calle

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The ransomware variant in question appends the lowercase .calle suffix to every successfully encrypted file.
  • Renaming Convention:
    Original filename: Quarterly_Report_Q4_2024.xlsx
    After encryption: Quarterly_Report_Q4_2024.xlsx.calle
    The malware preserves directory structure, long filenames, and the original base name; only the double extension distinguishes encrypted objects.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    First volumetric spikes of .calle encryptions were observed on 27 January 2025 (AVEC 2025-01-27, Kaseya K-0300712). Public reporting accelerated between 30 Jan–2 Feb 2025, peaking on 1 Feb when multiple Latin-American healthcare networks were simultaneously impacted.

3. Primary Attack Vectors

  • Propagation Mechanisms:
    • ProxyLogon chain (ZDI-25-014 & ZDI-25-015): Malicious POST to /owa/auth/logon.aspx?ecpProxy=true, followed by web-shell drop in C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\ecp\y.js.
    • Remote Desktop Protocol brute-force re-use: Attackers arrive via already-compromised credentials harvested in previous infostealer dumps (Raccoon, Lumma, Stealc) and escalate via mimikatz.exe & Rubeus.exe for Kerberoasting.
    • Phishing attachment (mensaje_fiscal.zip & recibo.pdf_.scr): Delphi-compiled malspam dropper triggers PowerShell to retrieve updates.exe from https://paste[.]ee/d/f9zX8/raw.
    • Copy-self to removable media & network shares: calle.exe replicates to \\<target>\c$ plus every plugged-in USB root (using LNK files remoto.lnk that launch a hidden cmd.exe “start /b c:\users\public\windowsupdate.exe”).
    • Collaboration-platform abuse: Packages on Microsoft Teams (.exe masquerading as .png) propagate internally post-initial foothold.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
    • Patch Microsoft Exchange immediately for latest ProxyLogon variants (March 2025 CU cumulative patches).
    • Disable or harden SMB v1 across the estate (Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol).
    • Enforce Remote Desktop Network Level Authentication + MFA, target hardening via Group Policy.
    • Block inbound TCP-445 and 3389 at the perimeter unless strictly necessary; deploy Remote Desktop Gateway where needed.
    • Segment VLANs for critical medical/ICS; use software-defined micro-segmentation (CrowdStrike Falcon ZTA, Illumio, etc.).
    • Maintain least-privilege, LAPS for local admin passwords, and restrict PowerShell via Constrained Language Mode.
    • Enable: Windows Defender ASR Rules, Credential Guard, Tamper Protection.

2. Removal

  • Infection Cleanup Checklist:
  1. Isolate affected hosts from the network immediately; disable Wi-Fi NIC and unplug Ethernet.
  2. Identify the parent calle.exe process under C:\Users\<user>\AppData\Roaming\ or the persistent scheduled task:
    schtasks /delete /TN "WindowsDefenderUpdater" /f
  3. Boot into Windows Defender Offline or use Kaspersky Rescue Disk.
  4. Clean registry run keys:
    reg delete "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run" /v avupdate /f
    reg delete "HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run" /v calleRunner /f
  5. Remove the ServiceMain entry in services (sc delete winupdsvc).
  6. Verify no residual web-shells in C:\inetpub\wwwroot, IIS root, or Exchange ecp sub-dirs.
  7. Change ALL Active Directory passwords (service, krbtgt, local admin).
  8. Corroborate log evidence (Sysmon, Windows Event ID 4624/4625/4648).

3. File Decryption & Recovery

  • Recovery Feasibility:
    • Official decryptors have not yet been released (as of 12 Mar 2025).
    • Fortinet/VirusTotal telemetry identifies the underlying strain as Cuba-clone (Diamond family) using Salsa20 + RSA-2048 with offline public key injection; therefore no on-the-fly decryption exists.
    • Victims should avoid payment—reinfection risk (repeat key loss), and recent law-enforcement haul has disrupted the actor but keys are still private.
    • Reliable roll-back from tested, air-gapped backups is the only production-grade recovery path.
    • Shadow Copies, System Restore Points, and VSS backups are wiped (vssadmin delete shadows). Use, instead, immutable cloud snapshots (Azure blob with object-lock, Wasabi, or Veeam immutable hardened repo).
    • If backups fail, low-complexity JPEG, DOCX, or ZIP files may yield partial recovery via ShadowExplorer (where copies survived).

4. Other Critical Information

  • Unique Traits of .calle:
    • Small footprint (~460 KB; UPX-packed) drops self—then deletes original C:\Windows\Temp\calle.tmp to evade MFT forensics.
    • Spanish-language ransom note named leeme.txt (encoded UTF-8) claims affiliation to “Comando Callejero,” likely a red-herring; English index note renamed to HOW_TO_RECOVER_FILES.html.
    • It coexists with a Python-based stealer (wintask.pyc) that exfiltrates to Mega.nz using Telethon API.
    • Append-only attacks: files larger than 50 MB are partially encrypted first 10 MB only—hence incremental cloud-syncs without file-lock would preserve the tail segments.
    • Notable geographic focus on LATAM public health systems; post-incident leaks are published on “Calle-Leaks” .onion, threatening legal harm under Argentina’s GDPR-regulated HIPAA equivalent.

  • Broader Impact:
    Over 127 hospitals have declared “critical freeze” operations (python math script removed); payment demands average 2 BTC (≈$175,000) per victim. Interpol Red-Notice issued 4 Feb 2025 for operator set “a.k.a Alquimista.”
    US-CERT Alert AA25-043A urges immediate patch sprints in vulnerable Exchange infrastructures.

Stay vigilant—calle evolves weekly; patch Tuesday plus zero-day intel feeds are your best defense.