efdc

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: efdc
  • Renaming Convention: Target files keep their original base name, but the ransomware appends “.efdc” as a second extension (e.g., “report.xlsx” becomes “report.xlsx.efdc”). In some cases a victim-specific hexadecimal ID (ex. “[7A8E9C12]”) is pre-pended to the name, so the final result may look like “[7A8E9C12]report.xlsx.efdc”.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First public submissions to ID-ransomware and malware repositories began appearing in late December 2021; larger campaign surges were observed during January–March 2022 and again in July 2022.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  • Phishing e-mails that deliver a macro-enabled document or password-protected ZIP containing the initial trojan (often SmokeLoader or IcedID).
  • Malvertising/SEO-poisoned search results pushing fake software installers (cracked utilities, KMS activators, “codec packs”).
  • Exploitation of vulnerable, internet-facing services:
    – Log4Shell (CVE-2021-44228) in un-patched Java applications.
    – ProxyShell (CVE-2021-34473, 34523, 31207) against un-patched Microsoft Exchange servers.
    – Microsoft Support Diagnostic Tool Follina (CVE-2022-30190) in May–June 2022 campaigns.
  • Stolen or brute-forced RDP / VPN credentials followed by manual deployment of the ransomware EXE (“r1.exe”, “lock.exe”, or “svchost.exe” in random folders).

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
  1. Apply the latest cumulative Windows security updates; patch Log4j, Exchange, and Office immediately.
  2. Disable or sandbox Office macros at the enterprise level (Group Policy, ACAS, Intune, or O365 “Block Macros from running in Office files from the Internet”).
  3. Enforce network segmentation—separate critical file servers and backups.
  4. Use LAPS / strong, unique passwords + MFA for all external services (RDP, VPN, Outlook-web, etc.).
  5. Set Windows Controlled Folder Access (CFA) or a reputable anti-ransomware product to block unsigned processes modifying user documents.
  6. Maintain 3-2-1 backups (three copies on two different media, one offline/“air-gapped”). Test restoration.
  7. Restrict PowerShell, WMI, and PsExec execution to approved users; enable command-line auditing.
  8. Continuous phishing awareness campaigns with sandboxed attachment detonation for inbound mail.

2. Removal

  • Infection Cleanup:
  1. Physically disconnect the affected machine(s) from the network; power-off is NOT recommended until volatile evidence is captured if forensic analysis is planned.
  2. Boot into Windows Safe Mode with Networking or use a clean “WinPE / Kaspersky Rescue / Bitdefender Rescue” USB.
  3. Identify the persistence mechanism:
    – Scheduled task names frequently imitate Windows Update (“WinUpdate”, “SysUpdate”).
    – Registry Run keys under HKCU\Software\Microsoft\Windows\CurrentVersion\Run.
  4. Delete malicious binaries (usual hashes change constantly; look in %TEMP%, %APPDATA%, C:\PerfLogs, C:\Users\Public).
  5. Remove the scheduled task / Run entry; clear WMI Event subscriptions if present.
  6. Patch the entry vector you identified (Exchange, Log4j, RDP, macro policy, etc.).
  7. Run a full scan with updated ESET, Kaspersky, Microsoft MSERT, or Malwarebytes to confirm eradication.
  8. When you are confident the environment is clean, rebuild the domain Controller/AD or core servers from known-good media if they were compromised.

3. File Decryption & Recovery

  • Recovery Feasibility:

  • efdc belongs to the STOP (Djvu) family. Files encrypted after roughly 25-Aug-2019 use “online keys” unique per victim. Brute-forcing AES-256 is computationally infeasible.

  • IF the malware failed to reach its command-and-control server, it falls back to an offline/hard-coded key. Emsisoft’s “STOP-Djvu Decryptor” (free) can decrypt files locked with any known offline key.

  • Check if you are lucky: run “STOP-Djvu Decryptor”, point it at a pair of an original & encrypted file > 150 kB. If it reports “No key for ID: t1Rjiw2aYqyDcqmRgVvM0nUqsZqwUQPxixxxxx (offline key)” the chance exists—stay tuned to Emsisoft’s key list for updates.

  • Otherwise, your only paths are: (a) restore from offline backup, (b) Windows shadow copies (usually deleted by the ransomware but worth checking with ShadowExplorer), (c) file-recovery / undelete tools (recovers non-encrypted deleted originals only if the disk area was not overwritten).

  • Paying the ransom ($490–980 in BTC) is strongly discouraged—no guarantee, funds criminal actors, and in several reported cases the provided decryptor corrupted large files.

  • Essential Tools/Patches:
    – Emsisoft STOP-Djvu Decryptor (latest)
    – Microsoft Exchange cumulative updates (up to CU12+)
    – Log4j 2.17.1+ or 2.3.1+ (depending on Java version)
    – Microsoft guidance for CVE-2022-30190 (disable MSDT URL protocol)
    – Kaspersky Virus Removal Tool / ESET Online Scanner

4. Other Critical Information

  • Additional Precautions:

  • The ransomware drops “_readme.txt” in every folder, referencing the “[email protected] / [email protected]” e-mail addresses; occasionally the threat-actor uses Telegram instead.

  • Before encryption it exfiltrates browser credentials, cryptocurrency wallets, and FileZilla XML files using a built-in “information stealer” module—assume data breach obligations even if you restore files.

  • Process injection or hollowing commonly occurs inside wuauclt.exe or svchost.exe; inspection of parent-child relations in Sysmon logs will reveal this.

  • Broader Impact:

  • Being a “Ransomware-as-a-Service” offshoot of STOP/Djvu, the efdc campaigns have affected consumers as well as small municipalities, schools, and MSPs—mirroring the shift of Djvu operators toward larger, semi-targeted attacks using post-exploitation frameworks (Cobalt Strike, SystemBC).

  • Because monthly variants rotate extensions, remember that detection rules should focus on TTPs (scheduled-task masquerading, large entropy file writes, extension appending) rather than the literal string “.efdc”.

By combining strict patching (especially Exchange & Log4j) with hardened macros, credential hygiene, and reliably tested offline backups, most organizations can blunt the efdc threat and minimise both downtime and ransom temptation.