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Latest Astaroth living-off-the-land attacks are even more invisible but not less observable

March 23rd, 2020 No comments

Following a short hiatus, Astaroth came back to life in early February sporting significant changes in its attack chain. Astaroth is an info-stealing malware that employs multiple fileless techniques and abuses various legitimate processes to attempt running undetected on compromised machines. The updated attack chain, which we started seeing in late 2019, maintains Astaroth’s complex, multi-component nature and continues its pattern of detection evasion.

Figure 1. Microsoft Defender ATP data showing revival of Astaroth campaigns

Heat map showing Astaroth encounters, with Brazil accounting for majority of encounters

Figure 2. Geographic distribution of Astaroth campaigns this year, with majority of encounters recorded in Brazil

When we first blogged about Astaroth’s methods, we noted how it completely lived off the land to avoid detection: only system tools that are already existing on the machine are ever executed. In fact, it was an unusual spike in activities related to Windows Management Instrumentation Command-line (WMIC) that prompted our investigation and eventually exposed the Astaroth campaign.

Astaroth now completely avoids the use of WMIC and related techniques to bypass existing detections. Instead, the attackers introduced new techniques that make the attack chain even stealthier:

  • Abusing Alternate Data Streams (ADS) to hide malicious payloads
  • Abusing the legitimate process ExtExport.exe, a highly uncommon attack vector, to load the payload

Astaroth exemplifies how living-off-the-land techniques have become standard components of today’s attacks intent on evading security solutions. However, as we mentioned in our previous blog on Astaroth, fileless threats are very much observable. These threats still leave a great deal of memory footprint that can be inspected and blocked as they happen. Next-generation protection and behavioral containment and blocking capabilities in Microsoft Defender Advanced Threat Protection (Microsoft Defender ATP) lead the charge in exposing threats like Astaroth.

In this blog, we’ll share our technical analysis of the revamped Astaroth attack chain and demonstrate how specific Microsoft technologies tackle the multiple advanced components of the attack.

Dismantling the new Astaroth attack chain

The attackers were careful to ensure the updates didn’t make Astaroth easier to detect; on the contrary, the updates only make Astaroth’s activities even more invisible.

One of the most significant updates is the use of Alternate Data Stream (ADS), which Astaroth abuses at several stages to perform various activities. ADS is a file attribute that allows a user to attach data to an existing file. The stream data and its size are not visible in File Explorer, so attacks abuse this feature to hide malicious code in plain sight.

Astaroth 2020 attack chain

Figure 2. Astaroth attack chain 2020

In the case of Astaroth, attackers hide binary data inside the ADS of the file desktop.ini, without changing the file size. By doing this, the attackers create a haven for the payloads, which are read and decrypted on the fly.

Screenshot comparing contents of desktop.ini before and after infection

Figure 3. Desktop.ini before and after infection

The complex attack chain, which involves the use of multiple living-off-the-land binaries (LOLBins), results in the eventual loading of the Astaroth malware directly in memory. When running, Astaroth decrypts plugins that allow it to steal sensitive information, like email passwords and browser passwords.

In the succeeding sections, we describe each step of Astaroth’s attack chain in detail.

Arrival

The attack begins with an email with a message in Portuguese that translates to: “Please find in the link below the STATEMENT #56704/2019 AND LEGAL DECISION, for due purposes”. The email contains a link that points to URL hosting an archive file, Arquivo_PDF_<date>.zip, which contains a LNK file with a similarly misleading name. When clicked, the LNK file runs an obfuscated BAT command line.

Email used in Astaroth campaign

Figure 4. Sample email used in latest Astaroth attacks

The BAT command drops a single-line JavaScript file to the Pictures folder and invokes explorer.exe to run the JavaScript file.

