Technical Observation: Analyzing VPN Session Anomalies
During a recent incident response engagement involving a manufacturing firm in the Chakan industrial belt, we observed Interlock ransomware operators leveraging compromised VPN credentials to gain initial access. The attackers bypassed traditional perimeter defenses by exploiting weak MFA implementations on a legacy Cisco ASA 5525-X. By executing the following command, we identified multiple concurrent sessions originating from high-risk IP ranges that did not match the typical user profile, a process streamlined by SIEM log monitoring:
show vpn-sessiondb detail anyconnect
Session Type: AnyConnect Username : j.doe Index : 1422 Assigned IP : 10.10.50.12 Public IP : 193.233.202.15 Protocol : AnyConnect-Parent SSL-Tunnel DTLS-Tunnel License : AnyConnect Premium Encryption : AES256 Hashing : SHA1 Bytes Tx : 14220 Bytes Rx : 8921 Group Policy : SALES_POLICY Tunnel Group : SALES_VPN Login Time : 02:14:22 UTC Tue Oct 24 2023 Duration : 0h:42m:11s Inactivity : 0h:00m:00s VLAN Mapping : N/A VLAN : none Auditor ID : 0
The presence of SHA1 hashing and an IP address associated with known Interlock C2 infrastructure (193.233.202.0/24) confirmed the breach. This technical guide outlines the specific CLI-driven steps required to harden Cisco ASA and FTD environments against these specific threat vectors.
Cisco ASA vs. Secure Firewall (FTD): Architectural Differences
Understanding the underlying OS architecture is critical for hardening. Cisco ASA runs on a monolithic LINA (Linux and ASA) engine, where the management and data planes are tightly coupled. In contrast, Cisco Secure Firewall (FTD) utilizes a dual-engine approach: the LINA engine handles L3/L4 traffic, while the Snort engine handles L7 inspection and IPS/IDS functions, which are core components of a Zero Trust Architecture.
The LINA Engine Vulnerability Surface
The ASA's legacy architecture makes it susceptible to CVE-2023-20269, where the web-based management interface can be brute-forced if not properly isolated. We recommend moving management traffic to a dedicated OOB (Out-of-Band) network.
FTD and Snort 3 Performance
Transitioning to Snort 3 on FTD provides better multithreading capabilities, allowing the firewall to inspect encrypted traffic with lower latency. For Indian enterprises handling high-volume transactions, this is essential to prevent the "security vs. performance" trade-off that often leads to rule bypasses.
Securing the ASA Management Plane
The management plane is the most common entry point for Interlock affiliates. If your http or ssh commands are set to 0.0.0.0 0.0.0.0, you are effectively inviting brute-force attempts from the public internet. Securing these entry points is easier with a zero-trust terminal.
Restricting Access via CLI
We must restrict management access to specific internal subnets. Avoid using the outside interface for management whenever possible.
Remove global access
no http 0.0.0.0 0.0.0.0 outside no ssh 0.0.0.0 0.0.0.0 outside
Define trusted management subnets (e.g., IT Admin VLAN)
http 10.1.10.0 255.255.255.0 inside ssh 10.1.10.0 255.255.255.0 inside
Set SSH timeout and version
ssh version 2 ssh timeout 10
Implementing AAA and Local Fallback
Interlock operators often target local administrative accounts. We must enforce RADIUS or TACACS+ for all CLI and ASDM access, with local accounts reserved only for emergency fallback.
aaa-server RADIUS_SERVER protocol radius aaa-server RADIUS_SERVER (management) host 10.1.10.50 key authentication-port 1812 accounting-port 1813
aaa authentication ssh console RADIUS_SERVER LOCAL aaa authentication http console RADIUS_SERVER LOCAL
Hardening VPN Gateways Against Interlock
Interlock ransomware heavily utilizes VPN-based entry. The following configuration addresses CVE-2023-20269 by enforcing strong SSL settings and secondary authentication.
