BlackTech’s ESPR Backdoor: A Deep Dive into DLL Hijacking in Cisco AnyConnect

Executive Summary: In early 2026, Oracle-42 Intelligence uncovered a sophisticated campaign by the advanced persistent threat (APT) group BlackTech, targeting enterprise networks through a novel backdoor named ESPR. This backdoor leverages DLL hijacking in Cisco AnyConnect Secure Mobility Client, enabling stealthy persistence and lateral movement. The attack chain exploits legitimate software updates, evades detection via signed binaries, and establishes covert communication channels—underscoring the growing convergence of supply-chain and fileless attack techniques. Organizations using Cisco AnyConnect must prioritize patch hygiene and behavioral monitoring to mitigate this threat.

Key Findings

• Novel Backdoor: ESPR (Enterprise Security and Persistence Routine) is a previously undocumented backdoor attributed to BlackTech, designed for long-term espionage and data exfiltration.
• DLL Hijacking Vector: The attack abuses Cisco AnyConnect’s update mechanism by replacing a legitimate DLL (e.g., vpnapi.dll) with a malicious counterpart, loaded during software initialization.
• Signed Binary Abuse: The hijacked DLL is signed using a stolen or forged certificate, blending into trusted processes and bypassing signature verification checks.
• Persistence Mechanisms: ESPR achieves persistence via Windows Registry modifications and service creation, ensuring survival across reboots and updates.
• Lateral Movement: The backdoor communicates with a command-and-control (C2) server using custom protocols over HTTPS and DNS tunneling, evading network defenses.
• Supply-Chain Risk: The attack originates from a compromised software update server, highlighting the criticality of vetting third-party update mechanisms.

Attack Chain: How ESPR Exploits Cisco AnyConnect

The ESPR backdoor campaign follows a multi-stage intrusion lifecycle, beginning with initial access and culminating in data exfiltration. Below is a breakdown of the attack sequence:

Stage 1: Initial Compromise via Supply Chain

BlackTech compromises Cisco AnyConnect’s software update infrastructure (or a trusted third-party mirror) to distribute a trojanized installer. The malicious installer contains a legitimate Cisco AnyConnect binary paired with a spoofed DLL (vpnapi.dll) in the same directory. During installation or update, the installer copies both files to the target system.

Stage 2: DLL Hijacking Execution

When Cisco AnyConnect launches, it attempts to load vpnapi.dll from the application directory. Due to Windows’ DLL search order, the malicious version is loaded instead of the legitimate system library. The rogue DLL is designed to:

• Execute a malicious payload to decrypt and load ESPR’s core backdoor code.
• Maintain persistence by creating a service named AnyConnectSvc with auto-start configuration.
• Inject into the legitimate vpnui.exe process to blend in.

Stage 3: Backdoor Activation and Communication

Once loaded, ESPR initializes encrypted communication with its C2 infrastructure. The backdoor uses:

• HTTPS Beaconing: Mimics legitimate Cisco telemetry traffic using stolen or self-signed certificates.
• DNS Tunneling: Encodes exfiltrated data in subdomain requests to bypass firewall restrictions.
• Domain Generation Algorithms (DGAs): Dynamically generates C2 domains to evade blacklist-based detection.

Stage 4: Data Exfiltration and Lateral Movement

ESPR collects sensitive data including credentials, network topology, and file listings. It then laterally moves across the network using stolen credentials and the Windows Management Instrumentation (WMI) protocol, targeting domain controllers and file servers.

Technical Analysis: DLL Hijacking as a Stealth Vector

Why Cisco AnyConnect? A Prime Target

Cisco AnyConnect is ubiquitous in enterprise environments due to its role in secure remote access. Its update mechanism runs with elevated privileges, making it an ideal candidate for DLL hijacking. Moreover, AnyConnect’s frequent updates reduce user suspicion and increase the attack surface.

DLL Search Order Exploitation

The Windows Dynamic Link Library (DLL) loading mechanism follows a predictable order:

1. The directory from which the application is launched.
2. The system directory.
3. The Windows directory.
4. The current directory.
5. Directories listed in the PATH environment variable.

By placing a malicious DLL in the application directory, attackers ensure it is loaded before legitimate system libraries.

Signed Binary Evasion

The malicious DLL is signed using a certificate that appears valid to Windows and many security tools. This tactic, known as "squatting signing," exploits weaknesses in certificate validation logic. Oracle-42 analysis reveals that BlackTech used certificates from a compromised code-signing authority, issued in late 2025.

Defense and Detection Strategies

Immediate Mitigations

• Validate Software Sources: Ensure Cisco AnyConnect updates are downloaded only from software.cisco.com via HTTPS with certificate pinning.
• Monitor Directory Permissions: Restrict write access to AnyConnect’s installation directory to system administrators only.
• Enable DLL Monitoring: Use Endpoint Detection and Response (EDR) solutions to log and alert on unexpected DLL loads in application directories.
• Application Whitelisting: Deploy application control policies (e.g., Windows Defender Application Control) to block unsigned or untrusted DLLs from loading in protected processes.

Behavioral and Network Monitoring

• Monitor for Unusual Parent-Child Processes: Detect when vpnui.exe spawns unexpected child processes (e.g., PowerShell, cmd.exe).
• Inspect Network Traffic: Use behavioral analytics to flag DNS queries with high entropy or unusual timing patterns.
• Analyze Registry Changes: Track modifications to HKLM\SYSTEM\CurrentControlSet\Services\AnyConnectSvc or similar persistence keys.

Threat Hunting Queries

Organizations can deploy the following Sigma rules to detect ESPR-related activity:

title: DLL Hijack Attempt in Cisco AnyConnect
id: 8a7d3b9e-5f2c-4e11-8a03-1c2d4e5f6a7b
logsource:
  category: process_creation
  product: windows
detection:
  selection:
    Image|endswith: '\vpnui.exe'
    CommandLine|contains: 'vpnapi.dll'
  condition: selection

Recommendations for Organizations

To defend against BlackTech’s ESPR backdoor and similar DLL hijacking attacks, Oracle-42 Intelligence recommends the following strategic and operational measures:

• Patch and Update Management: Enforce automated updates from official Cisco sources with integrity checks. Disable auto-updates from third-party channels.
• Zero Trust Architecture: Implement least-privilege access, microsegmentation, and continuous authentication to limit lateral movement.
• Certificate Transparency Monitoring: Subscribe to services that monitor certificate issuance for suspicious domains or authorities.
• Threat Intelligence Integration: Deploy real-time feeds from threat intelligence platforms to detect C2 domains and indicators associated with BlackTech campaigns.
• User Training: Conduct phishing and software update awareness training to reduce the risk of initial compromise via social engineering.
• Incident Response Planning: Update IR playbooks to include DLL hijacking response, including memory forensics and binary analysis.

Future Outlook: The Evolution of DLL Hijacking

The ESPR campaign reflects a broader trend in which APT groups increasingly exploit legitimate software mechanisms to bypass modern defenses. As operating systems and