Malware code showing GetObject technique

The dropped one-liner script uses the GetObject technique to fetch and run the much larger main JavaScript directly in memory:

Malware code showing BITSAdmin abuse

BITSAdmin abuse

The main script then invokes multiple instances of BITSAdmin using a benign looking command-line to download multiple binary blobs from a command-and-control (C2) server:

Malware code showing downloaded content showing ADS

The downloaded payloads are encrypted and have the following file names:

  • masihaddajjaldwwn.gif
  • masihaddajjalc.jpg
  • masihaddajjala.jpg
  • masihaddajjalb.jpg
  • masihaddajjaldx.gif
  • masihaddajjalg.gif
  • masihaddajjalgx.gif
  • masihaddajjali.gif
  • masihaddajjalxa.~
  • masihaddajjalxb.~
  • masihaddajjalxc.~
  • masihaddajjal64w.dll
  • masihaddajjal64q.dll
  • masihaddajjal64e.dll

Alternate Data Streams abuse

As mentioned, the new Astaroth attacks use a clever technique of copying downloaded data to the ADS of desktop.ini. For each download, the content is copied to the ADS, and then the original content is deleted. These steps are repeated for all downloaded payloads.

Malware code showing abuse of ADS to run script to find security products

Another way that Astaroth abuses ADS is when it runs a script to find installed security products. A malicious script responsible for enumerating security products is dropped and then copied as an ADS to an empty text file. The execution command-line looks like this:

ExtExport.exe abuse

The main script combines three separately downloaded binary blobs to form the first-stage malware code:

Malware code showing three blobs forming first-stage malware code

The script then uses a LOLBin not previously seen in Astaroth attacks to load the first-stage malware code: ExtExport.exe, which is a legitimate utility shipped as part of Internet Explorer. Attackers can load any DLL by passing an attacker-controlled path to the tool. The tool searches for any DLL with the following file names: mozcrt19.dll, mozsqlite3.dll, or sqlite3.dll. Attackers need only to rename the malicious payload to one of these names, and it is loaded by ExtExport.exe.

Malware code showing ExtExport.exe abuse

Userinit.exe abuse

The newly loaded DLL (mozcrt19.dll, mozsqlite3.dll, or sqlite3.dll) is a proxy that reads three binary ADS streams (desktop.ini:masihaddajjalxa.~, desktop.ini:masihaddajjalxb.~, and desktop.ini:masihaddajjalxc.~) and combines these into a DLL. The newly formed DLL is the second-stage malware code and is loaded in the same process using the reflective DLL loading technique.

The newly loaded DLL is also a proxy that reads and decrypts another ADS stream (desktop.ini:masihaddajjalgx.gif) into a DLL. This DLL is injected into userinit.exe using the process hollowing technique.

The newly loaded DLL inside userinit.exe is again a proxy that reads and decrypts another ADS stream (desktop.ini:masihaddajjalg.gif) into a DLL. This DLL is the malicious info-stealer known as Astaroth and is reflectively loaded inside userinit.exe. Hence, Astaroth never touches the disk and is loaded directly in memory, making it very evasive.

Astaroth payload

When running, the Astaroth payload then reads and decrypts more components from the ADS stream of desktop.ini (desktop.ini:masihaddajjaldwwn.gif, desktop.ini:masihaddajjalc.jpg, desktop.ini:masihaddajjala.jpg, desktop.ini:masihaddajjalb.jpg, and desktop.ini:masihaddajjali.gif).

Some of these components are credential-stealing plugins hidden inside the ADS stream of desktop.ini. Astaroth abuses these plugins to steal information from compromised systems:

  • NirSoft’s MailPassView – an email client password recovery tool
  • NirSoft’s WebBrowserPassView – a web browser password recovery tool

As mentioned, Astaroth also finds installed security products. It then attempts to disable these security products. For Microsoft Defender Antivirus customers, tamper protection prevents such malicious and unauthorized changes to security settings.

Comprehensive, dynamic protection against living-off-the-land, fileless, and other sophisticated threats with Microsoft Threat Protection

Attackers are increasingly turning to living-off-the-land techniques to attempt running undetected for as long as possible on systems. Because these attacks use multiple executables that are native to the system and have legitimate uses, they require a comprehensive, behavior-based approach to detection.

