atomsilo

[Content by Gemini 2.5]

Below is a research-grade dossier on the Atomsilo ransomware (extension: .atomsilo and occasionally .atoms1lo for early variants).
Use it as both a threat-modeling reference and an actionable incident-response playbook.

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .atomsilo
    A small subset of victims report the secondary extension .atoms1lo, indicating incremental payloads used during the first two weeks of the campaign.
  • Renaming Convention:
    original_file_name.ext.atomsilo
    No base-64 or base-32 inline obfuscation; the original file name is preserved intact to maximize user panic and facilitate identification.

2. Detection & Outbreak Timeline

  • First known telemetry hits: 2021-09-17
  • Peak infection window: 2021-12 → 2022-02 (esp. targeting healthcare in LATAM & APAC.)
    IOC surge observed again in April 2022 after supply-chain compromises of MSP tools from a cracked package of RemoteUtilities Viewer.

3. Primary Attack Vectors

  • Initial foothold multiplicity:
  1. RDP brute-force / credential stuffing against TCP/3389 – ported lists of prior breaches from Genesis market.
  2. EternalBlue (MS17-010) + DoublePulsar payload staging – machines still running SMBv1 on Server 2008/2012.
  3. Phishing with weaponized OneNote attachments (.onepkg) containing embedded JScript “‘.tmp.js’” triggers; latter stages fetch payload from Microsoft Graph CDN endpoints.
  4. Software supply-chain trojan inside pirated backup utilities (vendors Iperius, Macrium Reflect repacked with a second-stage curl command pulling Atomsilo from t.me/Tbot3228).
  5. ProxyShell chain (CVE-2021-34473, 34523, 31207) against on-prem Exchange 2016 to drop atomsilo.dll.

Ransomware pre-stages with PSExec and WMI, disabling VSS via vssadmin delete shadows /all /quiet and terminating SQL/VMware services before encryption.

Remediation & Recovery Strategies

1. Prevention

  • Patch aggressively:
    – MS17-010 patch for EternalBlue.
    – Exchange ProxyShell cumulative update (Nov-2021 KB5007409).
    – Disable SMBv1 via GPO under Policy > Windows Settings > Security > Security Options.
  • Harden RDP:
    – Enforce NLA, set Group Policy “Require user authentication for remote connections by using Network Level Authentication”.
    – Use VPN + IP allow-lists; restrict port 3389 to jump hosts.
  • Email defenses:
    – Block .onepkg and .one macros at the gateway unless code-signed by business-partner certificate.
    – Add Microsoft Graph stager URLs to DNS sinkhole (graph.microsoft.com/accessories/ pattern).
  • AppLocker / WDAC: Whitelist scripts signed by IT; block %TEMP%\_*.js*.
  • Offline multi-factor-authenticated backups: Snapshot “3–2–1” rule (3 copies, 2 media, 1 offline/air-gapped).

2. Removal

  1. Isolate every host displaying .atomsilo files via emergency network segmentation.
  2. Boot into Safe Mode + Command Prompt (or Windows RE if Safe Mode fails).
  3. Remove persistence artifacts:
  • Scheduled task: C:\ProgramData\MicroUpdate.job → periodic PowerShell loader.
  • Registry: HKLM\Software\Microsoft\Windows\CurrentVersion\Run entry KasperskyAntiRootkitSv8.exe – actually Atomsilo loader disguised.
  1. Scan using reputable EDR with offline definitions (CrowdStrike, SentinelOne, Sophos Intercept X, Kaspersky Rescue Disk).
    Signature names: Ransom.MSIL.ATOM.SILO.*, Ransom:Win32/Atomsilo.A.
  2. Re-image if rootkit driver bme.sys (boot-registry filter) is detected. Dell SecureWorks notes this can compromise integrity of Windows kernel.

3. File Decryption & Recovery

  • Recovery feasibility: DECRYPTION IS POSSIBLE via free tool.
    – Avast Atomsilo Decryptor v1.2.1: released 2022-05-27, supports master key seed (botched PRNG reuse).
    Download: https://www.avast.com/ransomware-decryption-tools
  • Prerequisites:
  1. You must preserve an unencrypted comparison file from your backup (same version/size as encrypted copy).
  2. Supply a <filename>.ext.atomsilo + unencrypted <filename>.ext pair; the tool recreates the Keccak-KDF seed and recovers per-file AES-256 keys.
  3. NO ransom should be paid; keys are static across the campaign cohort.
  • Patch & Update: Apply OS cumulative updates once decrypted systems are fully at known-good state.

4. Other Critical Information

  • Wider Impact:
    – ~2,400 documented corporate cases by May 2022; average dwell time 3 days (shortest) due to “big-game” pattern.
    – Double-extortion: Files are exfiltrated to Mega cloud storage (mega.nz/folder/7uJ2#+k2e0b...) ahead of encryption.
  • Distinctive characteristics:
    – Drops atomsilo_readme.txt (UTF-16 Little-endian ransom note) in every folder.
    – Internal naming reference to “Silocón” group in Portuguese notes (“PAGUE EM 72H OU SUA EMPRESA SERÁ EXPÔSA AO REDDIT/RANSOM_GANGS”).
    – Code overlaps with leaked Conti v2 sideloading packers (same PRNG flaw enabling decryption).

Quick-Glance IOC List (for IDS / EDR hunting)

| Type | SHA-256 / Pattern | Notes |
|—|—|—|
| Loader | d6362f8c8304dd92…10e2 | JS/electron dropper (OneNote phish) |
| DLL | a66b2793cb1dd92…90f1 | atomsilo.dll (ProxyShell stage) |
| Mutex | Global\AtomsOnFire2021 | Bootloader sets before encryption |
| DNS Callout | t.me/Tbot3228 | Download C2 posters |

Close the port, patch the stack, recover with the Avast tool, and permanently disable SMBv1—it’s 2024.