CVE-2025-20327: Cisco IOS Software Web UI DoS Vulnerability

CVE-2025-20327 Overview

CVE-2025-20327 is a denial of service (DoS) vulnerability in the web UI of Cisco IOS Software. An authenticated, remote attacker with low privileges can trigger the flaw by sending a crafted URL in an HTTP request. Successful exploitation forces the affected device to reload, interrupting network operations.

The weakness is classified as improper validation of specified syntactic structure [CWE-1287]. Cisco published the security advisory on September 24, 2025. The issue carries network attack vector characteristics with a scope change because a successful exploit affects availability beyond the vulnerable component itself.

Critical Impact
An authenticated low-privilege attacker can remotely reload a Cisco IOS device through its web UI, causing network outages across dependent infrastructure.

Affected Products

Cisco IOS Software (devices with the web UI feature enabled)
Refer to the Cisco Security Advisory for the full list of affected releases

Discovery Timeline

2025-09-24 - CVE-2025-20327 published to NVD
2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-20327

Vulnerability Analysis

The vulnerability resides in the HTTP request handling logic of the Cisco IOS web UI. When the device receives a crafted URL, the parser fails to validate the structure of the request before processing it. This parsing failure leads to an unhandled condition that crashes the device and triggers a reload.

Because the web UI runs as a privileged management interface, a forced reload interrupts routing, switching, and any dependent services running on the affected device. The CVSS scope is marked as changed because the impact extends to networks and clients behind the device, not just the management process itself.

Exploitation requires valid credentials, even at the lowest privilege tier. Read-only accounts that can authenticate to the web UI are sufficient to send the malicious request. The Exploit Prediction Scoring System (EPSS) currently estimates a 0.346% probability of exploitation in the next 30 days.

Root Cause

The root cause is improper input validation of the URL component of inbound HTTP requests. The web UI does not enforce the expected syntactic structure of the URL before passing it to downstream processing routines, resulting in a condition that forces the device to reload.

Attack Vector

The attack is delivered over the network as a single HTTP request. The attacker authenticates to the Cisco IOS web UI using any valid account, then issues a request containing a malformed URL. The device processes the crafted URL, reaches an invalid state, and reloads. No user interaction on the victim side is required.

No public proof-of-concept code is available at the time of publication. Technical specifics are limited to the disclosure in the Cisco Security Advisory.

Detection Methods for CVE-2025-20327

Indicators of Compromise

Unexpected device reloads or crashinfo files referencing the HTTP server process
Authenticated HTTP or HTTPS requests to the web UI containing malformed or non-standard URL structures
Web UI access from unexpected source addresses preceding a device reload event

Detection Strategies

Correlate syslog reload events (%SYS-5-RELOAD) with prior authenticated web UI sessions to identify suspicious sequences
Inspect HTTP access logs at upstream proxies or NetFlow collectors for malformed URLs targeting the IOS web UI on TCP/80 and TCP/443
Alert on repeated authentication attempts to the web UI from accounts that do not normally use it

Monitoring Recommendations

Forward IOS syslog and AAA authentication records to a centralized SIEM for correlation against device availability events
Track web UI session activity per user account and flag low-privilege accounts that issue administrative-style requests
Monitor device uptime metrics through SNMP or streaming telemetry to detect unscheduled reloads in near real time

How to Mitigate CVE-2025-20327

Immediate Actions Required

Apply the fixed Cisco IOS Software release identified in the Cisco Security Advisory
Disable the web UI (no ip http server and no ip http secure-server) on devices that do not require it
Restrict web UI access to trusted management subnets using access control lists
Audit local and AAA accounts with web UI access and remove unused or low-privilege accounts that are not required

Patch Information

Cisco has published fixed software releases through its standard advisory process. Refer to the Cisco Security Advisory for the specific fixed versions that correspond to each affected IOS train. Upgrade decisions should follow Cisco's recommended migration path for the platform in use.

Workarounds

Disable the HTTP and HTTPS servers on the device if the web UI is not in use
Apply an access class to the HTTP server to restrict source addresses: ip http access-class <acl>
Place the management interface in a dedicated VRF and filter inbound HTTP traffic at the network edge

bash

# Configuration example: disable web UI and restrict management access
configure terminal
 no ip http server
 no ip http secure-server
!
# If the web UI must remain enabled, restrict it with an ACL
ip access-list standard MGMT-WEBUI
 permit 10.10.0.0 0.0.0.255
 deny   any log
!
ip http access-class MGMT-WEBUI
ip http secure-server
end
write memory