⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Medium · CVSS 6.1

How to Fix CVE-2026-20104: Critical Vulnerability in Cisco IOS XE Software

Other vulnerabilities in the same area that are worth patching alongside this one:

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 6.1 - Medium
Actively exploited?Not currently listed in CISA KEV
Affected16.12.8, 16.12.6, 16.12.6a, 16.12.7, 17.3.3, 17.3.4, and others
Fixed inSee vendor advisory
Type (CWE)CWE-124: Buffer Underwrite ('Buffer Underflow')

What is CVE-2026-20104?

CVE-2026-20104 is a security flaw in Cisco IOS XE Software. 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.

Why this CVE matters

Unpatched network-facing software is the leading initial-access vector in public breach reporting. Treat any CVSS-9 class flaw on an internet-reachable system as urgent, regardless of whether public exploit code has been observed yet.

For deployments of Cisco IOS XE Software that have been exposed to the public internet during the disclosure window, the operating assumption should be that scanning has already happened. Even where exploitation has not been publicly observed, scanning for the vulnerable fingerprint is cheap and routine. Patching closes the door; log review and credential rotation close out the rest of the response.

Am I affected?

You are affected if your installation matches any of these version ranges:

Check your installed version against the list above. If you cannot determine the version, treat the system as affected and follow the upgrade path below.

On Cisco IOS / IOS XE systems, run show version to read the running image and compare against the affected ranges. For ASA / FTD, use show version from privileged EXEC mode.

How to fix CVE-2026-20104

  1. Read the vendor advisory in full: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-xe-secureboot-bypass-B6uYxYSZ
  2. Upgrade Cisco IOS XE Software to the patched build listed in the vendor advisory.
  3. Back up the configuration (and database, where applicable) before upgrading.
  4. Apply the patch in a maintenance window. For HA pairs, upgrade the standby node first, fail over, then upgrade the former primary.
  5. Restart the affected service so the patched binary loads, then verify the new version (see verification section).

Cisco device CLI


# Confirm the patched build against the vendor advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-xe-secureboot-bypass-B6uYxYSZ
# 1. Confirm the running image on the Cisco Cisco IOS XE Software device.
show version
show inventory

# 2. Stage the patched image from Cisco's Software Center, verify SHA512 before installing.
copy ftp://10.0.0.10/cisco-ios-xe-software-<patched-version-from-advisory>.bin flash:

# 3. Set boot variable and reload during a maintenance window.
configure terminal
no boot system
boot system flash:cisco-ios-xe-software-<patched-version-from-advisory>.bin
end
write memory
reload

# 4. After reboot, confirm the new image.
show version | include System image

Verify the fix landed


# Confirm the patched build against the vendor advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-xe-secureboot-bypass-B6uYxYSZ
# 1. Confirm the running version equals the advisory's fixed-in build.
#    (Use the platform-specific version probe from the commands above.)

# 2. Re-scan with your vulnerability scanner (Nessus, Qualys, Tenable, OpenVAS).
#    The scanner should no longer flag CVE-2026-20104 on the patched target.

# 3. Inspect recent service and kernel logs for crash-loops or rollback events.
journalctl --since "10 minutes ago" | tail -200
dmesg --since "10 minutes ago" | tail -100

If you cannot patch immediately

No official workaround exists beyond restricting network exposure to the affected component. Apply the vendor patch as the primary remediation.

How to verify the fix worked

If your installation was internet-reachable during the disclosure window, treat log review as part of the remediation rather than an optional follow-up. Look for log entries that do not match your normal request patterns, especially repeated requests to the same uncommon endpoint, and any administrative changes you cannot tie back to a known operator.

Frequently asked questions

Is CVE-2026-20104 being exploited in the wild?

Public exploitation has not been confirmed by CISA at the time of writing. Treat the patch as time-sensitive anyway; reports often lag actual abuse.

Will a WAF or IDS rule fully mitigate CVE-2026-20104?

No. Network-layer filters can reduce noise and slow opportunistic scanners, but they will not stop a determined attacker. The vendor patch is the only durable fix.

How long should I plan for the upgrade?

Typical vendor-documented upgrade windows for Cisco IOS XE Software run from a few minutes to under an hour depending on cluster size. Test in a staging environment first and follow the vendor's documented HA upgrade order.

References

*This guide was assembled from the official vendor advisory, the NVD record, and the CISA KEV catalog entry on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*