Security1 publisher2 min readPublished
A signed UEFI shell can disable the Secure Boot that trusted it
On a September 10 podcast, Eclypsium researchers walked from vulnerable signed UEFI shells to Fire Ant binaries wearing EDR agent names. The common thread is verification: a defender can check very little of that stack alone.
The Watch · Security desk
What happened
- Below the Surface episode 82, recorded September 10, 2026 with host Paul Asadoorian, Vlad Babkin and Chase Snyder, opened on newly discussed vulnerable UEFI shell behavior at vendors including AMI and Cisco UCS.
- The group compared that behavior to the earlier Framework UEFI shell issues and to the general risk of signed administrative tools that can be abused to disable or bypass Secure Boot.
- Snyder raised the Fire Ant campaign, in which binaries planted on Linux management hosts impersonated SentinelOne and Cybereason agents.
Compiled by The WatchSomething wrong?How this is made
Why it matters
- constraint With hashes alone, a shop cannot separate a correctly signed safe image from a correctly signed unsafe one, so its boot-chain assurance rests on the vendor's release process instead of its own check.
- exposure The reachable ground is the Linux management host and the appliance management plane, where the persistence is ordinary and the telemetry is thin.
- capability Large-scale firmware and binary analysis stops being the property of specialist teams, on both the finding and the exploiting side.
- decision Each extra trusted bootable environment, including Citrix's Linux recovery OS, is a management surface a shop has to take on deliberately or decline.
Secure Boot is an allow list. The firmware checks a signature, and anything the vendor signed runs before the kernel and before module policy. A UEFI shell signed by the platform vendor has exactly that standing. Eclypsium publishes its related work on signed UEFI shells under the name BombShell [4].
Babkin's framing is the useful part of the segment. Nobody on the call alleged vendor malice [5]. He argued that a vendor can run a strong security program and still sign something unsafe, because complex signing, release and firmware workflows produce mistakes [6].
The transparency discussion goes past a hash list for that reason. Babkin said publishing hashes alone is not enough, that the binaries themselves may need to be public, and that the problem gets much larger once you extend it beyond UEFI to firmware and full device images [7]. A published hash proves only that a file is the one the vendor shipped, and whether what the vendor shipped was safe is a separate question [1]. On network gear, the number of device-specific builds, embedded components and vendor variations makes that verification harder again [8]. Babkin also argued that vendors should stop keeping firmware behind paywalls and encryption, on the grounds that researchers finding and reporting bugs does less damage than attackers finding and quietly exploiting them [9].
Fire Ant's operators did not need a kernel implant to hold Linux management hosts. Asadoorian tied the campaign to Cisco IOS XE targeting, TACACS abuse, packet sniffing and the price attackers put on network-device credentials [14]. Process naming, /tmp, cron, systemd, vendor-specific startup mechanisms and thin visibility into appliances are enough on their own [15]. An attacker who lands on a Linux host inherits much of the same broad control the administrator has [16].
The same shape appears above the firmware. Asadoorian covered research that disables Windows Defender by registering a fake antivirus provider [12].
The episode is a discussion, and it does not include CVE identifiers, affected firmware versions, or evidence of exploitation in the wild [17].
Babkin said open and specialized security models are getting better at large-scale firmware and binary analysis [10]. That improvement is available to attackers as much as to defenders. Firmware has the slowest patch cycle.
Two hunts are available today without any vendor cooperation: process names on Linux management hosts that do not match the deployment inventory, and cron entries and systemd units that nobody in change control recognizes [15].
What to watch
- CVE assignments or advisories from AMI or Cisco with affected version lists would turn the shell discussion into a patch deadline.
- Any transparency log that covers signed UEFI images, and whether it publishes the binaries or only their hashes.
- Further Fire Ant reporting on which network-device credentials were taken and what the operators did with them.