Raw vector
CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:NSummary
CVE-2026-20104 is a medium-severity Buffer Underflow (CWE-124) vulnerability in Cisco IOS XE (inferred from references). Its CVSS base score is 6.1 (Medium).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 6th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SI-10 (Information Input Validation) — see the control section below for these in your framework.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-15435
Vulnerability Data
A vulnerability in the bootloader of Cisco IOS XE Software for Cisco Catalyst 9200 Series Switches, Cisco Catalyst ESS9300 Embedded Series Switches, Cisco Catalyst IE9310 and IE9320 Rugged Series Switches, and Cisco IE3500 and IE3505 Rugged Series Switches could allow…
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an authenticated, local attacker with level-15 privileges or an unauthenticated attacker with physical access to an affected device to execute arbitrary code at boot time and break the chain of trust. This vulnerability is due to insufficient validation of software at boot time. An attacker could exploit this vulnerability by manipulating the loaded binaries on an affected device to bypass some of the integrity checks that are performed during the boot process. A successful exploit could allow the attacker to execute code that bypasses the requirement to run Cisco-signed images. Cisco has assigned this security advisory a Security Impact Rating (SIR) of High rather than Medium as the score indicates because this vulnerability allows an attacker to bypass a major security feature of a device.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
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V1.4.1
Mitigating Controls (NIST 800-53 r5) AI
Developer testing and analysis can discover buffer underwrite flaws before deployment but does not stop their introduction.
Input validation can enforce bounds on indices or pointers before buffer writes, structurally stopping underwrite conditions.
Memory protection mechanisms limit the blast radius of an out-of-bounds write even if the coding flaw exists.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Secure SDLC practices directly prevent introduction of buffer underwrite flaws via coding standards, reviews, and testing.
Vulnerability identification processes can discover buffer underwrite issues but do not prevent their creation.
Patching removes instances of the weakness after discovery but does not address root-cause prevention in code.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing in development catches buffer-underflow defects before release.
Secure SDLC mandates input validation and bounds checking that directly prevent buffer underflow.
Application security requirements can specify buffer-size and pointer-safety rules.
Secure architecture and engineering principles require safe memory-handling patterns.
Secure coding standards explicitly forbid unsafe pointer arithmetic and mandate bounds checks.