April 11, 2021

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Cobalt Strike 4.3 – Command and CONTROL

Cobalt Strike 4.3 is now available. The bulk of the release involves updates to DNS processing but there are some other, smaller changes in there too.

DNS updates

We have added options to Malleable C2 to allow DNS traffic to be masked. A new dns-beacon block allows you to specify options to override the DNS subhost prefix used for different types of request. All existing options relating to DNS have also been moved inside this block. The affected options are: dns_idle, dns_max_txt, dns_sleep, dns_ttl, maxdns, dns_stager_prepend and dns_stager_subhost.

Something that you should be aware of is that the addition of the dns-beacon block means that in order for them to be processed, these existing options need to be defined inside the block. Values for these options outside of the dns-beacon block in existing profiles will be ignored.

dns-beacon {
    # Options moved into 'dns-beacon' group in 4.3:
    set dns_idle             "1.2.3.4";
    set dns_max_txt          "199";
    set dns_sleep            "1";
    set dns_ttl              "5";
    set maxdns               "200";
    set dns_stager_prepend   "doc-stg-prepend";
    set dns_stager_subhost   "doc-stg-sh.";
    
    # DNS subhost override options added in 4.3:
    set beacon               "doc.bc.";
    set get_A                "doc.1a.";
    set get_AAAA             "doc.4a.";
    set get_TXT              "doc.tx.";
    set put_metadata         "doc.md.";
    set put_output           "doc.po.";
    set ns_response          "zero";
}

Another change related to DNS is the addition of functionality to allow a DNS Beacon to egress using a specific DNS resolver, rather than the default DNS resolver for the target server. A new DNS Resolver field has been added to the DNS listener configuration dialog to facilitate this change.

Rounding out the DNS changes, a smaller change is the addition of a customization option related to how the server responds to NS record requests. We have noticed that some DNS resolvers do not allow the DNS Beacon to successfully egress to their team server due to unexpected NS record requests being injected into the communications. Prior to this release, the team server would drop the NS requests and if certain DNS resolvers fail to receive responses to those requests, DNS communications would fail. To get around this issue, we have added another option (ns_response) to the new dns-beacon block to allow the response to those requests to be customized.

Host rotation

We have made improvements to evasion in the DNS and HTTP/S Beacons by adding a host rotation strategy option. Prior to this release, a close examination of DNS and HTTP/S traffic would reveal a round robin pattern of host processing. A new host rotation strategy option in the listener configurations for the DNS and HTTP/S Beacons allows you to use different strategies for rotating through hosts. The options include the existing round robin rotation plus three new options – random, rotate on failure, and rotate after a set period of time.

Quality-of-life updates

Outside of the main DNS theme to the release, we have made a couple of a smaller, quality-of-life changes; the first of which is the addition of a PowerShell IEX option in Scripted Web Delivery. The new powershell IEX option outputs a shorter IEX command that can be pasted directly into a PowerShell console.

Another quality-of-life change is the option to prefix console messages with a timestamp. This option can be turned on or off via the console preferences dialog.

User agent handling

One final update to mention involves how requests from certain user agents are handled. The default behaviour of the team server prior to this release has been to block requests from user agents starting with “curl”, “lynx” or “wget” with a 404 response. We have received feedback that this causes problems for some users that want their server to be able to respond to requests from traffic that appears to be coming from certain user agents. To address this, we have added a block_useragents option to the http-config block within the Malleable C2 profile. This allows you to specify which user agents to respond to.

To see a full list of what’s new in Cobalt Strike 4.3, please check out the release notes. Licensed users can run the update program to get the latest version. To purchase Cobalt Strike or ask about evaluation options, please contact us for more information.