Microsoft Threat Protection combines and orchestrates into a single solution the capabilities of multiple Microsoft security services to coordinate protection, detection, response, and prevention across endpoints, email, identities, and apps.

In the case of Astaroth, Office 365 ATP detects the malware delivery via email. Using detonation-based heuristics and machine learning, Office 365 ATP inspects links and attachments to identify malicious artifacts.

On endpoints, next-generation protection capabilities in Microsoft Defender ATP detect and prevent some components of Astaroth’s new attack chain. Notably, through Antimalware Scan Interface (AMSI), Microsoft Defender ATP can inspect the encrypted malicious scripts used in the initial stages of the attack.

For the more sophisticated sections of the attack chain, behavioral blocking and containment capabilities provide dynamic protection that can stop malicious behaviors and process trees. Behavior-based protections are key to exposing living-off-the-land threats that abuse and hide behind legitimate processes. These protections identify suspicious behavior sequences and advanced attack techniques observed on the client, which are used as triggers to analyze the process tree using real-time machine learning models in the cloud.

Diagram showing preventive and behavior-based blocking & containment solutions against Astaroth

Figure 5. Preventive and behavior-based blocking & containment protections against Astaroth

These behavior-based detections raise alerts in Microsoft Defender Security Center. With behavioral blocking and containment, not only are evasive threats exposed, detected, and stopped; security operations personnel are also notified so they can thoroughly investigate and remediate the root cause.

Figure 6. Sample Microsoft Defender ATP alerts on behavior-based detections of Astaroth’s activities

Microsoft Defender ATP’s EDR capabilities also have very strong coverage of advanced techniques employed by Astaroth, including cross-process migration, code injection, and use of LOLBins.

Figure 7. Sample Microsoft Defender ATP EDR alert and process tree on Astaroth’s behaviors

We expect Astaroth to further develop and increase in complexity, as long-running malware campaigns do. We will continue to watch this evolving threat and ensure that customers are protected from future updates through durable behavior-based protections.

 

 

Hardik Suri

Microsoft Defender ATP Research Team

 

 


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Multi-stage downloader Trojan sLoad abuses BITS almost exclusively for malicious activities

December 12th, 2019 No comments

Many of today’s threats evolve to incorporate as many living-off-the-land techniques as possible into the attack chain. The PowerShell-based downloader Trojan known as sLoad, however, puts all its bets on BITS.

Background Intelligent Transfer Service (BITS) is a component of the Windows operating system that provides an ability to transfer files in an asynchronous and throttled fashion using idle bandwidth. Abusing BITS, which provides the ability to create self-contained jobs that can be prioritized and queued up and that can launch other programs, has become a prevalent attack technique. Recent sophisticated malware campaigns like Astaroth have found success in the use of BITS for downloading payloads or additional components, especially in systems where the firewall is not configured to block malicious traffic from BITS jobs.

sLoad, detected by Windows Defender Antivirus as TrojanDownloader:PowerShell/sLoad, is used by adversaries for exfiltrating system information and delivering additional payloads in targeted attacks. It has been around for a few years and has not stopped evolving. What hasn’t changed, though, is its use of BITS for all of its exfiltration activities, as well as command-and-control (C2) communications from handshake to downloading additional payloads.

Once sLoad has infiltrated a machine, it can allow attackers to do further, potentially more damaging actions. Using exfiltrated information, attackers can identify what security solutions are running and test payloads before they are sneaked into the compromised system or, worse, high-priced targets. sLoad uses scheduled tasks, which runs the malware every three minutes, opening the window of opportunity for further compromise—hence raising the risk for the affected machine—every time it runs. We have already seen the malware attempt to deliver several other, potentially more dangerous Trojans to compromised machines.

While several malware campaigns have leveraged BITS, sLoad’s almost exclusive use of the service is notable. sLoad uses BITS as an alternative protocol to perform data exfiltration and most of its other malicious activities, enabling the malware to evade defenders and protections that may not be inspecting this unconventional protocol. Cloud-based machine learning-driven behavioral blocking and containment capabilities in Microsoft Defender Advanced Threat Protection detect and block sLoad’s activities as Behavior:Win32/sLoad.A.

