In plain language
What this advisory means
A flaw in Cisco IOS XE's web management system lets a remote user with low access rights cause the device to restart unexpectedly, resulting in downtime. This happens because the system incorrectly handles errors when verifying certificates. There's no temporary fix, but Cisco has issued software updates to resolve this issue.
Technical explanation
How the issue affects the environment
The Cisco IOS XE Software's web-based management interface suffers from insufficient error handling related to certificate-based authentication. An authenticated, remote attacker with low privileges can exploit this vulnerability by presenting a malformed certificate during authentication. This triggers an error in the interface's processing code, causing the affected device to reload unexpectedly, leading to a denial of service (DoS). This vulnerability is tracked as CVE-2026-20311 and relates to CWE-126 (Buffer Over-read). The HTTP Server feature must be enabled with Personal Identity Verification (PIV) for the device to be vulnerable. There are no known workarounds; mitigating the issue requires upgrading to fixed software releases provided by Cisco.
Operational impact
Why teams should care
Successful exploitation causes the affected device to reload, interrupting network operations and resulting in downtime. This denial of service impacts availability, possibly degrading business communications and services dependent on the device until it is fully operational again. Since the vulnerability can be exploited by low-privileged authenticated users, internal or less-privileged threat actors could disrupt network infrastructure.
Immediate action
Cisco recommends upgrading affected Cisco IOS XE devices to the latest fixed software release that addresses this vulnerability. Use the Cisco Software Checker tool to identify vulnerable releases and obtain the appropriate updates. Since there are no workarounds, prompt patching is essential to mitigate the risk.
Affected and fixed releases
Temporary risk reduction
Cisco states there are no workarounds that address this vulnerability.
Evidence and validation checklist
- Vulnerability affects Cisco IOS XE Software with HTTP Server feature enabled and PIV authentication.
- Exploitation requires authenticated, remote attacker with low privileges.
- Exploitation involves authenticating with a malformed certificate causing device reload.
- No workarounds available; remediation is software upgrade.
- Cisco released advisory ID cisco-sa-xe-webui-dos-PtAODAWW on 2026-08-05.
- CVE-2026-20311 and CWE-126 are identifiers for this issue.
Authoritative reference
Cisco PSIRT Advisories
QCS detected and normalized this record from the official source. Vendor guidance remains authoritative.