Ransmoware Telefonica

Hunt Hypothesis

This detection rule identifies potential ransomware activity associated with the “Telefonica” campaign by monitoring for specific file encryption patterns and process execution anomalies within Azure Sentinel. Proactively hunting for this behavior is critical to validate the efficacy of existing low-severity alerts and ensure early identification of initial infection stages before they escalate into widespread data loss events.

YARA Rule

rule ransom_telefonica : TELEF
{
  meta:
    author = "Jaume Martin <@Xumeiquer>"
    description = "Ransmoware Telefonica"
    date = "2017-05-13"
    reference = "http://www.elmundo.es/tecnologia/2017/05/12/59158a8ce5fdea194f8b4616.html"
    md5 = "7f7ccaa16fb15eb1c7399d422f8363e8"
    sha256 = "2584e1521065e45ec3c17767c065429038fc6291c091097ea8b22c8a502c41dd"
  strings:
    $a = "RegCreateKeyW" wide ascii nocase
    $b = "cmd.exe /c"
    $c = "115p7UMMngoj1pMvkpHijcRdfJNXj6LrLn" ascii
    $d = "12t9YDPgwueZ9NyMgw519p7AA8isjr6SMw" ascii
    $e = "13AM4VW2dhxYgXeQepoHkHSQuy6NgaEb94" ascii
    $f = "tasksche.exe"
  condition:
    uint16(0) == 0x5A4D and $a and for all of ($b, $c, $d, $e, $f) : (@ > @a)
}

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 6 string patterns in its detection logic.

References

• http://www.elmundo.es/tecnologia/2017/05/12/59158a8ce5fdea194f8b4616.html
• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the Ransomware Telefonica detection rule in an enterprise environment, including suggested filters and exclusions:

• Enterprise Backup Agent Snapshot Operations

  • Scenario: The Veeam Backup & Replication agent or Commvault Data Protection Agent performs scheduled snapshot creation on critical file servers. These agents often execute mass file locking, attribute modification (e.g., setting the “Archive” bit), and rapid metadata updates across thousands of files within a short window, mimicking ransomware encryption behavior.
  • Filter/Exclusion: Create an exclusion rule for processes running under specific service accounts (e.g., DOMAIN\VeeamBackupSvc or DOMAIN\CommvaultAgent) and restrict the detection to file extensions commonly targeted by ransomware (.docx, .xlsx, .pdf) while excluding temporary backup lock files (.vbm, .dat).

• Windows Server Scheduled Defrag and Optimization Jobs

  • Scenario: The built-in Windows “Optimize Drives” scheduled task runs nightly on high-volume database servers. This process triggers the defrag.exe or fsutil utilities to reorganize clusters, which involves reading and rewriting file headers and metadata for large files, generating a spike in file modification events similar to ransomware activity.
  • Filter/Exclusion: Exclude the specific command line arguments associated with optimization tasks (e.g., containing -v, -o, or fsutil) and filter by the process path C:\Windows\System32\defrag.exe running under the SYSTEM account during standard maintenance windows (01:00 – 05:00 local time).

• Microsoft Endpoint Configuration Manager (SCCM) Content Updates

  • Scenario: During a major patch deployment cycle, SCCM (or Microsoft Intune) agents push large content packages