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SYNful Knock is a firmware-based backdoor targeting Cisco routers running IOS, publicly identified in September 2015. The implant consists of a modified IOS image that executes malicious code at startup, providing persistent remote access that survives device reboots. It preserves the original image size by overwriting selected legitimate IOS functions with malicious code. A specially crafted TCP SYN packet activates its covert communication mechanism, and a secret backdoor password bypasses normal authentication checks. Other authentication attempts continue through normal verification, helping conceal the compromise.
The implant supports downloading and loading additional modules from the Internet. These modules run in volatile memory and do not survive reboots, unlike the modified IOS image. Initial installation has involved attackers using default or weak administrative credentials; the implant does not inherently require exploitation of a zero-day vulnerability. SYNful Knock provides a durable foothold in network infrastructure for subsequent attacker activity.
SYNful Knock has been deployed by operators associated with Russia’s Federal Security Service Center 16 in network-infrastructure espionage operations. Confirmed implants identified in 2015 affected Internet-facing infrastructure in Ukraine, the Philippines, Mexico, and India.
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1 CVE Mallory has correlated with this family across public research and vendor advisories. Each row links to the full Mallory page for that vulnerability.
Static Tundra has been exploiting CVE-2018-0171, a CVSS 9.8 Cisco Smart Install vulnerability, to compromise unpatched network devices worldwide for intelligence gathering. The content also reports that Salt Typhoon leveraged this vulnerability against major U.S. telecommunications companies.
3 distinct threat actors attributed by public researchers. Open in Mallory to see the full evidence chain and overlapping campaigns.
Persistent Access Establishment: Deploy SYNful Knock firmware implants for long-term persistence.
Кастомный инструментарий - в 2015 году публично идентифицировано вредоносное ПО SYNful Knock, которое подразделение разворачивало на определённых устройствах Cisco ... SYNful Knock, публично идентифицированный в 2015 году, - конкретный пример: модифицированный IOS-образ исполняет имплант при каждой загрузке.
Persistent access is maintained with reused SNMP credentials or, when required, the SYNful Knock malware, which survives reboots.
9 distinct techniques documented for this family, organized by ATT&CK tactic.
Persistent access is maintained with reused SNMP credentials or, when required, the SYNful Knock malware, which survives reboots.
The FBI detected Russian FSB cyber actors exploiting Simple Network Management Protocol (SNMP) and end-of-life networking devices running an unpatched vulnerability (CVE-2018-0171) in Cisco Smart Install (SMI) to broadly target entities in the United States and globally.
Persistent access is maintained with reused SNMP credentials or, when required, the SYNful Knock malware, which survives reboots.
Remote access to the device can then be achieved by sending a specifically crafted TCP SYN packet, commonly referred to as a “magic packet.”
Persistent access is maintained with reused SNMP credentials or, when required, the SYNful Knock malware, which survives reboots.
Static Tundra has been observed leveraging a Cisco IOS firmware implant known as SYNful Knock to achieve persistent access to compromised systems. SYNful Knock is a modular implant that attackers inject into a Cisco IOS image and then load onto the compromised device.
4 indicators attributed across vendor reports, sandbox runs, and researcher write-ups. Full values are available in Mallory.
IPs, domains, and DNS infrastructure linked to this family.
16 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
Имплант для сетевых устройств Cisco, связанный с модификацией образа IOS для обеспечения скрытого и устойчивого несанкционированного доступа при каждой загрузке устройства.
A malware implant/backdoor used to maintain persistent access on compromised Cisco network devices, including surviving device reboots.
Custom malware/backdoor deployed to certain Cisco devices by the FSB Center 16 unit to compromise networking infrastructure.
Firmware implant that Static Tundra deploys on compromised network devices to establish long-term persistent access. The reference describes its deployment after Smart Install exploitation, rather than identifying the implant itself as exploiting the vulnerability.
Match every observed IP, domain, and hash against your live telemetry.
Named campaigns wielding this family, with evidence pinned to each claim.
CVEs this family uses for access and lateral movement.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Every documented technique, ranked by evidence weight.
Reddit, Mastodon, and CTI community discussion around this family.