Buffer underflow in Cisco IOS XE - CVE-2026-20104
Published: March 25, 2026
Vulnerability identifier: #VU124563
CSH Severity: Medium
CVSS v4: 7 [CVSS:4.0/AV:P/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-20104
CWE-ID: CWE-124
Exploitation vector: Local access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows an attacker with physical access to execute arbitrary code at boot time and break the chain of trust.
The vulnerability exists due to insufficient validation of software at boot time in the bootloader when manipulating loaded binaries. An attacker with physical access can modify the device's binaries to bypass integrity checks during boot and execute unsigned code, breaking the chain of trust.
Successful exploitation allows execution of arbitrary code that bypasses the requirement to run Cisco-signed images.
Affected software
Cisco IOS XE
How to mitigate CVE-2026-20104
Install security update from vendor's website.
Cisco IOS XE - addressed in versions 17.15.5, 17.18.2