 2021. To find the flag, grep for CSR in the decrypted traffic:

Figure 13: finding the flag inside the decrypted traffic

Conclusion

The major difference between DNS Cobalt Strike traffic and HTTP Cobalt Strike traffic, is how the encrypted data is encoded. Once encrypted data is recovered, decrypting it is very similar for DNS and HTTP.

About the authors

Didier Stevens is a malware expert working for NVISO. Didier is a SANS Internet Storm Center senior handler and Microsoft MVP, and has developed numerous popular tools to assist with malware analysis. You can find Didier on Twitter and LinkedIn.

You can follow NVISO Labs on Twitter to stay up to date on all our future research and publications.

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