CVE-2026-20281 in Session Initiation Protocol Software
Summary
by MITRE • 09/02/2026
A vulnerability in Cisco Desk Phone 9800 Series, Cisco IP Phone 7800 and 8800 Series, and Cisco Video Phone 8875 that are running Cisco Session Initiation Protocol (SIP) Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device.
This vulnerability is due to improper memory management when an affected device processes HTTP packets. An attacker could exploit this vulnerability by sending a continuous stream of crafted HTTP packets to the device. A successful exploit could allow the attacker to cause the affected device to continuously consume memory, resulting in a DoS condition. A manual reboot of the device is required to recover from this condition. Note: For this vulnerability to be exploitable, the phone must be registered to Cisco Unified Communications Manager (Unified CM) and have Web Access enabled. Web Access is disabled by default.
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Analysis
by VulDB Data Team • 09/02/2026
The identified security flaw resides within the HTTP processing subsystem of specific Cisco endpoint devices, including the Desk Phone 9800 Series, IP Phones in the 7800 and 8800 series, as well as the Video Phone 8875. These devices operate on Cisco Session Initiation Protocol software and are designed to handle various network requests for configuration, status monitoring, and administrative access. The core technical deficiency is characterized by improper memory management during the parsing and handling of incoming HTTP packets. This flaw indicates a failure in the application layer to correctly allocate or release system resources when processing malformed or specifically crafted input data. When an attacker sends a continuous stream of these crafted HTTP packets, the device fails to manage its internal buffers efficiently, leading to uncontrolled memory consumption. As the vulnerable process continues to accept and attempt to parse this malicious traffic, it steadily consumes available RAM until critical thresholds are breached. This resource exhaustion directly results in a denial of service condition where the phone becomes unresponsive or crashes entirely due to lack of system resources required for normal operation.
The operational impact of this vulnerability is significant for organizations relying on these endpoints for voice and video communication services. A successful exploitation leads to immediate service disruption, as the affected device ceases to function correctly under the load generated by the attack. The primary consequence is that legitimate users lose access to telephony services provided through the compromised endpoint. Recovery from this state is not automatic; it requires manual intervention in the form of a physical or remote reboot of the device. This necessity for manual recovery introduces operational overhead and potential downtime, particularly if multiple devices are targeted simultaneously during an attack campaign. The inability to automatically recover without human interaction exacerbates the severity of the denial of service, as network administrators must physically visit each affected site or have local staff perform reboots, delaying restoration of services.
From a threat modeling perspective, this vulnerability aligns with CWE-401, which describes missing release of memory after effective usage, and CWE-787, indicating out-of-bounds write that can lead to heap corruption or resource exhaustion depending on the specific implementation details not fully disclosed in the summary. In terms of adversary tactics, this exploit maps to MITRE ATT&CK technique T1499, Endpoint Denial of Service, specifically under subtechniques involving application layer attacks. The attacker leverages the network interface to flood the target with requests that trigger the memory leak, effectively degrading or destroying system availability without needing authentication credentials for the initial exploitation phase. However, it is crucial to note that the exploitability of this vulnerability is constrained by specific environmental prerequisites. The targeted phone must be registered to a Cisco Unified Communications Manager instance and have Web Access enabled on its interface. Since Web Access is disabled by default in standard configurations, the attack surface is significantly reduced for most deployments unless an administrator has explicitly enabled remote web management capabilities.
Mitigation strategies should prioritize both technical controls and administrative policies to reduce exposure. The primary defense involves ensuring that Web Access remains disabled on all endpoints where it is not strictly required for operational purposes. Administrators should audit their fleet of Cisco phones to identify any devices with this feature enabled and disable it through the device configuration profiles or local settings if possible. For environments where remote web access is necessary, network segmentation strategies must be employed to restrict HTTP traffic from untrusted networks to these endpoints. Implementing firewall rules that limit source IP addresses allowed to reach the management interfaces of the phones can prevent unauthorized actors from sending the crafted packets required for exploitation. Additionally, keeping firmware up to date with vendor-provided patches is essential, as Cisco typically releases updates to address such memory management flaws in subsequent software revisions. Monitoring network traffic for anomalous HTTP request patterns directed at phone IP addresses can also aid in early detection of attempted exploits before significant resource exhaustion occurs.