Detect Reduce Key Space in Google Chronicle
Adversaries may reduce the level of effort required to decrypt data transmitted over the network by reducing the cipher strength of encrypted communications on compromised network devices. By reducing RSA modulus sizes (e.g., from 2048 to 512 bits), configuring weak Diffie-Hellman groups (group 1 or group 2), selecting DES/3DES over AES, or enabling RC4 cipher suites, adversaries make encrypted VPN traffic and management sessions feasible to brute-force without possessing the private key. This technique is typically deployed after gaining access to the network device CLI via T1059.008 and is often combined with T1601 (Modify System Image) to survive reboots. The primary risk is passive interception of IPsec VPN tunnels, SSH management sessions, and SSL/TLS control-plane traffic that protects device authentication credentials and network routing information.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1600 Weaken Encryption
- Sub-technique
- T1600.001 Reduce Key Space
- Canonical reference
- https://attack.mitre.org/techniques/T1600/001/
YARA-L Detection Query
rule detect_weaken_encryption {
meta:
author = "Argus"
description = "Detects cryptographic weakening on network devices"
severity = "HIGH"
technique = "T1600"
events:
$e.metadata.event_type = "NETWORK_UNCATEGORIZED"
re.regex($e.principal.hostname, `.*`) nocase
(
re.regex($e.network.application_protocol, `syslog`) or
$e.metadata.product_name = "Cisco IOS"
)
(
re.regex($e.network.session_id, `no crypto|encryption des|esp-des|modulus 512|modulus 768|null-encryption|ip ssh version 1|group 1 |group 2 `) nocase or
re.regex($e.principal.resource.name, `no crypto|encryption des`) nocase
)
condition:
$e
}Google Chronicle YARA-L 2.0 rule for T1600.001 detection using UDM event types and field matching. Detects adversary reduction of cryptographic key space on network devices by monitoring three data s
Data Sources
Required Tables
False Positives & Tuning
- Legacy network devices (Cisco ASA 5505, older IOS versions) that only support DES or 1024-bit RSA due to hardware limitations — these will trigger on existing configurations, not new adversary changes
- Authorized penetration testing or security assessments where engineers intentionally configure weak crypto to test detection coverage
- IPsec site-to-site VPN interoperability requirements with legacy partner organizations that mandate DH group 2 or 3DES in IKE phase 1 policy
- Scheduled key rotation procedures where the team temporarily generates a smaller key before importing the final production key
- Automated network configuration management tools (Ansible, SolarWinds NCM, Cisco DNA Center) that apply baseline templates containing older cipher suite definitions
Other platforms for T1600.001
Testing Methodology
Validate this detection against 4 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Cisco IOS: Generate Weak 512-bit RSA Key via Netmiko (Lab)
Expected signal: Cisco IOS Syslog: `%CRYPTO_ENGINE-5-KEY_ADDITION: A key named ARGUS-TEST has been generated or imported by crypto-engine`. TACACS+ accounting (if configured): username, source IP, and command `crypto key generate rsa label ARGUS-TEST modulus 512` logged with timestamp. SSH session event: `%SSH-5-SSH2_USERAUTH` showing authentication from the management IP.
- Test 2Cisco IOS: Configure Weak IKE Policy with DH Group 2 and DES via Expect Script
Expected signal: Cisco IOS Syslog: `%SYS-5-CONFIG_I: Configured from console by admin on vty0 (source_ip)`. Multiple CONFIG_I messages in rapid succession for each sub-command. TACACS+ accounting: individual commands `encryption des`, `hash md5`, `group 2` within `crypto isakmp policy 99` context. `show crypto isakmp policy` output will show policy 99 with DES/MD5/Group2.
- Test 3OpenSSL: Demonstrate 512-bit RSA Key Space Exhaustion (Conceptual Lab)
Expected signal: Linux auditd EXECVE records: `openssl genrsa` and `openssl rsa` process creation events if auditd is configured for command execution auditing. Sysmon for Linux (if deployed): Event ID 1 process creation for openssl binary with argument genrsa and 512. File creation event for /tmp/argus-t1600001/weak_rsa_512.pem. This test does NOT generate network device syslog events — it demonstrates the key size concept on a Linux host.
- Test 4Cisco IOS: Configure Weak IPsec Transform Set with ESP-DES via Python Netmiko
Expected signal: Cisco IOS Syslog: `%SYS-5-CONFIG_I: Configured from console by admin on vty0 (mgmt_ip)`. TACACS+ accounting: commands `crypto ipsec transform-set ARGUS-WEAK-TEST esp-des esp-md5-hmac` and `mode tunnel` logged with attribution. `show crypto ipsec transform-set ARGUS-WEAK-TEST` output confirms `{ esp-des esp-md5-hmac }` indicating the weak configuration is active.
References (8)
- https://attack.mitre.org/techniques/T1600/001/
- https://blogs.cisco.com/security/evolution-of-attacks-on-cisco-ios-devices
- https://community.cisco.com/t5/security-blogs/attackers-continue-to-target-legacy-devices/ba-p/4169954
- https://learn.microsoft.com/en-us/azure/sentinel/connect-syslog
- https://learn.microsoft.com/en-us/azure/sentinel/connect-common-event-format
- https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/security/a1/sec-a1-xe-3se-3850-cr-book/sec-a1-xe-3se-3850-cr-book_chapter_01.html
- https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-131Ar2.pdf
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1600/T1600.md
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