AA20-227A: Phishing Emails Used to Deploy KONNI Malware

Original release date: August 14, 2020

Summary

This Alert uses the MITRE Adversarial Tactics, Techniques, and Common Knowledge (ATT&CK®) framework. See the ATT&CK for Enterprise framework for all referenced threat actor techniques.

The Cybersecurity and Infrastructure Security Agency (CISA) has observed cyber actors using emails containing a Microsoft Word document with a malicious Visual Basic Application (VBA) macro code to deploy KONNI malware. KONNI is a remote administration tool (RAT) used by malicious cyber actors to steal files, capture keystrokes, take screenshots, and execute arbitrary code on infected hosts.

Technical Details

KONNI malware is often delivered via phishing emails as a Microsoft Word document with a malicious VBA macro code (Phishing: Spearphising Attachment [T1566.001]). The malicious code can change the font color from light grey to black (to fool the user to enable content), check if the Windows operating system is a 32-bit or 64-bit version, and construct and execute the command line to download additional files (Command and Scripting Interpreter: Windows Command Shell [T1059.003]).

Once the VBA macro constructs the command line, it uses the certificate database tool CertUtil to download remote files from a given Uniform Resource Locator. It also incorporates a built-in function to decode base64-encoded files. The Command Prompt silently copies certutil.exe into a temp directory and renames it to evade detection.

The cyber actor then downloads a text file from a remote resource containing a base64-encoded string that is decoded by CertUtil and saved as a batch (.BAT) file. Finally, the cyber actor deletes the text file from the temp directory and executes the .BAT file.

MITRE ATT&CK Techniques

According to MITRE, KONNI uses the ATT&CK techniques listed in table 1.

Table 1: KONNI ATT&CK techniques

Detection

Signatures

CISA developed the following Snort signatures for use in detecting KONNI malware exploits.

alert tcp any any -> any $HTTP_PORTS (msg:"HTTP URI contains '/weget/*.php' (KONNI)"; sid:1; rev:1; flow:established,to_server; content:"/weget/"; http_uri; depth:7; offset:0; fast_pattern; content:".php"; http_uri; distance:0; within:12; content:!"Referrer|3a 20|"; http_header; classtype:http-uri; priority:2; metadata:service http;)

alert tcp any any -> any $HTTP_PORTS (msg:"KONNI:HTTP header contains 'User-Agent|3a 20|HTTP|0d 0a|'"; sid:1; rev:1; flow:established,to_server; content:"User-Agent|3a 20|HTTP|0d 0a|"; http_header; fast_pattern:only; content:"POST"; nocase; http_method; classtype:http-header; priority:2; metadata:service http;)

alert tcp any any -> any $HTTP_PORTS (msg:"KONNI:HTTP URI contains '/weget/(upload|uploadtm|download)'"; sid:1; rev:1; flow:established,to_server; content:"/weget/"; http_uri; fast_pattern:only; pcre:"/^/wegetx2f(?:upload|uploadtm|download).php/iU"; content:"POST"; http_method; classtype:http-uri; priority:2; reference:url,blog.talosintelligence.com/2017/07/konni-references-north-korean-missile-capabilities.html; metadata:service http;)

Mitigations

CISA recommends that users and administrators consider using the following best practices to strengthen the security posture of their organization’s systems. Any configuration changes should be reviewed by system owners and administrators prior to implementation to avoid unwanted impacts.

• Maintain up-to-date antivirus signatures and engines. See Protecting Against Malicious Code.
• Keep operating system patches up to date. See Understanding Patches and Software Updates.
• Disable file and printer sharing services. If these services are required, use strong passwords or Active Directory authentication.
• Restrict users’ ability (permissions) to install and run unwanted software applications. Do not add users to the local administrators’ group unless required.
• Enforce a strong password policy. See Choosing and Protecting Passwords.
• Exercise caution when opening email attachments, even if the attachment is expected and the sender appears to be known. See Using Caution with Email Attachments.
• Enable a personal firewall on agency workstations, configured to deny unsolicited connection requests.
• Disable unnecessary services on agency workstations and servers.
• Scan for and remove suspicious email attachments; ensure the scanned attachment is its “true file type” (i.e., the extension matches the file header).
• Monitor users’ web browsing habits; restrict access to sites with unfavorable content.
• Exercise caution when using removable media (e.g., USB thumb drives, external drives, CDs).
•  Scan all software downloaded from the internet prior to executing.
• Maintain situational awareness of the latest threats and implement appropriate access control lists.
• Visit the MITRE ATT&CK Techniques pages (linked in table 1 above) for additional mitigation and detection strategies.

For additional information on malware incident prevention and handling, see the National Institute of Standards and Technology Special Publication 800-83, “Guide to Malware Incident Prevention and Handling for Desktops and Laptops.”

Resources

• d-hunter – A Look Into KONNI 2019 Campaign
• MITRE ATT&CK – KONNI
• MITRE ATT&CK for Enterprise

Revisions

• August 14, 2020: Initial Version