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Beware of Slow Downloads

March 9, 2018

I often receive emails that ask about slow file downloads with the Beacon payload. Here are the symptoms:

  • It takes multiple hours to grab a few megabytes
  • The sleep time makes no difference
  • File uploads are fast and not affected by this “slowness”

When I get these emails, I usually ask the user about their Malleable C2 profile. Malleable C2 is a technology to change the network and memory indicators for Cobalt Strike’s Beacon payload. In some cases, it can alter the tool’s behavior too.

I sometimes get a reply that the operator is using a custom profile derived from one of the Malleable C2 profiles on my Github repository. Inevitably, I’ll learn that the base profile is a profile that uses HTTP GET requests to download tasks from AND send data back to Cobalt Strike’s team server.

Cobalt Strike’s Beacon payload downloads tasks from its team server via an HTTP GET (or POST) request. The payload limits itself to 1MB of encrypted data per request. This is enough to download most task packages in one request.

By default, Cobalt Strike’s Beacon payload sends data back to Cobalt Strike’s team server with an HTTP POST request. In this default, the Beacon payload embeds its encrypted data into the body of the POST request. Here, the limit is again, 1MB. If you’re downloading a file, Beacon will deliver it in 512KB pieces. This 1MB limit is enough to send a 512KB file piece and some output in one HTTP POST request.

The default is what most Cobalt Strike users are used to and it’s the behavior most Cobalt Strike users expect when they use the HTTP and HTTPS Beacon payloads.

Cobalt Strike 3.6 extended Malleable C2 to allow operators to change where, in the HTTP request, Beacon embeds data it sends back to the team server. The default is still to embed data into the body of an HTTP POST request. But, you also have the flexibility to embed Beacon’s data into the URI, an HTTP header, or a URI parameter. You can also change the HTTP verb associated with this request too. This is amazing flexibility to put into an operator’s hands.

The above flexibility has consequences though. I can stick 1MB of data into the body of an HTTP POST request, no problem. I can’t stick 1MB of data into a URI, an HTTP header, or a URI parameter. That won’t work. What does Cobalt Strike’s Beacon do in these situations? Beacon chunks its output.

The chunker will divide any data, destined for the team server, into ~100 byte chunks. Each piece is sent back to the team server in its own HTTP request. This is where the behavior change comes.

I can send a 512KB file piece in the body of one HTTP POST request. That same file piece requires over 5,240 HTTP requests when divided into 100 byte chunks. Beacon does not make these HTTP requests in parallel. Rather, it makes one request, and waits for the response. It then makes the second request and waits for its response. This happens until all needed HTTP requests are made. The latency associated with each request is the thing that affects your download speed.

If you’ve seen this behavior in your use of Cobalt Strike, I hope this blog post helps clarify why you’re seeing it.

2 comments

  1. Does this problem applied to cs 3.10 and i will like to check if it can be purchase for use
    thanks


    • I wouldn’t call this a “problem”. It’s a logical, but often missed, side effect of using a URI, header, or URI parameter to send large quantities of information to a payload controller.



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