CVE-2026-20125 in IOS
Summary
by MITRE • 03/25/2026
A vulnerability in the HTTP Server feature of Cisco IOS Software and Cisco IOS XE Software Release 3E could allow an authenticated, remote attacker to cause an affected device to reload unexpectedly, resulting in a denial of service (DoS) condition. This vulnerability is due to improper validation of user-supplied input. An attacker could exploit this vulnerability by sending malformed HTTP requests to an affected device. A successful exploit could allow the attacker to cause a watchdog timer to expire and the device to reload, resulting in a DoS condition. To exploit this vulnerability, the attacker must have a valid user account.
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Analysis
by VulDB Data Team • 04/12/2026
This vulnerability exists within the HTTP server implementation of Cisco IOS and IOS XE software versions, specifically affecting the 3E release line. The flaw represents a classic input validation weakness that can be exploited remotely by authenticated users to disrupt network operations. The vulnerability stems from inadequate sanitization of user-supplied data within the HTTP request processing pipeline, creating a condition where malformed input can trigger unexpected system behavior.
The technical exploitation occurs through carefully crafted HTTP requests that manipulate the device's internal state management mechanisms. When an authenticated user submits malformed HTTP requests, the system fails to properly validate the input parameters, leading to a cascade of events that ultimately causes the watchdog timer mechanism to expire prematurely. This watchdog timer expiration triggers an automatic device reload process, effectively causing the network infrastructure to become temporarily unavailable. The vulnerability requires legitimate authentication credentials, which means unauthorized access attempts would be blocked by standard authentication mechanisms, but once authenticated, an attacker can leverage this flaw to disrupt services.
The operational impact of this vulnerability extends beyond simple service disruption to potentially affect critical network infrastructure reliability. Network administrators may experience unexpected outages during peak traffic periods, leading to service degradation for end users. The vulnerability's authentication requirement provides some protection against casual exploitation but does not eliminate the risk entirely, as compromised legitimate accounts could be used to trigger the DoS condition. This type of vulnerability aligns with CWE-20, which describes "Improper Input Validation" as a fundamental weakness in software design that allows malformed data to bypass validation checks and cause system instability.
Organizations should implement immediate mitigation strategies including applying the latest security patches from Cisco to address the root cause of the vulnerability. Network segmentation and access control measures should be enhanced to limit the scope of potential exploitation, while monitoring systems should be configured to detect unusual reload patterns that might indicate exploitation attempts. The ATT&CK framework categorizes this vulnerability under privilege escalation and denial of service tactics, where the authenticated attacker leverages system weaknesses to gain control over device availability. Security teams should also consider implementing rate limiting and input validation controls at network boundaries to prevent exploitation attempts before they reach the vulnerable device. Regular security assessments and vulnerability scanning should be conducted to identify similar input validation flaws across the network infrastructure, as this represents a common attack vector that affects multiple network device vendors.