CVE-2025-20240: Cisco IOS XE Software XSS Vulnerability

CVE-2025-20240 Overview

CVE-2025-20240 is a reflected cross-site scripting (XSS) vulnerability in the Web Authentication feature of Cisco IOS XE Software. The flaw allows an unauthenticated, remote attacker to inject arbitrary script into a user's browser session on an affected device. Exploitation requires user interaction, specifically convincing a target to click a crafted link. A successful attack can lead to session cookie theft and hijacking of authenticated web sessions. The vulnerability is tracked under [CWE-692] and stems from improper sanitization of user-supplied input processed by the Web Authentication component.

Critical Impact
An attacker can steal user cookies and execute arbitrary script in the context of the affected Cisco IOS XE web interface, enabling session hijacking against network administrators.

Affected Products

Cisco IOS XE Software with the Web Authentication feature enabled
Refer to the Cisco Security Advisory for the complete list of affected versions
Devices exposing the web-based authentication portal to untrusted networks

Discovery Timeline

2025-09-24 - CVE-2025-20240 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-20240

Vulnerability Analysis

The vulnerability exists in the Web Authentication (WebAuth) feature of Cisco IOS XE Software. WebAuth serves an HTTP-based captive portal that intercepts client traffic and prompts for credentials before granting network access. The portal reflects one or more request parameters back into the HTML response without sufficient encoding or sanitization. An attacker who controls the parameter value can inject JavaScript that executes in the victim's browser under the origin of the Cisco device.

Because the flaw is reflected rather than stored, exploitation depends on delivering a crafted URL to a victim. Successful execution allows theft of session cookies, manipulation of the rendered page, and issuance of authenticated requests on behalf of the user. The scope change component of the CVSS vector reflects that injected script can impact resources beyond the immediately vulnerable component.

Root Cause

The root cause is improper neutralization of special characters in user-supplied input before it is written to the HTTP response body. The Web Authentication handler treats untrusted query string or form values as safe HTML content. Standard output encoding for HTML contexts is not applied, allowing <script> tags and event handler attributes to survive into the rendered page.

Attack Vector

Exploitation is network-based and requires user interaction. An attacker crafts a URL that targets the WebAuth endpoint on an affected Cisco IOS XE device and embeds a malicious payload in a reflected parameter. The attacker then delivers the link through phishing, chat, or an embedded resource on a controlled site. When an authenticated administrator or captive-portal user clicks the link, the payload executes in their browser, exfiltrating cookies to an attacker-controlled endpoint or performing actions against the device's management interface.

No verified proof-of-concept code is publicly available. Refer to the Cisco Security Advisory for vendor-provided technical details.

Detection Methods for CVE-2025-20240

Indicators of Compromise

HTTP requests to Cisco IOS XE WebAuth endpoints containing URL-encoded <script>, onerror=, onload=, or javascript: payloads in query parameters
Outbound browser requests from administrator workstations to unfamiliar domains immediately after visiting a Cisco device management URL
Web server access logs on the device showing unusually long or heavily encoded parameter values directed at the authentication portal

Detection Strategies

Deploy web application firewall or IDS signatures that flag reflected XSS payload patterns targeting WebAuth URIs
Inspect proxy and NetFlow logs for referrer chains where a phishing domain precedes access to a Cisco IOS XE management IP
Correlate administrator browser telemetry with concurrent authentication events on network devices to identify session hijacking attempts

Monitoring Recommendations

Enable HTTP access logging on Cisco IOS XE devices and forward logs to a centralized SIEM for parameter inspection
Monitor for anomalous cookie exfiltration patterns from administrator endpoints, such as short outbound POST requests carrying base64 blobs
Alert on any external delivery of URLs referencing internal management IP addresses or hostnames of network devices

How to Mitigate CVE-2025-20240

Immediate Actions Required

Apply the fixed Cisco IOS XE Software release identified in the Cisco Security Advisory
Restrict access to the WebAuth and web management interfaces using access control lists that permit only trusted management networks
Train administrators to avoid clicking links that reference network device management addresses from untrusted sources

Patch Information

Cisco has published a security advisory with fixed software versions. Consult the Cisco Security Advisory cisco-sa-webui-xss-VWyDgjOU to identify the appropriate upgrade path for each affected platform and train.

Workarounds

Disable the Web Authentication feature on devices where it is not required for guest or client onboarding
Terminate WebAuth on a dedicated interface isolated from administrative traffic and untrusted user segments
Enforce short session lifetimes and require re-authentication for administrative changes to reduce the value of stolen cookies

bash

# Restrict HTTP/HTTPS management access to a trusted subnet
ip access-list standard MGMT-ACL
 permit 10.10.0.0 0.0.255.255
 deny   any log
!
ip http access-class MGMT-ACL
ip http secure-server
ip http server