Full Report
A sudden CPU spike turned out to be the first clue of an in-progress RansomHub ransomware attack. Varonis breaks down how their team traced the attack from fake browser updates to domain-admin takeover, ultimately stopping the attack before files were encrypted. [...]
Analysis Summary
# Incident Report: RansomHub Ransomware Attack Thwarted by CPU Spike Alert
## Executive Summary
This incident involved a RansomHub affiliate attempt that was detected via an anomalous CPU spike on a server. Attackers gained initial access via a fake browser update (malicious JavaScript) and rapidly established persistence, deployed a multi-layered SOCKS proxy for C2, and began lateral movement and credential harvesting before the Varonis response team contained the threat. The attack was neutralized before any files were encrypted or significant business impact occurred.
## Incident Details
- **Discovery Date:** Undisclosed (Detected via CPU spike)
- **Incident Date:** Undisclosed (Occurred over 48 hours leading up to remediation)
- **Affected Organization:** Customer of Varonis (Not disclosed)
- **Sector:** Undisclosed
- **Geography:** Undisclosed
## Timeline of Events
### Initial Access
- **Date/Time:** Attack initiation time undisclosed.
- **Vector:** Malicious JavaScript payload disguised as a legitimate browser update downloaded and executed by a user.
- **Details:** Initial execution triggered automated reconnaissance, AD enumeration, local system querying, and memory credential hunting.
### Persistence & C2 Establishment
- **Timing:** Within minutes of initial execution.
- **Action:** Second-stage malware deployed as a recurring Scheduled Task.
- **Action:** A legitimate Python distribution was downloaded to `%LOCALAPPDATA%\ConnectedDevicesPlatform`, along with a 10-layer, multi-stage encrypted Python script.
- **Action:** The encrypted script, designed with anti-analysis features (VM/Debug detection), was unpacked, ultimately deploying a SOCKS proxy to expose the corporate network to attacker infrastructure.
### Lateral Movement & Reconnaissance
- **Timing:** Immediately following initial compromise.
- **Action:** Adversary began scanning network shares for credential-containing files (e.g., RDP, OVPN files, KeePass Vaults).
- **Action:** Attacker manipulated organizational email signatures ($env:APPDATA\Microsoft\Signatures) by embedding malicious image references, potentially setting up future NTLM relay attacks against other users.
- **Action:** Discovery included active enumeration of Active Directory users and computers.
### Detection & Response
- **Detection:** The first clue was a **sudden spike in CPU activity** on a compromised server.
- **Response:** Varonis IR team investigated, hunted, contained, and remediated the threat over the subsequent 48 hours.
- **Outcome:** The attack was stopped before files were encrypted, resulting in zero business downtime.
## Attack Methodology
- **Initial Access:** User execution of a malicious JavaScript payload disguised as a browser update.
- **Persistence:** Installation of a recurring Scheduled Task; installation of Python components in user profile space (`%LOCALAPPDATA%`).
- **Privilege Escalation:** Actions included hunting for credentials in memory and scanning for files containing authentication data (Implied path to escalation, though not explicitly detailed as achieved).
- **Defense Evasion:** Multi-layered (10-stage) encryption with randomized variables; inclusion of VM detection, Debug detection, and Process Tracing detection within the malware.
- **Credential Access:** Hunting for credentials in memory; potential future NTLM credential harvesting via modified email signatures.
- **Discovery:** Enumerating Active Directory users/computers; querying local system info; scanning network shares for credential material.
- **Lateral Movement:** Deployment of a SOCKS proxy using a compromised host as a transport pivot to bridge attacker infrastructure with the internal network.
- **Collection:** Scanning for specific credential-related file types.
- **Exfiltration:** The C2 relay (SOCKS proxy) would facilitate data exfiltration, though the attack was stopped pre-exfiltration.
- **Impact:** Threat neutralized prior to encryption or major operational impact. **(Goal was Ransomware deployment)**.
## Impact Assessment
- **Financial:** Minimal, as remediation occurred before encryption or downtime.
- **Data Breach:** Data collection/reconnaissance was underway, but no confirmed large-scale exfiltration had occurred before intervention.
- **Operational:** Zero business downtime achieved due to advanced intervention.
- **Reputational:** Not publicly disclosed.
## Indicators of Compromise
- **Network Indicators:** SOCKS proxy communication tunneling traffic to attacker infrastructure.
- **File Indicators:** Malicious JavaScript payload; Python installation in `%LOCALAPPDATA%\ConnectedDevicesPlatform`; Multi-stage encrypted Python script.
- **Behavioral Indicators:** High CPU utilization spike on affected server; creation of recurring Scheduled Tasks; manipulation of **all** user email signature files in `$env:APPDATA\Microsoft\Signatures`.
## Response Actions
- **Containment:** Immediate action taken by the Varonis team to isolate and halt the ongoing compromise activities.
- **Eradication:** Unpacking and analysis of the custom malware to fully understand and remove persistence mechanisms and C2 channels.
- **Recovery:** Security posture was restored with zero business downtime.
## Lessons Learned
- **Visibility is Critical:** A seemingly innocuous event (CPU spike) can be the first indicator of advanced, in-progress compromise.
- **Advanced Malware Techniques:** Attackers utilize highly obfuscated and multi-layered encryption routines specifically designed to thwart standard analysis tools.
- **Low-and-Slow Targeting:** The adversary immediately targeted high-value items like AD configuration and credential storage upon initial foothold.
- **Supply Chain/Phishing Risk:** User-initiated execution of malicious files disguised as "updates" remains a primary entry vector.
## Recommendations
- **Enhance Monitoring:** Implement enhanced monitoring centered around anomalous baseline deviations (like CPU spikes) which may signal an underlying threat activity like malware unpacking or C2 beaconing.
- **Restrict Execution:** Implement controls (e.g., application whitelisting) to restrict execution from user profile directories (like `%LOCALAPPDATA%`) and prevent the installation of non-standard software distributions.
- **Email Security:** Implement protections or scanning specifically against suspicious image/HTML content embedded in email signatures, which can be used in NTLM relay attempts.
- **Credential Hunting Detection:** Establish detection rules for internal reconnaissance commands (PowerShell execution targeting memory, AD enumeration, scanning for credential file types).