Detect ROMMONkit in Elastic Security
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/
Elastic Detection Query
sequence by host.name with maxspan=1h
[any where event.dataset == "cisco.ios" and
(
message : ("ROMMON", "rommon", "rom monitor", "bootldr", "BOOT variable", "confreg", "config-register", "0x2142", "0x2100", "0x0")
or message : ("tftp", "TFTP", "copy tftp", "archive download-sw", "upgrade rom-monitor", "upgrade rommon")
or (message : ("SYS-5-RELOAD", "Reload requested", "SYS-6-BOOTTIME") and message : ("boot system", "BOOT path-list", "startup-config"))
)
]
// Alternatively, single-event detection for direct ROMMON manipulation signals:
any where
event.dataset in ("cisco.ios", "cisco.asa", "cisco.ftd") and
(
message : ("ROMMON", "rommon", "rom monitor", "bootldr", "BOOT variable") or
message : ("upgrade rom-monitor", "upgrade rommon", "copy tftp flash", "archive download-sw") or
message : ("confreg", "config-register", "0x2142") or
(message : ("SYS-5-RELOAD", "Reload requested") and message : ("boot system", "BOOT path-list"))
)Detects ROMMONkit activity (T1542.004) by monitoring Cisco IOS syslog events forwarded to Elasticsearch for ROMMON firmware manipulation indicators: TFTP-based firmware uploads, ROMMON variable changes, configuration register modifications, and suspicious device reloads correlated with boot path changes. Sequences events per device within a 1-hour window to catch multi-step compromise.
Data Sources
Required Tables
False Positives & Tuning
- Authorized network engineers performing scheduled ROMMON upgrades during maintenance windows — coordinate with network ops team to suppress alerts for approved change tickets
- TFTP transfers of legitimate IOS images during planned software refresh campaigns — these may trigger the TFTP keyword rules even without ROMMON context
- Automated configuration backup tools (e.g., Oxidized, RANCID) triggering 'Configured from' or 'startup-config' messages that match boot config keywords
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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