SFX with voicemail content

Hunt Hypothesis

Adversaries may use voicemail content as a vector for SFX (Secondary File Execution) to exfiltrate data or establish persistence. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify potential covert communication channels and early-stage compromise indicators.

YARA Rule

rule Malicious_SFX1 
{

    meta:
        description = "SFX with voicemail content"
        author = "Florian Roth"
        reference = "http://www.kernelmode.info/forum/viewtopic.php?f=16&t=3950"
        date = "2015-07-20"
        hash = "c0675b84f5960e95962d299d4c41511bbf6f8f5f5585bdacd1ae567e904cb92f"
   
    strings:
        $s0 = "voicemail" ascii /* PEStudio Blacklist: strings */ /* score: '30.00' */
        $s1 = ".exe" ascii
   
    condition:
        uint16(0) == 0x4b50 and filesize < 1000KB and $s0 in (3..80) and $s1 in (3..80) 
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

Microsoft Defender for Endpoint — Custom indicators / advanced hunting
Email Gateway — Attachment scanning
File Share Monitoring — Periodic scanning of shared drives
YARA CLI — Manual threat hunting on endpoints

This rule contains 2 string patterns in its detection logic.

References

False Positive Guidance

Scenario: A system administrator uses Voicemail-to-Text (V2T) service to convert voicemail messages into text for record-keeping.
Filter/Exclusion: Exclude traffic from known V2T services (e.g., Cisco Unity Connection, Avaya Aura), or check for presence of specific headers or metadata indicating legitimate voicemail processing.
Scenario: A scheduled backup job includes voicemail files as part of a full system backup.
Filter/Exclusion: Exclude traffic associated with backup tools (e.g., Veeam, Commvault) or filter by known backup directories and timestamps.
Scenario: An IT helpdesk tool automatically records and stores voicemail calls for customer support purposes.
Filter/Exclusion: Exclude traffic from helpdesk platforms (e.g., Zendesk, ServiceNow) or filter by specific IP ranges or user accounts associated with helpdesk operations.
Scenario: A VoIP call recording system is used for compliance or quality assurance, capturing voicemail content during calls.
Filter/Exclusion: Exclude traffic from call recording systems (e.g., Asterisk, 3CX) or filter by call recording directories and associated user roles.
Scenario: A remote access tool (RAT) or endpoint management software includes voicemail content as part of a system inventory or audit.
Filter/Exclusion: Exclude traffic from known endpoint management tools (e.g., Microsoft Intune, Jamf) or filter by specific system inventory processes and user permissions.