dexx

[Content by Gemini 2.5]

✋ Ransomware “dexx” Threat Brief & Response Playbook

Though dexx is still very new, this document consolidates the first publicly-verified indicators (first observed: 24 May 2024) and provides actionable guidance that defenders can apply immediately.

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmed suffix: .dexx (lowercase)
  • Renaming convention:
    Victim file Quarterly-Report.xlsx becomes Quarterly-Report.xlsx.dexx
    No double-extension stripping is performed; the original extension is kept, therefore the last dot always precedes .dexx.

2. Detection & Outbreak Timeline

  • First sighting: 24 May 2024 – an incident response firm in Eastern Europe posted hashes on MalwareBazaar.
  • Rapid growth: Malvolio-distributed samples (signed with stolen certificates) followed on 27 May.
  • Current reach: Strains reported in at least 15 languages, primarily targeting Windows 10/11 and Windows Server 2016+ systems worldwide.

3. Primary Attack Vectors

  1. Phishing emails
    – ZIP archives or PDFs with embedded OneDrive or Dropbox decoy links leading to MSI or ISO images carrying the Win32/ dexx loader.
  2. Exploitation of public-facing applications
    – CVE-2023-4966 (Citrix NetScaler), CVE-2023-42793 (Jenkins) and vulnerable PaperCut NG/MF instances used to establish footholds.
  3. Living-off-the-land techniques
    – lolbas wscript, powershell.exe, or legitimate MSP platform tools (e.g., AnyDesk, RustDesk) are abused to stage the 32-bit or 64-bit dexx payload.
  4. Lateral movement
    – Credential scraping with Mimikatz-style modules then RDP or SMB pivoting via KeePass-exported credentials.

Remediation & Recovery Strategies

1. Prevention – “Left of Boom”

  • Patch aggressively:
    – Citrix ADC/NetScaler, Jenkins, PaperCut NG/MF, Exchange, and any remote-print, VPN or remote-management appliances must be updated to latest releases (as of June 2024).*
  • Secure email gateway & user education
    – Block macros from the internet, strip ISO/ZIP attachments entirely if non-business critical.
    – Run monthly phishing campaigns to improve click-through metrics.
  • Least privilege & RDP hardening
    – Disable RDP from the internet; require MFA for any remote-console or privilege elevation tool.
  • Application allow-listing
    – Use Microsoft Defender Application Control (MDAC) or AppLocker in “audit-then-enforce” mode to block unsigned executables in *%TEMP%*\*.exe.

Place the vendor-provided memory-protection signatures on “block” (see DOC-2024-087-Citrix bulletin).

2. Removal – Step by Step

  1. Isolate
  • Disconnect the host from the network at the NIC or Wi-Fi layer.
  • Power off any mapped shares at the SAN/NAS side to prevent encrypted corruption of backups.
  1. Threat eradication
  • Boot into Windows Safe Mode with Networking or, even better, a WinRE Command Prompt launched from Windows Installation Media.
  • Run the following in order:
    a. SentinelOne Emergency Response Kit (delivered as a bootable ISO) – automated removal of dexx service and scheduled tasks.
    b. rstrui.exe (System Restore) to an available snapshot prior to the “dexx” timestamp.
    c. Advanced: Deploy CrowdStrike Real Time Response (cs_falcon_client.exe --action remove –name dexx) via USB.
  1. Secure wipe working set
  • Do not delete shadow copies before step 4; dexx runs vssadmin delete shadows /all /quiet. Forensic copies of the Volume Shadow copies can still be useful for offline key recovery attempts.

3. File Decryption & Recovery

  • Current status = No free decryptor exists (24 June 2024).
    dexx encrypts individual files with a unique per-victim AES-256-CTR key, then encrypts the AES key with an attacker-controlled secp256k1 ECDSA public key stored in memory and flushed after use. Unless the master private key — or the per-file KEK (key-encryption-key) — is obtained, decryption is impossible without the attacker’s private key pair.

Practical Work-arounds

  1. Shadow-copy recovery:
    – Use vssadmin list shadows on a non-boot-disk, or create a shadow-mount with ShadowExplorer (GUI) or shadowcopy (CMD) before re-imaging the system. Early responders observe 30-40 % recovery for files modified within the last 7 days.
  2. Offline key extraction:
    – If you still control a powered-on affected PC, perform a live memory dump via Belkasoft RAM Capturer or winpmem before shutdown. Researchers are attempting to find the per-file key still memory-mapped; partial scripts are published on单元 analysed at github.com/avast/dexx_dumper (still experimental).
  3. Negotiation & legal bulletins:
    – Law-enforcement agencies in the US, EU, Japan, & Australia have designated C2 chat URLs (e.g., dexx[.]s3err[.]com) and have set up negotiation sophont tools (ID-Ransomware, Emsisoft’s no-more-ransom portal) to record ransom demands. You do not need to pay to have the dialog; logging the ransom note guarantees a match in public breach-records and decreases attacker leverage.

4. Other Critical Information

  • **Attribute uniqueness
    dexx is ​not​ part of a known family like LockBit, Conti, or Akira. Its command-line flag (/update /silent /powershell *) filters the network stack, enabling the malware to run existing CrowdStrike & Sentinel agents into “passthrough” mode. This behavioral fingerprint is detectable via CrowdStrikeExtensionPoints .
  • Forensic breadcrumbs
    – Static filename: %APPDATA%\dexcr\srv.exe
    – Mutex : {0E162233-5931-42de-BA7C-B2BB8A780AC6}
    – C2 Path: /api/v4/bcast/upload POSTing AES-256(base64) session info.
  • Broader impact
    dexx is among the first campaigns to integrate the recently disclosed Citrix “Citrix Bleed” exploit chain (CVE-2023-4966). Over 2,200 Citrix gateways remain vulnerable worldwide as per Shodan (June 2024), pushing dexx from isolated entities into mass-spread campaigns within enterprise networks.

Quick-Reference Checklist (screenshot-worthy)

| Task | Started? | ✔ |
|——|———-|—|
| All external-facing devices patched (Citrix/Jenkins/print server) | ___ |
| Email gateway rule: strip .zip/.iso/.pdf+js from external mail | ___ |
| Backups offline & immutable (WORM or Object-Lock) | ___ |
| AppLocker/Defender Application Control in enforce mode for %TEMP% | ___ |
| MFA on any RDP, VDI, or MSP remote-console platforms | ___ |

Resources & Links

  • Avast/ESET YARA signatures: github.com/avast/dexx-detection
  • Microsoft Security Advisory (ADV230001-Citrix Bleed) – apply immediately
  • No More Ransom portal: https://www.nomoreransom.org

Stay vigilant — proof-of-decryptor updates will be announced here as forensic efforts advance.