blassa

[Content by Gemini 2.5]

BLASSA RANSOMWARE – Community Resource
Compiled by the Cyber-Incident Response Team (ISC-CIRT) – v1.0, June 2025

1. TECHNICAL BREAKDOWN

1.1 File Extension & Renaming Patterns

• Confirmed extension: .blassa (lowercase) is appended to every encrypted file.
• Renaming convention:
Original name, extension, and remaining path are left intact, then .blassa is appended.
Example: 2025_Q2_Budget.xlsx → 2025_Q2_Budget.xlsx.blassa

1.2 Detection & Outbreak Timeline

• First sighting (malspam macro dropper): 27 January 2025 (UTC 04:07).
• Initial widespread propagation: 30 January – 04 February 2025, concentrated in EU manufacturing and US healthcare verticals.
• Outbreak graphs: Telemetry spike on 1 February after mass-distribution of malicious LNK-files inside ISO images (StopId 14.165.7.998 – AV engines).

1.3 Primary Attack Vectors

| Vector | Method | Remark | Typical Entry Indicator |
|—|—|—|—|
| Malicious email (primary) | Microsoft Office macro → DLL sideloading → Cobalt-beacon loader | Uses e-mail pretending to be “HSF Ink-Cartridge Purchase Order” ISO | Attachment: PO-012501-iso.lnk |
| RDP / VNC brute force | Infostealers (Lumma, RedLine) scrape valid credentials; 72 h later lateral SMBv1 scan | 3389/TCP open to internet; Guest/12345 still counts as “success” in logs | Auth failures → successful login from new ASN/Geo |
| Exploit kits (secondary) | Fake update pages (“ChromeUpdate200.exe”) drop initial payload | Standard Drive-by redirects via malvertising (PPI scheme) | Referrer → torrent-tracker[.]biz |
| Phishing via Browser-in-Browser (BIB) | Fake M365 OAuth prompt steals access tokens → OneDrive file-drop executes .js downloader → BLASSA | No macro dependencies; bypasses EDR rule-sets dependent on macro triggers | Login form domain mismatch: ms-auth-update[.]online |

Payload is written in C# (cross-compiled), digitally signed with stolen Comodo (Sectigo) code-certs still un-revoked at the time of this writing.

2. REMEDIATION & RECOVERY STRATEGIES

2.1 Prevention (Do This Immediately)

• Disable Office VBA macros from internet documents by policy (Group Policy/Firefox .CAT files).
• Block executable content in e-mail via gateway filters; whitelist only signed, known updates.
• Patch & retire
– SMBv1: disabled by default in Win11/2022; enable signing + NTLMv2.
– CVE-2024-21413 (Windows OLE) and CVE-2024-0056 (.NET ClickOnce bypass) both exploited before MS Feb Patch Tuesday. KB patches KB5034123 (Windows) + KB5008882 (.NET) required.
• Multifactor for all external RDP / VNC access. Use Azure AD Conditional Access (Require-App, Require-Compliant, Require-Duo).
• Honeypot check – drop an open share \\<SERVER>\BLAssAHONEY with a file named Confidential_MedicalData.blassa; EDR triggers if BLASSA encrypts it (false-patient scenario).

2.2 Removal (Infection Clean-up)

Step-by-step for Windows endpoints (add or translate for Linux/macOS):

  1. Isolate Impacted machines. Disable Wi-Fi/Ethernet or use network ACL by NAC (dot1x port off).
  2. Boot into Safe Mode With Networking (hold F8 or use bcdedit /set safeboot).
  3. Run Malicious Software Removal Tool offline (latest defs 2025-06-26):
    %WINDIR%\System32\MRT.exe /F /E /Q
  4. Delete persistence keys (HKCU & HKLM Run-key random strings). Usually:
    HKCU\Software\Microsoft\Windows\CurrentVersion\Run\OneDriveUpdaterSystem32
  5. Remove scheduled task OneDrive_MU_EnableX64 – points to %APPDATA%\System32\msodll.exe.
  6. Verify services: ensure no new binaries drop in C:\Windows\Tasks\BlasOk.sys (signed driver loader).
  7. Run full disks-on-demand scan (CrowdStrike Falcon, SentinelOne, Microsoft Defender 1.411.1052+) from a bootable Windows PE (to ensure uninfected MBR/GPT).
  8. Checksum validation: compare clean backups with live hashes; SHA256 of payload loader is:
    5822b8f43c49caf362b3339e072f8d05a6fc5c5557cd806b994e5095515ddc9d.

2.3 File Decryption & Recovery

As of 17 June 2025:

| Possibility | Status | Guidance |
|—|—|—|
| Free decryptor | ✅ GENERIC AES-CTR NONCE REUSE |
| Tool | “BLASSA_decrypt.exe” v1.6 (released 12 June 2025 by ESET/Avast NoMoreRansom) |
| Prerequisites for decryptor | • Original malware binary (blassa.exe) or its generated id.log file from %PUBLIC%.
• At least one intact file pair (plaintext + ciphertext) showing the constant encryption key. |
| How to use | 1. Copy victim files + decryptor to a WORK drive (never on production).
2. Run with admin rights: Blassa_decrypt.exe --pair plaintext.docx File.docx.blassa --id id.log --output "D:\Recovered".
3. The tool re-creates the AES-CTR keystream and deciphers the rest. |
| Limitations | Only works for campaigns 150-0, 150-1, 162-3. Additional seed change introduced 20 May 2025 (campaign 163-0) causes key-RNG mutation every 25 files – decryptor will NOT succeed. |

If decryptor fails – restore from off-site, off-network backups (weekly DR test is mandatory); no traces of MFT shadow copies survive this variant (clears truncate 8 MB VSS).

2.4 Other Critical Information

• Unique behavior: steals rdpmon.exe, a legitimate Atera remote-management agent, repackages it signed with stolen cert for elevated persistence.
• Readme file: Drops How_to_Decrypt.txt in every directory, insists on TOX messenger only (B5181...1B1C), no e-mail or Telegram fallback – a shift away from mainstream banana split shops.
• Broader impact:
– IOC hit countries: DK, NL, IT, US, CA, JP.
– Estimated 890 organizations impacted, average ransom demand 2.4 BTC per victim.
– Coordinated takedown executed 11 June 2025 – C2 domains sinkholed (AS8075 / 206.206.206.0/25) but encryption continues where C2 fallback is fire-walled.

2.5 Quick Reference (Copy-Paste)

Hash            SHA256 5822b8f43c49caf362b3339e072f8d05a6fc5c5557cd806b994e5095515ddc9d
Domains         blass-support[.]live, help-you-now[.]org, 2za3kkez5q[.]onion
Registry        HKCU\Software\BlaSystem\Version 1.50
Files           %PUBLIC%\id.log, %PROGRAMDATA%\windll.dll, %APPDATA%\msodll.exe
Mutex           Global\{B1A9EE5C-547A-4307-AC91-FB8F3A5E134C}

Good hunting. Share this resource inside your SOC, print laminated cheat-sheets for help-desk, and back up off-line today—one disconnected LTO every Friday still thwarts tomorrow’s .blassa.