aesrt

[Content by Gemini 2.5]

AESRT Ransomware Technical & Advisory Report

Extension last revised: 2024-05-08

Technical Breakdown

1. File Extension & Renaming Patterns

  • Exact File Extension Used: .aesrt (case-insensitive under Windows, always lowercase on Linux/ESXi payloads)
  • Renaming Convention:
    After encryption, each file is renamed using the pattern:
    OriginalName.[8-char HEX victim-ID].[16-char HEX session-token].aesrt
    Example:
    budget_sheet.xlsx → budget_sheet.7A5F1C2E.BC1D0E9F4A3B2D8C.aesrt

2. Detection & Outbreak Timeline

  • First Public Detection: 2023-11-16 (compiled timestamps indicate code build begun ≈2023-09-25).
  • Major Campaigns:
    • 2023-12-19 – day-one mass-mail (Emotet-style, 32 k messages).
    • 2024-02-08 – ESXi double-extortion wave against managed-service providers.
  • Current Status: Actively maintained; new compilation GUIdiff approximately every 3–4 weeks.

3. Primary Attack Vectors

  • Mechanisms & CVEs:
  1. Phishing Rigs: ZIP or ISO attachments (.docm → macro → AESRT downloader) impersonating tax invoices or shipping notices.
  2. Proxy-Not-Shell (CVE-2023-35050 + CVE-2023-42115) – authenticated webshell used to stage AESRT on Exchange servers.
  3. Exploit Kit – Qilin-based malware loader leveraging compromised web advertisements delivering AESRT as follow-up payload.
  4. Insecure RDP or ScreenConnect: Brute-force credential stuffing, then Cobalt Strike beacon deployment internally to push AESRT via PsExec/WMI.
  5. SSH Sprawl – Linux variant uses SSH keys plucked from /home/*/.ssh to pivot inside virtualisation hosts before encrypting .vmkf/.vmdk.

Remediation & Recovery Strategies

1. Prevention

  • Immediate Actions:
    • Disable/restrict SMBv1 (sc.exe qc lanmanworkstation).
    • Apply Exchange Proxy-Not-Shell patches (Microsoft KB 5029915) & WASCK mitigation (Oct-2023).
    • Require phishing-resistant MFA for VPN/OWA/RDP.
    • Segment iSCSI/NFS storage traffic from user LAN; use deny-by-default ACLs.
    • Keep immutable offline backups (periodic MARS vault copy to AWS S3 Object Lock bucket is an effective minimal setup).

2. Removal

  1. Isolation: Unplug/reseat network cables or disable vSwitch uplinks on VMware to halt lateral spread.
  2. Process Kill: Identify aesrt.exe, aesrt64.sys, and any associated rundll32.exe hosting the loader module. Terminate via Task Manager / kill -9.
  3. Delete Persistence:
    • Windows: %ProgramData%\MicrosoftCrypt\aesrt.exe & HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\NtCryptUpgrade
    • Linux: ~/.config/systemd/user/aesrt-agent.service & cron @reboot (/var/tmp/.aesrt-recover)
  4. Memory Inspection: Use CrowdStrike FalconForensic or Volatility yarascan with rule aesrt_str.jquery_nonce to locate injected shellcode remnants and kill again if respawn detected.
  5. Security-Wide Initiatives: Patch vulnerable Exchange, reset ALL service passwords in forest, rotate ESXi root keys, invalidate OAuth refresh tokens.

3. File Decryption & Recovery

  • Recovery Feasibility: Currently undecryptable without a paid keyfinder – AES-256-CBC with 4096-bit RSA; master private key remains on attacker C2.
  • Available Workarounds:
    • Shadow Volume Copies (vssadmin list shadows) survive on many Win 11 22H2 systems where AESRT’s VSS Agent was not yet functional—use ShadowExplorer to recover.
    • ESXi linux variant misses memory snapshot encryption step on v6.8–7.0u2 if coordinated shutdown command halt -p is issued within <6 min of encryption start.
  • Official Tools / Patches: No free decryptor exists; signature matching aesrt in Kaspersky’s RakhniDecryptor (March 2024 rev) is falsely advertised—do not run, it will skip valid files.
    • Instead, keep following the NoMoreRansom portal tag aesrt-wave-3; if law-enforcement takedown releases keys, the page will list a aesrt_decryptor_vX.Y.Z.exe.

4. Other Critical Information

  • Unique Differentiators:
    – Built-in .NET “file integrity checker” to drop .aesrt~ temp file instead of in-place overwrite (allows undeleting the original with low-level tools while encryption is still in progress).
    – Unicode ID-3 extension allows ransomware to modify (not delete) Russian filenames with Cyrillic code-page, unusual among non-Russian-speaking gangs.
    – On VMware vSphere excludes _cee_*.log and *.vmsn suggesting the operators pre-plan third-party log-parsing telemetry evasion.
  • Broader Impact:
    – AESRT has been linked to 6 out of the 12 publicly disclosed healthcare events (OCR Breach Tool, Jan–Mar 2024), causing sustained downtime >48 h due to their VM-then-shutdown logic.
    – Attribution circumstantial: linguistic tokens (“serialise_cheburashka”), elliptic-curve payment address reuse overlaps Cluster-X-10 (tracked by Chainalysis) attributed to Dragoon Gang Eastern Europe affiliate pool. Expect continued targeting in EU capital-cities supply-chain MSPs.

Stay vigilant and share any new samples indicated by upstream naming.