In this blog we’ll share our analysis of the multiple ways in which sLoad is abusing BITS and share how Microsoft Defender Advanced Threat Protection defeats these advanced malware techniques.

Stealthy installation via multiple cascaded scripts

sLoad is known to infect machines using spear-phishing emails and a common but effective detection evasion technique: the cascaded scripts. One script drops or downloads one or more scripts, passes control to one of these scripts, and repeats the process multiple times until the final component is installed.

Over time, we’ve seen some variations of this technique. One sLoad campaign used the link target field of a LNK file to run PowerShell commands that extracts and runs the first-stage PowerShell code, which is appended to the end of the LNK file or, in one instance, the end of the ZIP file that originally contained the LNK file. In another campaign, the first-stage PowerShell code itself uses a download BITS job to download either the sLoad script and the C2 URL file or the sLoad dropper PowerShell script that embeds the encrypted sLoad script and C2 URL file within itself.

In the most recent attacks, for the first stage, sLoad shifted from using PowerShell script to VBScript. The randomly named VBScript file is simply a proxy that builds and then drops and runs a PowerShell script, always named rr.ps1. This is none other than the same sLoad PowerShell dropper mentioned earlier that embeds the encrypted sLoad script and C2 URL file within itself.

In most variations of the installation, the sLoad dropper script is the last intermediate stage that performs the following actions, and eventually decrypts and runs the final sLoad script:

  1. Creates an installation folder in the %APPDATA% folder named after the first 6 characters of the Win32 Product UUID. 
  2. Drops an infection marker file named _in, and during the successive executions, uses the LastWriteTime on this file to check whether the malware is installed within last 30 mins, in which case, it terminates. 
  3. Drops the encrypted sLoad script and the C2 URL file as config.ini and web.ini, respectively. 
  4. Builds and drops two more randomly named scripts: one VBScript and one PowerShell script. 
  5. Uses schtasks.exe to create a scheduled task named AppRunLog to run the randomly named VBScript from the previous step with decryption key supplied as a command line parameter; deletes the previously created related tasks (if found) before creating this one. The scheduled task is configured to start at 7:00 AM and run every 3 mins. 

The dropped VBScript that runs under the scheduled task is yet another proxy that simply runs the dropped PowerShell script with the same command line parameter (the decryption key). The PowerShell script decrypts the contents of the previously dropped config.ini in the memory into another piece of PowerShell code, which it then runs. This is the final component, the script detected as TrojanDownloader:PowerShell/sLoad, that uses BITS to perform every important malicious activity.

BITS abuse

The sLoad PowerShell script (the final component) then abuses BITS to carry out all of the following activities:

Finding an active C2 server

The malware decrypts the contents of previously dropped web.ini into a set of 2 URLs and creates a BITS download jobs to test the connection to these URLs. It then saves the URL that responds in the form of a file that contains a message “sok”, being downloaded as part of created BITS job. This ensures that the handshake is complete.

If none responds, the script appends the number “1” to the domain names in both URLs, saves the encrypted data back to the web.ini file, and exits from the script. As a result, the next time the scheduled job runs, the script uses the modified web.ini to obtain the modified URLs to attempt connecting to an active C2. With each unsuccessful attempt of connecting with C2s, the number appended to the domain names is increased by increments of 1 until it reaches 50, at which time it resets to 1. This technique offers a bit of a cushion and ensures continued contact between a compromised machine and a C2, in case the primary C2 is blocked.

This prevents the malware infrastructure from losing a compromised host if the primary C2 is blocked. It’s also interesting to see how the URLs used to reach C2 are structured to appear related to CAPTCHA verification, an attempt to escape watchful eyes.

