The first entry in a cross-reference table is always free and has a generation number of 65,535

Hunt Hypothesis

The hypothesis is that an adversary may be manipulating cross-reference table entries to mask malicious activity by exploiting the known behavior of the first entry having a generation number of 65,535. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify potential tampering or evasion tactics that could indicate advanced persistent threats.

YARA Rule

rule invalid_xref_numbers : PDF raw
{
        meta:
			author = "Glenn Edwards (@hiddenillusion)"
			version = "0.1"
			description = "The first entry in a cross-reference table is always free and has a generation number of 65,535"
			notes = "This can be also be in a stream..."
			weight = 1
		
        strings:
                $magic = { 25 50 44 46 }
                $reg0 = /xref\r?\n?.*\r?\n?.*65535\sf/
                $reg1 = /endstream.*\r?\n?endobj.*\r?\n?startxref/
        condition:
                $magic in (0..1024) and not $reg0 and not $reg1
}

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 3 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: A backup tool (e.g., Veeam, Commvault) initializes a cross-reference table during a backup job.
  Filter/Exclusion: Check if the process is associated with a known backup tool or if the event occurs during scheduled backup windows.

• Scenario: A system administrator manually edits the cross-reference table using a tool like dumpbin or odbcconf during maintenance.
  Filter/Exclusion: Exclude events where the process is initiated by a known admin tool or user with elevated privileges.

• Scenario: A scheduled job (e.g., SQL Server Agent job) runs a script that modifies cross-reference tables as part of database maintenance.
  Filter/Exclusion: Filter out events where the process is associated with a known job scheduler or script execution tool.

• Scenario: A log management tool (e.g., Splunk, ELK Stack) processes logs and generates a cross-reference table entry as part of its indexing process.
  Filter/Exclusion: Exclude entries where the process is associated with a log management tool or its associated services.

• Scenario: A virtualization platform (e.g., VMware vCenter, Hyper-V) creates a cross-reference table entry during VM snapshot creation or configuration changes.
  Filter/Exclusion: Exclude events where the process is related to virtualization management tools or VM lifecycle operations.