Detect ROMMONkit in CrowdStrike LogScale
Adversaries may abuse the ROM Monitor (ROMMON) by loading unauthorized firmware with adversary code to provide persistent access and manipulate Cisco network device behavior in a way that is extremely difficult to detect. ROMMON is a Cisco network device firmware that functions as a boot loader, boot image, or boot helper to initialize hardware and software when the platform is powered on or reset. An adversary may upgrade the ROMMON image locally or remotely via TFTP with adversary code and restart the device to overwrite the existing ROMMON image. This provides persistence that survives IOS upgrades and standard remediation, and has been observed in the wild via the SYNful Knock implant campaign targeting Cisco ISR routers. Because ROMMON executes before the operating system loads, malicious code embedded at this layer can intercept and modify IOS behavior, inject backdoors, and evade integrity checks.
MITRE ATT&CK
- Tactic
- Defense Evasion Persistence
- Technique
- T1542 Pre-OS Boot
- Sub-technique
- T1542.004 ROMMONkit
- Canonical reference
- https://attack.mitre.org/techniques/T1542/004/
LogScale Detection Query
// T1542.004 — ROMMONkit: Network device ROMMON firmware manipulation
// CrowdStrike Falcon LogScale — targets network device syslog forwarded via Falcon LogScale collector
// or via syslog forwarder integration
#repo="network_devices" OR #repo="syslog" OR #vendor="cisco"
| regex(field="@rawstring", regex="(?i)(rommon|rom\s+monitor|bootldr|boot\s+variable|confreg|config-register|0x2142|0x2100|upgrade\s+rom-monitor|upgrade\s+rommon|copy\s+tftp|archive\s+download-sw|SYS-5-RELOAD|Reload\s+requested)", strict=false)
| eval(
IsRommonChange = if(match(lower(@rawstring), /rommon|rom monitor|bootldr|boot variable|confreg|config-register|0x2142/), "1", "0"),
IsTFTPTransfer = if(match(lower(@rawstring), /tftp|copy tftp|archive download-sw|upgrade rom-monitor|upgrade rommon/), "1", "0"),
IsReload = if(match(@rawstring, /SYS-5-RELOAD|Reload requested|SYS-6-BOOTTIME/), "1", "0"),
IsBootVarChange = if(match(lower(@rawstring), /boot system|boot path-list|startup-config/), "1", "0")
)
| eval(
SuspicionScore = toint(IsRommonChange) + toint(IsTFTPTransfer) + toint(IsReload) + toint(IsBootVarChange)
)
| where SuspicionScore > 0
| table([_timeparsed, host, #repo, @rawstring, IsRommonChange, IsTFTPTransfer, IsReload, IsBootVarChange, SuspicionScore])
| sort(field="_timeparsed", order="desc")CrowdStrike LogScale (CQL) query detecting ROMMONkit (T1542.004) by processing Cisco IOS and generic syslog events forwarded to Falcon LogScale. Applies regex matching across ROMMON manipulation, TFTP firmware transfer, reload, and boot configuration change patterns. Computes a SuspicionScore for downstream alerting logic — score >= 2 indicates high-confidence firmware tampering activity.
Data Sources
Required Tables
False Positives & Tuning
- Authorized ROMMON firmware upgrades initiated by the network operations team — cross-reference alert timing against the change management system (ServiceNow, Jira) before escalating
- Cisco IOS software upgrades using the 'archive download-sw' command which triggers the TFTP transfer pattern but is a standard upgrade mechanism when performed from an approved server
- Router password recovery procedures using config-register 0x2142 performed by IT staff — common procedure after administrative credential loss; verify against a helpdesk ticket
Other platforms for T1542.004
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 1Verify Current ROMMON Version and Boot Variables
Expected signal: Cisco IOS syslog: `%SYS-6-PRIVCMD` (if privilege accounting enabled) for each privileged exec command. TACACS+ accounting records for the enable session and each show command. AAA accounting logs showing the source IP and username. No TFTP or reload events generated.
- Test 2TFTP Image Transfer to Network Device (Lab Only)
Expected signal: Cisco IOS syslog: `%TFTP-6-TRANSFER: Received 1234 bytes` or `%COPY-5-UPROMPRMT: 1234 bytes copied in 2.345 secs`. CommonSecurityLog/Syslog in SIEM will show the TFTP transfer message with source IP 192.168.100.99. TACACS+ accounting logs the `copy tftp` command with source IP. NetFlow captures UDP/69 session from 192.168.100.99 to device management IP.
- Test 3Configuration Register Modification (Lab Only)
Expected signal: Cisco IOS syslog: `%SYS-5-CONFIG_I: Configured from console by <user> on <terminal>` after the config change. `show bootvar` output includes `Configuration register is 0x2142`. TACACS+ accounting logs the `config-register 0x2142` command. Syslog forwarded to SIEM contains the CONFIG_I message with the configuration terminal session details.
- Test 4ROMMON Environment Variable Inspection via ROMMON Prompt (Lab Only)
Expected signal: Cisco IOS syslog before reload: `%SYS-5-RELOAD: Reload requested by <user> on vty0. Reload Reason: Reload command.` After reload: `%SYS-6-BOOTTIME: Time taken to reboot after reload = <seconds> seconds`. TACACS+ logs the `reload` command. Syslog gap during ROMMON phase (ROMMON does not forward syslog). After IOS boots: logging resumes with startup sequence messages.
References (7)
- https://attack.mitre.org/techniques/T1542/004/
- 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://www.mandiant.com/resources/blog/synful-knock-detecting-cisco-ios-rootkits-with-host-based-forensics
- https://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-software-releases-121-mainline/7181-rommon-integrity.html
- https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/security/d1/sec-d1-cr-book/sec-cr-r1.html
- https://github.com/nsacyber/Hardware-and-Firmware-Security-Guidance
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