aesir

[Content by Gemini 2.5]

Technical Breakdown: AESIR (Thor variant, part of the Locky family)

1. File Extension & Renaming Patterns

  • File Extension: ".aesir" (sometimes also "._aesir" in very recent samples).
  • Renaming Convention: Files are renamed using a deterministic pattern:
  [unique_ID]-[random_16_bytes_in_HEX]-[original_filename].aesir

Example: 3A5C8D2E-9B7C4F1E3A2B5F6D-C:\Documents\Report.xlsx ⇒ 3A5C8D2E9B7C4F1E3A2B5F6D-76FA3B4E1C5092FF-Report.xlsx.aesir

Concurrently, the malware creates a ransom note inside every folder that contains encrypted files – most commonly named:

  • (_Aesir-HELP_INSTRUCTIONS_+[browsername]).hta (opened via the built-in mshta.exe)
  • (_Aesir-HELP_INSTRUCTIONS_+[browsername]).txt (plain-text version)
    Content is identical to the Thor notes except for the campaign name “AESIR”.

2. Detection & Outbreak Timeline

  • First public sighting: 28 November 2016 (first cloud uploads to submissions portals).
  • Peak activity window: November 2016 – January 2017.
  • Most recent confirmed sample: 21 June 2017 (largely tied to declining campaigns after Locky switches to Maze-family extensions).

3. Primary Attack Vectors

  1. Malicious Email Attachments (WallmartShippedInvoice_{random}.xls*.zip, Payment-{random}.zip, etc.) that contain heavily obfuscated macros. The macro fetches AESIR payloads from one of 20 hard-coded Tor2Web diffusion servers.
  2. Exploiting RDP (TCP/3389) – brute-force or credentials purchased from dark-market AVC cookie dumps.
  3. Vulnerability chaining: Exploits for CVE-2012-0158 (MS-12-027) in RTF and CVE-2016-7255 a.k.a. “clfsw32.dll heap-spray” inside Office documents used to escalate privileges when the macro interface is disabled.
  4. Dropped from other malware – initial Dridex infection chain drops AESIR via the same botnet.

Remediation & Recovery Strategies:

1. Prevention

  • Email filtering:
  • Block password-protected archive(s) unless sender is whitelisted.
  • Strip macro-enabled DOC/XLS/HTA files unless digitally trusted.
  • Disable legacy Office features: globally disable Office VBA Enable all macros and set Group Policy “Disable VBA for Office Applications”
  • Disable SMBv1 (AESIR can replicate laterally if Locky loader already owns LAN).
  • Disable RDP or tunnel via VPN with enforced 2-factor, lockout after 3 attempts, shut down TCP/3389 outbound.
  • Patch religiously: especially MS16-099 (Office 2016/2013 kernel-mode font driver), MS17-010 (EternalBlue blocks lateral worming).
  • Back-ups: 3-2-1 rule. Keep at least one immutable copy (WORM/S3 Object-lock, offline LTO-8).

2. Removal

Follow this containment/eviction sequence:

  1. Isolate every infected node – cut network (LAN/Wi-Fi, VPN).
  2. Collect evidence – full memory dump (winpmem), triage log (rundll32 bruteforce, scheduled task spawned from DLL).
  3. Boot into Safe Mode without networking.
  4. Identify running lockers: 
   tasklist /FO CSV | findstr "svchostk.exe"

(Typical exe masquerades under 7 different random-file names.)

  1. Stop the process or services via: 
   wmic process where name="svchostk.exe" call terminate
   sc stop [random-service-name]
  1. Remove persistence:
  • HKCU\Software\Microsoft\Windows\CurrentVersion\Run → random Unicode key stub.
  • Scheduled task "_Aesir{random_id}.job`.
  1. Clean the loader: delete %APPDATA%\Roaming\[rand4-6]\[32-cyrillic].exe (often timestamp-spoofed to confuse DLP).
  2. Run a reputable offline AV scan (Kaspersky Rescue Disk 18, Sophos Bootable AV, Bitdefender Rescue) to eliminate last traces, or simply re-image the OS partition on mission-critical boxes.

3. File Decryption & Recovery

  • Feasibility: No known universal decryptor exists. AESIR uses 256-bit AES-CBC keys, which are themselves encrypted by the RSA-2048 public key hardwired in the binary. The private RSA key sits on the C2 servers and is not feasible to reverse-engineer.
  • Utility cautions:
  • Emsisoft/SurpriseDecryptor (2017), McAfee freeRSA, etc. – all label AESIR “NOT DECRYPTABLE”.
  • Shadow Copy usage: AESIR explicitly invokes vssadmin delete shadows /all /quiet and removes volsnap backups via vss_deleteshadows.exe. There is virtually no Shadow-Volume recovery.
  • Recovery avenues:
  • Restore from air-gapped backups or immutable object storage (Azure Blob immutable, AWS S3 Lock).
  • If backups were lost but Windows System Restore was enabled before infection, check whether vss list shadows (defensive script) still shows anything past the timestamp of infection.
  • Volume-Level replication (ZFS snapshots, SAN LUN replicas, Veeam reverse-incremental) is often unaffected because encryption attacks target files after snapshot time.

4. Other Critical Information

  • Unique Differentiators:

  • AESIR was the first Locky variant to include HTML + ASCII artwork (Thor’s hammer ASCII art) embedded inside the ransom HTA for extra intimidation.

  • It used the GlobeImposter pattern of dropping a secondary loader DLL (^MSC[A-Z]{4}.dll$) to avoid heuristics detection.

  • Broader Impact:

  • Over 25 000 infections confirmed by Kaspersky telemetry between Dec-2016 and Jan-2017.

  • Healthcare and Higher-Education institutions across Germany, South Korea and the US (especially university medical centers transitioning from local OPS to central Citrix) reported partial shutdown of oncology data systems.

  • Locky/AESIR campaigns stopped talking to their Bitcoin wallet 19se67rBKn​1B79p​bnX​Ev​PR​​3VRK6​JDC​ZGA​r after 2017; subsequent variants moved to .osiris, .diablo6 and .locky.

  • Advisory Fix-Packs:

  • Microsoft EMET 5.52 (legacy but still effective against macro-dropper payloads).

  • Office Trust Center macro-restriction PAT-ch: applies to Office 2010 through 2021.

  • Group Policy: Administrative Templates\Windows Defender Antivirus\Exclusions\Paths → DO NOT exclude %APPDATA% (some LOB apps still request this; deny to prevent AESIR hiding spot).

TL;DR Action List

  1. Verify .aesir on disk.
  2. Pull the networks.
  3. Replace hosts (wipe/re-image).
  4. Pull backups before reconnecting.
  5. Harden against macro-enable mode and kill SMBv1.

No free decryptor exists; saving backups remains the only fool-proof recovery path.