erop

[Content by Gemini 2.5]

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: Files encrypted by the “erop” ransomware are unequivocally re-suffixed with the lowercase four-letter extension .erop
    Example: Quarterly_Report.xlsx ➔ Quarterly_Report.xlsx.erop

  • Renaming Convention:
    – The original file name and every internal extension are preserved; the malware only appends .erop to the right-most position.
    – Files located inside network shares are also renamed using the same rule, making encrypted data instantly visible across mapped drives.
    – No e-mail address, victim-ID, or random string is inserted into the file name (a behaviour that distinguishes erop from many Dharma/Phobos offshoots).

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First publicly documented submissions to ID-Ransomware and VirusTotal appeared on 21-Jan-2024, with a detectable volume spike during the last week of January 2024. The campaign remains active in-the-wild as of April 2024.

3. Primary Attack Vectors

erop is a direct derivative of the STOP/Djvu family; its distribution therefore leverages the long-established STOP/Djvu playbook:

  • Software “warez” & crack sites – laced installers for Adobe products, game mods, cheating tools, KMS activators, etc.
  • Pay-Per-Install malvertising chains – poisoned Google/Bing ads that redirect to fake “software-update” landing pages pushing ISO/MSI/EXE payloads.
  • No signs of worm-like SMB/EternalBlue code – infection is user-assisted, not network self-propagating.
  • Follow-on info-stealers – operators habitually drop RedLine or Vidar trojan a few minutes after erop execution, exfiltrating credentials before encryption.

Remediation & Recovery Strategies

1. Prevention

  • Disable Windows Explorer’s “Hide extensions for known file types” – helps users spot multi-extension tricks such as setup.exe.exe.erop.
  • Use application whitelisting (WDAC/AppLocker) to block execution from %TEMP%, %LOCALAPPDATA%, and user-writable folders.
  • Patch third-party software aggressively; STOP variants routinely arrive bundled with older, exploitable builds of Java, Adobe AIR, and Visual C++ redistributables to distract AV while erop runs.
  • Strip e-mail attachments of ISO, IMG, MSI, and CAB at the mail-gateway level.
  • Enforce multi-factor authentication on any border-facing remote-access tool (RDP, AnyDesk, ScreenConnect, …) – not a primary erop vector but stops secondary hands-on-keyboard activity.

2. Removal (step-by-step)

  1. Physically disconnect the machine from Ethernet/Wi-Fi to prevent further file-share encryption.
  2. Boot into Safe Mode with Networking.
  3. Identify the launching process:
  • Look for a recent unsigned EXE in %LOCALAPPDATA%\[random]\ or C:\Users\Public\.
  • STOP variants commonly use names like igfxmtc.exe, svhost.exe, or build.exe.
  1. Terminate the malicious process, then delete its folder and the Run/RunOnce registry key it created (HKCU\Software\Microsoft\Windows\CurrentVersion\Run).
  2. Remove the scheduled task “Time Trigger Task” (another STOP hallmark).
  3. Install reputable AV/AM (Defender, Malwarebytes, ESET, etc.), update signatures, and run a full scan to purge the dropper and the usually-accompanying info-stealer.
  4. Before rebooting normally, clear the %SystemRoot%\System32\winevt\Logs directory if event-log tampering is observed, then re-enable Windows Event Log service.

3. File Decryption & Recovery

  • Current feasibility:
    – Some files encrypted by erop can be decrypted for free—but only if the malware used an OFFLINE key (hard-coded because the command-and-control server was unreachable during infection).
    – If an ONLINE key was used (majority of recent cases), each victim’s decryption key is unique and cannot be reproduced without the criminal’s private RSA key.

  • Tool-set:

  1. Download Emsisoft StopDecrypter (https://www.emsisoft.com/ransomware-decryption-tools/stop-djvu) – updated weekly with newly recovered offline keys.
  2. Launch the tool, point it at a pair of an encrypted + unencrypted file (≥128 kB) and click “Start”.
    • If the status line reports “Decryption is impossible (online key)” for your .erop extension, no universal decryptor exists at this time.
  3. Check the vendor’s “Personal ID” page. Victims whose ID ends in “t1” are offline-key victims—keep checking the site; when analysts seize a server and obtain that specific key, Emsisoft silently pushes it to the tool.
  • Parallel recovery:
    – Immediately create a bit-for-bit image of affected drives so any future key-release can still be applied against intact ciphertext.
    – Use file-carving utilities (PhotoRec, Scalpel) against un-allocated clusters—STOP variants only encrypt the first 0x500000 bytes (~5 MB) of each file; larger plain-text shadows may still reside slack-space.

4. Other Critical Information

  • Differentiators:
    – erop is visually identical to other recent STOP/Djvu strains (nobu, urnb, cadq, …) except for the new extension.
    – The ransom note file name remains unchanged: _readme.txt (standard STOP language demanding $490/$980).
    – Operators’ contact e-mails currently rotate through [email protected], [email protected], and [email protected].

  • Wider impact:
    – STOP/Djvu (erop included) is the #1 consumer-level ransomware by submission count; home users, small architectural/engineering offices and schools with limited IT staff form the bulk of victims.
    – Because the malware frequently arrives together with credential-stealers, secondary business-e-mail-compromise (BEC) attacks often follow weeks later when victims have supposedly “recovered”; reset all passwords and review SaaS/remote access logs post-cleanup.

Stay vigilant, keep offline backups disconnected when not in use, and share IoCs (SHA-256, campaign URLs, ransom-note samples) with the community—collective insight accelerates key recovery efforts for every erop victim.