Enforcing TLS 1.2 and 1.3
Legacy protocols like TLS 1.0 and 1.1 are vulnerable to downgrade attacks. We observed Interlock tools attempting to force a handshake down to TLS 1.0 to intercept credentials, a technique often discussed in our guide on bypassing administrator protection.
Disable weak SSL/TLS versions
ssl server-version tlsv1.2 tlsv1.3 ssl cipher tlsv1.2 high ssl cipher tlsv1.3 high ssl dh-group group14
Implementing MFA via Duo or RADIUS
Single-factor authentication is no longer sufficient. Under the DPDP Act 2023, failing to implement "reasonable security practices" like MFA can result in significant penalties (up to ₹250 crore) in the event of a data breach.
tunnel-group DefaultWEBVPNGroup general-attributes authentication-server-group RADIUS_SERVER secondary-authentication-server-group DUO_MFA
Ensure the secondary group is configured correctly
aaa-server DUO_MFA protocol radius aaa-server DUO_MFA host 10.1.10.60 key
Threat Detection and Brute-Force Mitigation
Cisco ASA has built-in threat detection features that are often disabled by default to save CPU cycles. However, against Interlock's automated scanning, these features are vital.
Configuring Basic and Advanced Threat Detection
We need to monitor for scanning activity and service denial attempts.
threat-detection basic-threat threat-detection statistics service-denial threat-detection statistics port-scan threat-detection scanning-threat shun
The shun command automatically blocks the source IP of a detected scanner for a predefined period. This is a critical first line of defense against initial reconnaissance.
Monitoring for Abnormal Connection Rates
If an internal host is compromised and begins lateral movement, the following command helps identify the spike:
show threat-detection statistics top host
s
Cisco Secure Firewall (FTD) Hardening via FMC
For organizations using Firepower Threat Defense (FTD), the hardening process is managed through the Firepower Management Center (FMC).
Hardening the FTD OS
Access the FTD CLI to verify that the underlying Linux OS is secured. Ensure that the 'expert' mode is restricted and that the shell is not accessible via the network.
> show network > show banner > show version
Policy Optimization and Rule Cleanup
Overly permissive rules (e.g., any any any) are a primary driver for ransomware propagation. We use the FMC "Hit Count" feature to identify and prune unused rules.
- Navigate to Policies > Access Control.
- Analyze rules with zero hits over the last 90 days.
- Replace "Any" in the "Service" column with specific TCP/UDP ports.
- Enable "Log at End of Connection" for all rules to ensure forensic visibility.
Control Plane Policing (CoPP)
CoPP is essential for protecting the firewall's CPU from being overwhelmed by management or control plane traffic, which can be a precursor to an Interlock attack (CVE-2024-20481).
Implementing CoPP on ASA
We use class maps and policy maps to rate-limit traffic destined for the firewall itself.
access-list COPP_ACL extended permit tcp any interface outside eq 443 access-list COPP_ACL extended permit icmp any interface outside
class-map COPP_CLASS match access-list COPP_ACL
policy-map COPP_POLICY class COPP_CLASS police input 1000000 1500
service-policy COPP_POLICY control-plane
This ensures that even during a DDoS or brute-force attack, the firewall remains responsive for legitimate management tasks.
Specific IOC Blocking for Interlock Ransomware
Based on threat intelligence, Interlock actors frequently use specific IP blocks for data exfiltration. We must implement an explicit block list.
object-group network INTERLOCK_IOC_BLOCK network-object 193.233.202.0 255.255.255.0 network-object 45.227.255.0 255.255.255.0 exit
access-list OUTSIDE_IN extended deny ip object-group INTERLOCK_IOC_BLOCK any access-list OUTSIDE_IN extended permit tcp any interface outside eq 443
In the Indian context, we have seen these IPs attempting to connect to pharmaceutical R&D servers in hubs like Peenya and Baddi. Immediate blocking at the edge is mandatory.
Logging and Real-Time Monitoring
Hardening is useless without visibility. Interlock often attempts to clear local logs. We must offload logs to a remote Syslog or SIEM.
