ThreatFox: KrDownloader IOCs

Hunt Hypothesis

This detection rule identifies adversary activity involving the KrDownloader tool by monitoring for three specific indicators of compromise (IOCs) known to facilitate initial access and payload delivery. Proactively hunting for these IOCs in Azure Sentinel is critical because KrDownloader often serves as a precursor to advanced threats, allowing SOC teams to intercept early-stage intrusions before they escalate into full-scale compromises.

IOC Summary

Malware Family: KrDownloader Total IOCs: 3 IOC Types: sha1_hash, md5_hash, sha256_hash

KQL: Hash Hunt

// Hunt for files matching known malicious hashes
// Source: ThreatFox - KrDownloader
let malicious_hashes = dynamic(["0a4750223a0c04e5a41b474eaf776a4b10129f498b1cf76693b2e54955361f8c", "4783e4bcbd97b4927afae1d8160222b53452831c", "7825ed42fd7c9d92b93f420b1653e8e9"]);
DeviceFileEvents
| where SHA256 in (malicious_hashes) or SHA1 in (malicious_hashes) or MD5 in (malicious_hashes)
| project Timestamp, DeviceName, FileName, FolderPath, SHA256, InitiatingProcessFileName
| order by Timestamp desc

Required Data Sources

References

• ThreatFox — KrDownloader

False Positive Guidance

Here are specific false positive scenarios and corresponding exclusions for the ThreatFox: KrDownloader IOCs detection rule in an enterprise environment:

• Legitimate Admin Script Execution via PowerShell

  • Scenario: A system administrator runs a custom deployment script using PowerShell.exe to download configuration templates or update manifests from an internal repository. The script utilizes the standard Windows Invoke-WebRequest cmdlet, which may trigger network connections matching KrDownloader’s IOCs (specifically certain User-Agent strings or TLS fingerprints).
  • Suggested Filter: Exclude processes where the parent process is PowerShell.exe and the command line contains specific internal domain keywords (e.g., -URI "https://internal-corp.com/..."). Additionally, filter out if the process hash matches a known whitelisted script hash stored in your CMDB.

• Automated Patch Management via SCCM or Intune

  • Scenario: The enterprise’s Configuration Manager (SCCM) or Microsoft Intune agent initiates a background download task to fetch software updates or application packages from the central distribution point. These agents often use standard HTTP/HTTPS clients that share network signatures with KrDownloader, particularly when downloading large payloads during maintenance windows.
  • Suggested Filter: Create an exclusion for processes named ccmsetup.exe (SCCM) or Microsoft.Insights.Client.exe (Intune). Further refine by excluding traffic destined for specific internal IP ranges known to host the distribution points, ensuring only external connections matching KrDownloader IOCs are flagged.

• Scheduled Backup Agent Connectivity

  • Scenario: A scheduled job runs nightly using Veeam Backup & Replication or Commvault to connect to a cloud storage gateway or an off-site repository. The backup agent establishes persistent connections that may match the specific SSL/TLS certificate fingerprints or port usage associated with KrDownloader IOCs, especially if