CVE-2025-20335 is a medium-severity vulnerability affecting multiple Cisco IP phone models, including the Cisco Desk Phone 9800 Series, Cisco IP Phone 7800 and 8800 Series, and Cisco Video Phone 8875. The root cause of this vulnerability is improper access control related to directory permissions within the Cisco Session Initiation Protocol (SIP) software stack. Specifically, the flaw allows an unauthenticated remote attacker to write arbitrary files to certain directories on the underlying operating system of the affected devices. Exploitation requires that the Web Access feature be enabled on the phone, which is disabled by default. An attacker can exploit this by sending a crafted request to the vulnerable device, bypassing authentication controls due to the lack of proper verification mechanisms. The ability to write arbitrary files can lead to unauthorized modification of device configurations or potentially implant malicious code, which could be leveraged for further attacks such as persistent compromise or lateral movement within an enterprise network. The vulnerability affects a wide range of software versions spanning multiple major releases, indicating a long-standing issue in the Cisco SIP software. Although no known exploits are currently reported in the wild, the vulnerability’s network accessibility and lack of required privileges make it a significant risk if Web Access is enabled. The CVSS v3.1 score is 5.3 (medium), reflecting the network attack vector, no required privileges or user interaction, but limited impact confined to integrity without affecting confidentiality or availability directly.

For European organizations, this vulnerability poses a risk primarily to enterprises and public sector entities that deploy Cisco IP phones extensively as part of their unified communications infrastructure. Successful exploitation could allow attackers to modify device files, potentially disrupting voice communications or enabling persistent footholds within corporate networks. This could lead to espionage, interception of sensitive communications, or use of compromised devices as pivot points for broader network attacks. Given the reliance on VoIP systems in sectors such as finance, government, healthcare, and critical infrastructure across Europe, the impact could be significant if exploited at scale. The risk is heightened in environments where Web Access is enabled for remote management without adequate network segmentation or access controls. Additionally, compromised phones could undermine trust in communication systems and cause operational disruptions. However, the absence of known exploits and the default disabled state of Web Access somewhat mitigate immediate widespread impact. Organizations with legacy or unpatched Cisco IP phones remain vulnerable, especially if remote management features are enabled without proper security controls.

1. Immediately verify whether Web Access is enabled on all Cisco IP phones in the environment. Disable Web Access if it is not strictly necessary for operational purposes. 2. For devices requiring Web Access, restrict access to trusted management networks only, using network segmentation and firewall rules to limit exposure. 3. Apply all available Cisco patches and firmware updates addressing this vulnerability as soon as they become available. Monitor Cisco security advisories closely for patch releases. 4. Implement strict access control policies and multi-factor authentication for any remote management interfaces to reduce the risk of unauthorized access. 5. Conduct regular audits of device configurations and logs to detect any unauthorized file modifications or suspicious activity. 6. Employ network intrusion detection systems (IDS) or intrusion prevention systems (IPS) tuned to detect anomalous SIP or HTTP requests targeting IP phones. 7. Educate IT and security teams about the risks associated with enabling Web Access on IP phones and enforce change management policies to prevent inadvertent exposure. 8. Consider deploying endpoint protection solutions capable of monitoring and alerting on unauthorized file system changes on IP phones where supported.