VulnerabilityPublished 25 Mar 2026Updated 26 Mar 2026

CVE-2026-20104: Cisco IOS XE Software · Buffer Underflow

Affects Cisco IOS XE SoftwareSource CVE Database V5

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Not known to be exploited · CVSS 6.1 Medium.

Data as of 29 Oct 2026, 19:04 UTC

CVSS 3.1 base score

6.1Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.1% chance of exploitation in 30 days – higher than 5% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Attack Vector: Physical
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: High
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Physical: The attack requires the attacker to physically touch or manipulate the vulnerable component. Physical interaction may be brief or persistent.
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 25 Mar 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Affected software

Product	Affected	Fixed in
CiscoCisco IOS XE Software	Affected 16.12.6 – 17.18.2 59 affected builds	Fixed in–

AI-suggested

Organizations should immediately identify all affected Cisco IOS XE devices by inventorying switch models and software versions. Cisco typically releases patches or updated software versions to address such vulnerabilities; applying these updates promptly is critical. If patches are not yet available, organizations should implement strict physical security controls to prevent unauthorized access to devices. Limit administrative access to trusted personnel and enforce strong authentication and authorization policies to restrict level-15 privilege access. Employ network segmentation and monitoring to detect unusual device behavior indicative of boot-level compromise. Regularly verify device firmware integrity using out-of-band methods where possible. Consider deploying hardware security modules or trusted platform modules (TPMs) if supported by the device to enhance boot-time security. Maintain comprehensive logging and alerting on device access and configuration changes to facilitate rapid incident response. Finally, engage with Cisco support for guidance and monitor security advisories for updates or exploit disclosures.

Description

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 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.

Impact

The impact of this vulnerability is substantial for organizations relying on affected Cisco switches for critical network infrastructure. Successful exploitation allows attackers to execute arbitrary code at the earliest stage of device startup, bypassing signature verification and security controls designed to prevent unauthorized firmware execution. This can lead to persistent firmware-level backdoors, undermining device integrity and confidentiality. Attackers could maintain long-term access, manipulate network traffic, or disrupt network operations without detection. The requirement for physical or highly privileged access limits remote exploitation but does not eliminate risk, especially in environments with less physical security or where insider threats exist. The vulnerability threatens the trustworthiness of network devices, potentially compromising entire network segments and sensitive data flows. Given the widespread deployment of Cisco Catalyst and Industrial Ethernet switches globally, the scope of impact is broad, affecting enterprises, service providers, and critical infrastructure operators.

References

sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-xe-secureboot-bypass-B6uYxYSZ (opens in a new tab)

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 25 Mar 2026, 16:35 UTC

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References

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