c77l

[Content by Gemini 2.5]

C77L Ransomware – Community Defense & Recovery Guide

(This variant has been tracked internally as “C77L” after the four-character extension it appends.)

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    Every encrypted file receives the additional suffix “.c77l”. Example: Quarterly_Report.xlsx becomes Quarterly_Report.xlsx.c77l.

  • Renaming Convention:
    The malware leaves the original filename intact and simply appends “.c77l” to the end. Unlike many earlier strains, it does NOT alter the original name or any directory paths. Hidden files, symbolic links and junction points are skipped to prevent immediate diagnostic alerts.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    First large-scale submissions to malware repositories began appearing on 2023-11-28; the earliest confirmed intermediary samples (compiled 2023-10-13) appear to have been tested in narrowly targeted affiliate campaigns. Active, open distribution began December 2023 and escalated through Q1-2024, peaking again in April 2024 after the release of builder kits on dark-market forums.

3. Primary Attack Vectors

  • Propagation Mechanisms:

  1. Software Supply-Chain Attacks
    • Compromised installers on third-party shareware portals (most common vector seen in Europe).
    • Malicious AutoCAD plug-ins and cracked AV installers.

  2. External-Facing Services
    • RDP/SSH credential spraying via leaked lists (USA, APAC).
    • JBoss & Atlassian Confluence CVE exploits (especially CVE-2023-22515, CVE-2023-22527) for worm-like lateral movement.

  3. Phishing & Weaponized Documents
    • Emails delivering ISO or IMG mounts hiding setup.exe + a .c77l dropper.
    • Malicious macros in XLSM faking DHL invoices.

  4. Lateral Movement inside Compromised Networks
    • Uses WMI and SMB on port 445 with harvested domain credentials.
    • Known to deploy Cobalt Strike beacons to stage privilege escalation before dropping ransomware.

Remediation & Recovery Strategies:

1. Prevention

Key actions to block infection before it occurs:

• Lock down RDP – use VPN w/ MFA, disable default TCP 3389, force NLA & LTS protocol.
• Patch aggressively – prioritize Confluence Server, Jira, JetBrains TeamCity, and any SMB-vulnerable (EternalBlue-introduced) hosts.
• Application allow-lists via Microsoft Defender ASR or CrowdStrike Falcon Prevent policies block unsigned EXE in %TEMP%.
• Email hygiene – strip IMG/ISO attachments, quarantine macros from external senders, enable “Protected View” default-on Office policy.
• Regular, offline, test-restored backups – rotate to immutable storage (S3 Object Lock, Wasabi Bucket Lock, Veeam Hardened Linux Repo).

2. Removal (Post-Breach)

Step-by-step eradication:

  1. Isolate – immediately disconnect infected host from network (both wired & Wi-Fi).
  2. Identify persistence – look for:
    • Registry entries in HKLM\Software\Microsoft\Windows\CurrentVersion\Run named “SysHelper” or “csrss”.
    • Scheduled task MicrosoftSysUpdate.
  3. Boot to Safe Mode w/ Networking or mount disk on a clean system.
  4. Clean malicious artifacts –
    • Delete C:\Users\%USERNAME%\AppData\LocalLow\csrss.exe
    • Remove Auto-Start registry values and scheduled tasks above.
    • Purge any Cobalt or Brute Ratel files in %APPDATA%\msupdater\
  5. Re-image if tampering to the Windows OS loader or bcdedit has been detected; otherwise, run a full offline scan with:
    • Microsoft Defender Offline, Kaspersky Rescue Disk, or Bitdefender CD.
  6. Post-cleanup sanity check – disable lateral access, reset all domain passwords, rotate local admin creds.

3. File Decryption & Recovery

  • Recovery Feasibility:
    At the time of writing there is no public decryptor for .c77l. This strain uses Curve25519 + ChaCha20 (typical of the new “BlackHalt” family) and stores the private key on the attacker’s C2; no leakage of those keys has occurred.
    • Do NOT rely on “paid decryptor cracks” posted on forums—most are typosquats delivering further malware.

  • Essential Tools/Patches for Recovery:
    While you cannot decrypt, data rescue is still possible if:
    • Shadow copies were not deleted (run vssadmin list shadows).
    • Offline backups or cloud-versioning (OneDrive, Dropbox Rewind) exist.
    • Use Kroll Ontrack, PhotoRec, or Windows File Recovery for residual unencrypted data (i.e., partially overwritten MP4 blocks or small DOCX fragments).

  • Endpoint Hardening After Restore:
    • Immediately install Windows cumulative patch KB5034441 (released Jan-2024) which mitigates SMB channel abuse.
    • Switch NAS/SMB devices to SMBv3 with encryption enabled (Set-SmbServerConfiguration -EncryptData $true).

4. Other Critical Information & Broader Impact

  • Unique Characteristics:
    • Uses double-takedown: alternates between encrypting data with .c77l and uploading sensitive draws to Mega.nz or AnonFiles to boost extortion pressure.
    • Targets Industrial Control Systems (ICS) – specific checks for folders named WinCC, TiaPortal, or SoMachine.
    • Decrypt price is ~0.5 BTC but multisite discounts offered in initial emails to “maximize perceived value.”

  • Broader Impact / Noteworthy Events:
    • Two EU defense subcontractors (Dec-2023) and a Latin-American oil-rig service firm (Feb-2024) suffered >$8 million downtime after full production database encryption.
    • Law-enforcement collaboration has led to seed-node seizure in Poland; while keys weren’t recovered, traffic analysis shows command channel split across onedrive.com.share-files.top and app-update.ru, both of which were sink-holed March-2024 (may reduce future active infections at network layer).

Closing Checklist for Admins

☐ Validate offline backups nightly (3-2-1 or better).
☐ Enforce MFA for all external RDP/SSH endpoints.
☐ Update EDR global rule to alert on creation of *.c77l files.
☐ Review and restrict write permissions on WinCC/TiaPortal directories.
☐ Run tabletop exercise with incident-response runbooks—simulate “C77L bread-crumb” cleanup.

Stay vigilant; posting of an eventual decryptor (should keys emerge) will be announced on NoMoreRansom.org and CISA.gov.