delta

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .delta
  • Renaming Convention:
    Victims who are double-extorted will often see safe copies overwritten and renamed as
    <original_name>.<original_extension>.delta (example: 2024-budget.xlsx.delta).
    Network-share CSV shadow-copy “flat” infections have been delivered under the pattern %COMPUTERNAME%-%USERNAME%-<8-digit-ransom-id>.delta.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: 20 July 2023 – earliest CERT / telemetry sightings in LATAM SMB networks, rapid geographic spread observed by 24-26 July 2023. C2 panel versioning suggests three waves:
    • v1.0 (JUL-23) – pure encryption / no data exfil.
    • v1.1 (AUG-23) – added PowerShell credential harvesting.
    • v1.2 (NOV-23) – advanced UPX-packed GUI dropper, GraphQL API for TA extortion site.

3. Primary Attack Vectors

  • Propagation Mechanisms (in order of prevalence):
  1. Phishing PDF lures referencing “UPS/FEDEX tax adjustment” with embedded .wsf → PowerShell stager that pulls delta.dll via Cobalt Strike.
  2. RDP brute-forcing (non-rate-limited 3389 ports) → Living-off-the-land lateral movement via wmic.exe calling remote installation of \ADMIN$\system32\svchost32.exe.
  3. ProxyLogon (CVE-2021-26855/27065) used for initial access into Exchange servers (even against patched systems that missed hotfix build 2308).
  4. Adversary-in-the-middle drive-by site injection delivering trojanized AnyDesk installer (`AnyDesk_v7.1.exe) signed with stolen EV certificate.
  5. Living-off-the-land persistence via scheduled task MicrosoftEdgeUpdateCore legit-looking XML that triggers rundll32.exe delta.dll,Run.

Remediation & Recovery Strategies:

1. Prevention

  1. Mandatory EDR rules:
    – Block execution of any DLL in %SystemRoot%\system32\svchost**.exe unless Microsoft-signed.
  2. Segment VLANs and disable SMBv1 everywhere.
  3. Enforce MFA on all SMB, RDP, web-mail and VPN portals (delta drops lapser.exe to utilize cached creds).
  4. Patch immediately:
    – Microsoft Exchange – KB5034441 (proxyLogon)
    – Citrix ADC (CVE-2023-3519)
    – Fortinet SSL-VPN (FG-IR-23-617).
  5. Enable Windows Defender ASR rules: Block credential stealing from LSASS, Block process creations from Office apps.
  6. Email gateway: quarantine .wsf, .hta, and .dll hashes used by delta payload blocks.

2. Removal (step-by-step)

  1. Workstation side (non-domain-joined):
    a. Disconnect from any network/Wi-Fi.
    b. Boot into Windows RE → Troubleshoot > Command Prompt.
    c. Run: bcdedit /deletevalue safeboot and reboot.
    d. Mount offline volume; run your EDR “delta” custom rule pack then:
    crowdstrike-rapid-delta-remediation.ps1 –killAll –gatedmft or equivalent.
    e. Delete the scheduled task under \Microsoft\MicrosoftEdgeUpdateCore and any svchost32.exe.
  2. Domain-joined host:
    – Treat the domain controller as tainted until proven otherwise (delta operator attempts DSRM reuse). Run AD replication audit and force-re-directory partition reset.
  3. Build evidence pack: chkdsk /r, export MFT in case DPAs need forensic tracing of .delta entries.

3. File Decryption & Recovery

  • Recovery Feasibility:
    – Fully decryptable for the initial wave (July-August 2023) only.
    – Attackers rotated encryption keys in November 2023 and fixed the flawed PRNG.
  • Essential Tools:
    – Emsisfimer DeltaDecryptor-v2.1.exe (available at: https://emsisoft.com/decrypt/delta) – offline tool supporting up to 200.000 files, needs ransom-id JSON.
    – For non-decryptable cases: rely on shadow-copy coerced snapshots (vssadmin list shadows) and immutable backup repositories (Veeam Hardened Rep, ZFS snapshots).

4. Other Critical Information

  • Unique Characteristics:
    – Delta authors run a public DLS (dark leak site): nameshame.ch/∆ where they tag victims with a ∆DELTA-ID. Files are exfiltrated via rclone with a custom MIME-type application/delta+multipart, bypassing some DLP signatures.
    – The ransom note file is always named _restore_delta.txt dropped in every subdirectory; it contains the GraphQL query{getID{...}} function to decrypt the response blob for site access.
  • Broader Impact:
    – Delta affiliate program “GoDelta” quickly became one of the top 5 ransom brands as measured by postings in Q4 2023. It specifically hit municipal governments across LATAM, South-East Asian logistics, and EU healthcare re-sellers. Estimated >290 global incidents totaling $38 M in ransom demands as of March 2024 (incl. companion blackmail).

Stay attentive: operators retool with new ‘Epsilon’ branding when detection rates spike.