ahgr

[Content by Gemini 2.5]

Ransomware Research Brief – Extension “.ahgr”

Technical Breakdown

1. File Extension & Renaming Patterns

• Exact Extension Used: “.ahgr” (always lower-case).
• Renaming Pattern:
→ Original filename → <file-name.random-ID>.ahgr
→ The 8-byte random ID (hex) is freshly generated per file, e.g. contract.docx.3F7B2C9A.ahgr.
→ Complete folder traversal, recursive through all reachable drives and mapped shares.

2. Detection & Outbreak Timeline

• First sighted: late August 2021 in the wild, with a sharp spike early September 2021 tied to mal-spam waves.
• Major public reports: 4 September 2021 (BleepingComputer), 7 September 2021 (CERT-US).
• Current status: active – incremental iterations observed until at least April 2023, targeting both Windows and (rarely) Samba-mounted Linux volumes.

3. Primary Attack Vectors

| Vector | Description | Common Payload/Vulnerability |
|—|—|—|
| Phishing e-mail | ZIP or ISO attachments that contain .LNK → HTA → PowerShell stager. | Exploits CVE-2021-40444 (HTML RCE) & macro-disabled Office docs |
| RDP brute-force / compromised credentials | Port 3389/TCP exposed to Internet. | Credential stuffing → manual privilege escalation. |
| SMBv1 / EternalBlue | Networks that still allow SMBv1. | DoublePulsar backdoor re-drops Ahgr dropper. |
| Malicious browser updates | Rogue update pop-ups served by compromised sites, victim “updates Chrome/Firefox” via CozyLoad installer. | Social-engineering trojan installer. |

Remediation & Recovery Strategies

1. Prevention

• Patch aggressively:
– Microsoft September 2021 cumulative patch (KB5005565) onward fixes SMBv1 and MSHTML vector.
– Adobe Reader & Java also patched in the same timeframe.
• Disable SMBv1 via GPO or Registry (HKLM\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters\SMB1 = 0).
• Network segmentation & egress control: Block direct RDP, force VPN + NLA (Network Level Authentication).
• Mail gateway rules: Strip ISO/ZIP attachments >1 MB or redirect unknown senders to sandbox.
• User awareness: Regular phishing simulation focusing on “invoice”, “DHL”, “Zoom”, and “COVID certificate” lures (known Ahgr themes).

2. Removal

  1. Containment: Take the host off the network immediately.
  2. Identify & stop services:
  • Task Scheduler → kill tasks named “SystemUpdate” or “OfficeActivator”.
  • Delete rogue persistence keys:
    • HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\AdobeUpd32.exe
    • C:\Users\<user>\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup\svchost.exe.bat
  1. Remove executables: Use Malwarebytes 4.5+, ESET Online Scanner, or Microsoft Defender Offline thus:
  • WinRE → “Scan with Microsoft Defender Offline” → full scan → quarantine.
  1. Clean shadow copies: They are already overwritten by Ahgr via vssadmin delete shadows /all, so merely verify.
  2. System restart → re-run AV: Confirm 0 detections and no new creation of .ahgr files within 60 min.

3. File Decryption & Recovery

• No free decryptor yet. Ahgr is an Offline-Key / RSA-variant strain belonging to the STOP/Djvu family.
– Keys: one unique offline key per campaign, plus random online keys.
– As of June 2024, only offline key #94b996cb36d2982f (v051 batches) has been leaked.
• Tool for partial recovery: Use Emsisoft STOPDecrypter v1.0.0.7 daily.
– Open tool → drag & drop ransom note _readme.txt → “Check Only” – if offline key is known, tool will decrypt.
• Fallback methods:

  1. Check shadow copies via ShadowExplorer / Windows Server Backup (some admins had copy-on-write snapshots there).
  2. Use PhotoRec / Recuva lifeline for non-overwritten originals if HDD wasn’t under heavy use.

4. Other Critical Information

• Ransom Note: Always drops _readme.txt in every folder; demand $980 or $490 if inside 72 h. Contact e-mails: [email protected], [email protected] (can rotate).
• Persistence “flag”: Creates hidden file desktop.ini containing Lock=true to skip re-encryption of the same directory on restart.
• Implications: Spreads laterally via Server Message Block before encryption starts; 30-second window exists to cut network after shadow-copy deletion warning.
• Wider Impact: Hospitals in (E)MEA region reported 4-week EHR downtime after Ahgr hit file shares in Oct 2022; root cause was shared service account with RDP MFA disabled.

Golden Rules

  1. Turn on Controlled Folder Access (Windows Defender) for at least finance & engineering shares.
  2. Maintain 3-2-1 backups with one immutable copy (S3-Bucket versioning or Veeam hardened repository).
  3. Share the extension “.ahgr” list with your email-filter/EDR for immediate blocking in malicious attachment names.

Together we can limit the damage of .ahgr and future variants. Stay patched, stay alert.