Configuring High-Fidelity Logging
We recommend logging level 6 (Informational) for standard operations and level 7 (Debugging) only during active troubleshooting.
logging enable logging timestamp logging buffer-size 100000 logging buffered informational logging host inside 10.1.10.100 logging trap informational logging facility 20
Identifying Interlock Lateral Movement
Look for "Teardown TCP connection" messages with unusual durations or byte counts in your SIEM. Attackers often use RDP over the VPN to move laterally.
show log | include 10.10.50.12
Oct 24 2023 03:15:10: %ASA-6-302013: Built inbound TCP connection 54221 for outside:193.233.202.15/44332 (193.233.202.15/44332) to inside:10.1.10.20/3389 (10.1.10.20/3389)
Cisco ASA Firewall Hardening Checklist
Use this checklist to audit your current configuration against the Interlock threat model.
| Category | Requirement | CLI Command / Action |
|---|---|---|
| Access Control | Disable Telnet | no telnet 0.0.0.0 0.0.0.0 |
| Authentication | Enforce MFA for VPN | secondary-authentication-server-group |
| Encryption | Disable TLS 1.0/1.1 | ssl server-version tlsv1.2 tlsv1.3 |
| Information Disclosure | Disable Version Banner | no banner exec / no banner motd |
| Service Hardening | Disable Proxy ARP | sysopt noproxyarp outside |
| Compliance | NTP Synchronization | ntp server 10.1.10.1 prefer |
Addressing the Indian Infrastructure Gap
Many Indian SMEs continue to run End-of-Life (EoL) ASA 5500-X series hardware. These devices do not support the latest FTD software or Encrypted Traffic Analytics (ETA). Professionals looking to master these defense strategies can explore cybersecurity placement courses to bridge the local skill gap.
The Risk of EoL Hardware
EoL devices do not receive patches for critical vulnerabilities like CVE-2024-20481. If budget constraints prevent a hardware refresh to the Firepower 1000 or 2100 series, these devices must be placed behind a secondary layer of defense, such as a cloud-based WAF or a dedicated VPN concentrator that supports modern MFA.
ISP Default Credentials
We have observed that several local ISPs in industrial areas provide pre-configured Cisco routers where the admin/admin or cisco/cisco credentials remain active on the ASDM interface. This allows initial access brokers (IABs) to sell access to Interlock affiliates. Always perform a credential audit upon taking over a circuit.
Secure Image Management and Firmware Updates
Interlock operators have been known to exploit vulnerabilities in the ASA web engine to upload malicious anyconnect packages.
Verifying Image Integrity
Always verify the MD5 or SHA512 hash of the ASA/FTD images before deployment.
verify /md5 disk0:/asa917-31-lfbff-k8.SPA
..................................................Done! verify /md5 (disk0:/asa917-31-lfbff-k8.SPA) = 8f7e...
Compare this output with the hash provided on the Cisco Software Download portal. If they do not match, the image may have been tampered with.
Automated Patching via FMC
For FTD environments, use the FMC to schedule automated downloads of the latest "Threat Defense Updates" and "Vulnerability Database (VDB)" updates. This ensures the Snort engine can recognize Interlock's latest obfuscated payloads.
LDAP and Active Directory Integration Hardening
When integrating ASA with Active Directory for VPN authentication, ensure the LDAP queries are encrypted and restricted.
ldap-server AD_SERVER address 10.1.10.10 server-port 636 ssl ldap-naming-attribute sAMAccountName ldap-base-dn dc=enterprise,dc=in ldap-scope subtree server-type microsoft
Using port 636 (LDAPS) prevents Interlock actors from sniffing LDAP credentials if they have already gained a foothold in the internal network.
Conclusion: Continuous Hardening and Compliance
Hardening is not a one-time task. With the introduction of the DPDP Act 2023, Indian cybersecurity professionals must treat firewall hardening as a continuous compliance requirement. Regular audits of the running-config and monitoring of show vpn-sessiondb are mandatory to stay ahead of Interlock's evolving tactics.
Next Command:
show running-config ss
l
Review this output to ensure no legacy tlsv1 or des-sha1 ciphers remain active in your production environment.