Fetching a new list of C2s

For continued exfiltration of information, it’s important to maintain contact with an active C2. As the malicious domains cannot stay up running for a long time, the malware packs a functionality to refresh the list of C2 every time the scheduled task runs. Using a BITS download job, the malware downloads a new copy of web.ini from the active C2 to provisions a new set of C2s for future use.

Exfiltrating system information

Once an active C2 is identified, the malware starts collecting system information by performing the following:

  • saves the output of “net view” command
  • enumerates network drives and saves the provider names and device ids
  • produces the list of all running processes
  • obtains the OS caption
  • looks for Outlook folder, as well as Independent Computing Architecture (ICA) files, which are used by Citrix application servers to store configuration information

It then creates a BITS download job with the RemoteURL built using the URL for active C2 and the system information collected up this point.

Crafting URLs infused with stolen info is not a novel attacker technique. In addition, creating a BITS job with an extremely large RemoteURL parameter that includes non-encrypted system information stands out and is relatively easy to detect. However, this malware’s use of a download job instead of an upload job is a clever move to achieve stealth.

Deploying additional payloads

Because the malware exfiltrates system information using a BITS download job, it gets an opportunity to receive a response in the form of a file downloaded to the machine. It uses this opportunity to obtain additional payloads from the C2.

It sleeps and waits for the file to be downloaded. If the downloaded file instructs to download and invoke additional PowerShell codes, the supplied URL is used for the task. If not, then the URL is assumed to be pointing to an encoded PE image payload. The malware creates another BITS download job to download this payload, creates a copy of this newly downloaded encoded file, and uses another Windows utility, certutil.exe, to decode it into a portable executable (PE) file with .exe extension. Finally, it uses PowerShell.exe to run the decoded PE payload. One more BITS download job is created to download additional files.

Spying

The malware comes built with one of the most notorious spyware features: uploading screenshots. At several stages during the installation as well as when running additional payloads, the malware takes several screenshots at short intervals. It then uses a BITS upload job to send the stolen screenshots to the active C2. This is the only time that it uses an upload job, and these are the only files it uploads to the C2. Once uploaded, the screenshots are deleted from the machine.

Conclusion: Multiple layers of protection against multi-stage living-off-the-land threats

sLoad is just one example of the increasingly more prevalent threats that can perform most of their malicious activities by simply living off the land. In this case, it’s a dangerous threat that’s equipped with notorious spyware capabilities, infiltrative payload delivery, and data exfiltration capabilities. sLoad’s behavior can be classified as a Type III fileless technique: while it drops some malware files during installation, its use of only BITS jobs to perform most of its harmful behaviors and scheduled tasks for persistence achieves an almost fileless presence on compromised machines.

To defeat multi-stage, stealthy, and persistent threats like sLoad, Microsoft Defender ATP’s antivirus component uses multiple next-generation protection engines on the client and in the cloud. While most threats are identified and stopped by many of these engines, behavioral blocking and containment capabilities detects malicious behaviors and blocks threats after they have started running:

These detections are also surfaced in Microsoft Defender Security Center. Security operations teams can then use Microsoft Defender ATP’s other capabilities like endpoint detection and response (EDR), automated investigation and response, Threat and Vulnerability Management, and Microsoft Threat Experts to investigate and respond to attacks. This reflects the defense-in-depth strategy that is central to the unified endpoint protection provided by Microsoft Defender ATP.

As part of Microsoft Threat Protection, Microsoft Defender ATP shares security signals about this threat to other security services, which likewise inform and enrich endpoint protection. For example, Office 365 ATP’s intelligence on the emails that carry sLoad is shared to and used by Microsoft Defender ATP to build even stronger defenses at the source of infection. Real-time signal-sharing across Microsoft’s security services gives Microsoft Threat Protection unparalleled visibility across attack vectors and the unique ability to provide comprehensive protection against identities, endpoints, data, cloud apps, and infrastructure.

 

Sujit Magar
Microsoft Defender ATP Research Team

 

 


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Questions, concerns, or insights on this story? Join discussions at the Microsoft Defender ATP community.

Read all Microsoft security intelligence blog posts.

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