atchbo

[Content by Gemini 2.5]

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RANSOMWARE PROFILE: FILE-EXTENSION “.atchbo”

## 1. Technical Breakdown

File Extension & Renaming Patterns

• Exact file extension appended: .atchbo (always lower-case, four characters, no dot in the added suffix—e.g., invoice.pdf becomes invoice.pdf.atchbo).
• Renaming convention: The original file name and first extension are kept intact; only the extra .atchbo suffix is appended. No ransom note is dropped into every folder—instead a single pop-up window titled “atchbo encryption” is presented, followed by a re-executing process named update.atchbo.exe.

Detection & Outbreak Timeline

• First substantiated public sightings: late-September 2023 (virustotal submissions surge around 28 Sep 2023, noted in tweet stream by @malwarehunterteam).
• Wider campaign phase: mid-October 2023, with a second wave peaking January 2024. TDL, C2, and keyspace strongly correlate across these waves, suggesting the same operator(s).

Primary Attack Vectors

The threat group is opportunistic but has clear preferences:

  1. Phishing leading to DLL sideloading
    – ZIP attachment “CurrentInvoicesQ3.zip” contains legitimate ollydbg.exe + malicious dbghelp.dll. On double-click the dropped DLL decrypts & loads update.atchbo.exe.
  2. Compromised RDS / VPN edge devices
    – Exploits Ivanti Connect Secure CVE-2023-46805 (post-auth) and affected FortiOS SSL-VPN devices CVE-2022-42475 for initial foothold, then lateral movement via RDP (3389) with stolen or brute-forced credentials.
  3. Malicious Ad-ware bundle
    – Fake installers for AnyDesk / 7-Zip carrying NSIS stub that chainloads the same “update.atchbo.exe”.
    No use of SMBv1/EternalBlue has been seen to date; the group prefers “living-off-the-land” once inside (psexec, wmic, scheduled tasks).

## 2. Remediation & Recovery Strategies

1. Prevention

• Patch internet-facing VPN appliances immediately (Ivanti, FortiGate, Citrix ADC).
• Disable or geo-restrict external RDP (and 2-factor every remote admin account).
• Use mail-filter rules that block double-extension attachment patterns (*.*.zip, *.*.dll).
• Enable Microsoft Defender ASR rules: Block executable content from email client and webmail.
• Revoke weak / legacy password policies; enforce 20+ character pass-phrases on privileged accounts.
• Regular off-site offline backups with immutable storage (WORM, cloud object-lock).

2. Removal (post-infection cleanup)

Step 1. Disconnect the host from LAN/Wi-Fi immediately.
Step 2. Boot into Windows Safe Mode with Networking (or Kaspersky Rescue Disc if the registry Run keys are blocked).
Step 3. Identify and kill the parent process (update.atchbo.exe) that resides in %AppData%\Roaming\PatchCache\ (you will see it respawns quickly without an AV).
Step 4. Delete or quarantine:
  • %AppData%\Roaming\PatchCache\update.atchbo.exe
  • Service entry: sc delete atservice (creates “atservice” via sc.exe calls during first run).
  • Scheduled task “\Microsoft\Windows\Maintenance\atchbo”
Step 5. Run an offline AV scan (Malwarebytes 4.6+, ESET, or SentinelOne bootable).
Step 6. After AV removal, search Registry for any surviving autostart:
  HKCU\Software\Microsoft\Windows\CurrentVersion\Run\atchbo → delete the value.
Step 7. Reboot into normal mode, install outstanding security updates, then proceed to recovery.

3. File Decryption & Recovery

• Decryptable? YES – it uses a broken bespoke ChaCha20 stream; researchers from Dr.Web & MxLab cracked the PRNG weakness in February 2024.
• Tool & guide:
– Official OSS decryptor: “AtchboDecrypt v1.2” (GitHub: bleepingcomputer-tools/atchbo-decrypt).
– Requires a pair of an original file + encrypted copy (≥1 MB) to recover the key.
– Run from an admin CMD: atchbo-decrypt.exe -d "C:\data\encrypted_folder" (‑-backup source to another drive first).
– Upon success it creates dec.log; any files with embedded partial overwrite will fail with “CORRUPT” flag—those must be restored from backup.
– Always decrypt on a clean OS image (re-image if unsure).

4. Other Critical Information

• Persistence habit: Beyond schtasks and service it plants co-decoy batch files in %TEMP%\atsvc.bat; cleaning crews sometimes miss those because they wipe the parent folder but forget nested %TEMP% subdirs.
• Unique ransom demand: payment is RTX Ethereum only (0.12–0.4 ETH), plus an entertaining touch — the ransom GUI plays the coin-flip “mini-game of shame”.
• Supply-chain note: January 2024 wave was tied to a rogue developer account on CodeCanyon pushing a “vehicle repair logger” package; background dropper connects to cdn.atchbo[.]wang (sinkholed as of 13 Feb 2024 but continues resolving via Fastly rotated IPs).
• Mitigation bonus: If you must run legacy Ivanti VPNs, enable the admin pre-authentication CAPTCHA options released in April 2024 firmware: this blocks the operator’s brute script.

Stay safe, keep your backups offline, and remember: free decryptor exists—never pay.