Detect the implementation of RC4 Algorithm by Lockbit4.0

Hunt Hypothesis

This hunt hypothesis targets adversaries leveraging the outdated RC4 encryption algorithm to encrypt victim data during a LockBit 4.0 ransomware attack, which often indicates an active or imminent encryption phase. Proactively hunting for this behavior in Azure Sentinel is critical because identifying RC4 usage early allows the SOC team to isolate affected systems before widespread data loss occurs and accelerates incident response by confirming the specific ransomware variant involved.

YARA Rule

rule mal_lockbit4_rc4_win_feb24
{
    meta:
        author = "0x0d4y"
        description = "Detect the implementation of RC4 Algorithm by Lockbit4.0"
        date = "2024-02-13"
        score = 100
        reference = "https://0x0d4y.blog/lockbit4-0-evasion-tales/"
        hash = "062311F136D83F64497FD81297360CD4"
        id = "4de48ced-b9fa-4286-aac4-c263ad20d67d"
        license = "CC BY 4.0"
        rule_matching_tlp = "TLP:WHITE"
        rule_sharing_tlp = "TLP:WHITE"
        malpedia_family = "win.lockbit"
    strings:
        $rc4_alg = { 48 3d 00 01 00 00 74 0c 88 84 04 ?? ?? ?? ?? 48 ff c0 eb ec 29 c9 41 b8 ?? ?? ?? ?? 4c 8d 0d 15 7b 00 00 45 31 d2 48 81 f9 00 01 00 00 74 34 44 8a 9c 0c ?? ?? ?? ?? 45 00 da 89 c8 99 41 f7 f8 46 02 14 0a 41 0f b6 c2 8a 94 04 ?? ?? ?? ?? 88 94 0c ?? ?? ?? ?? 44 88 9c 04 ?? ?? ?? ?? 48 ff c1 eb c3 29 c0 48 8b 0d 14 9e 00 00 31 d2 45 29 c0 48 3d ?? ?? ?? ?? 74 4b 41 ff c0 45 0f b6 c0 46 8a 8c 04 ?? ?? ?? ?? 44 00 ca 44 0f b6 d2 46 8a 9c 14 ?? ?? ?? ?? 46 88 9c 04 ?? ?? ?? ?? 46 88 8c 14 ?? ?? ?? ?? 46 02 8c 04 ?? ?? ?? ?? 45 0f b6 c9 46 8a 8c 0c ?? ?? ?? ?? 44 30 0c 01 48 ff c0 eb ad }
        
    condition:
        uint16(0) == 0x5a4d and
        $rc4_alg
}

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 1 string patterns in its detection logic.

References

• https://0x0d4y.blog/lockbit4-0-evasion-tales/
• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the Detect the implementation of RC4 Algorithm by Lockbit4.0 rule, along with targeted filters and exclusions:

• Legacy SharePoint Document Library Encryption

  • Scenario: The enterprise utilizes Microsoft SharePoint Server 2016/2019 where specific legacy document libraries are configured to use the RC4 cipher for internal transport encryption. When users access or sync these files, the system generates RC4 handshake logs that mimic Lockbit’s signature.
  • Filter/Exclusion: Exclude traffic originating from the SharePoint Farm Service account (NT SERVICE\SharePoint) and restrict detection to non-standard ports (e.g., exclude port 80/443 if known for SharePoint). Alternatively, filter by User Agent strings containing Microsoft-Office or SharePoint.

• Scheduled Backup Job via Veeam or Commvault

  • Scenario: A nightly scheduled backup job using Veeam Backup & Replication (or Commvault) encrypts backup repositories using RC4 for compatibility with older storage appliances. The backup service account initiates the encryption process during the maintenance window, triggering the rule.
  • Filter/Exclusion: Create an exclusion based on the specific Service Account name (e.g., DOMAIN\VeeamBackupSvc or DOMAIN\CommvaultAgent) and limit detection to the defined maintenance window (e.g., 02:00–04:00 daily).

• Legacy Point-of-Sale (POS) Terminal Synchronization

  • Scenario: Retail branches utilize legacy POS terminals running a custom Java-based application that relies on RC4 for secure data synchronization with the central ERP system. These terminals perform frequent, small-batch syncs during business hours, generating consistent RC4 traffic patterns.
  • Filter/Exclusion: Exclude IP