CVE-2015-4226 in 9900 Phone
Summary
by MITRE
The packet-storing feature on Cisco 9900 phones with firmware 9.3(2) does not properly support the RTP protocol, which allows remote attackers to cause a denial of service (device hang) by sending malformed RTP packets after a call is answered, aka Bug ID CSCur39976.
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Analysis
by VulDB Data Team • 05/22/2022
The vulnerability identified as CVE-2015-4226 affects Cisco 9900 IP phones running firmware version 9.3(2) and represents a critical denial of service weakness in the device's packet-storing functionality. This issue specifically impacts the Real-time Transport Protocol implementation within the phone's communication stack, creating a scenario where legitimate network traffic can be exploited to disrupt normal device operations. The vulnerability manifests when malformed RTP packets are transmitted to the device after a call has been successfully answered, causing the phone to become unresponsive and effectively hang. This behavior constitutes a significant operational risk for organizations relying on these devices for critical communication infrastructure.
The technical flaw resides in the insufficient validation and handling of RTP packet structures within the Cisco 9900 phone's firmware implementation. According to CWE classification, this vulnerability aligns with CWE-129, which addresses improper validation of input boundaries, and CWE-248, which covers exposure of unintended multiple paths to a resource. The device fails to properly sanitize incoming RTP packets, particularly those containing malformed headers or unexpected payload structures, leading to a state where the phone's processing engine becomes trapped in an unrecoverable loop or encounters a critical error that results in complete system hang. The RTP protocol's design for real-time multimedia transmission does not adequately account for malformed packet scenarios in this specific implementation, creating an attack surface that remote adversaries can exploit without requiring authentication or privileged access.
From an operational perspective, this vulnerability presents a substantial risk to enterprise communication systems as it enables remote attackers to disrupt critical business operations through simple network-based attacks. The attack requires minimal technical expertise and can be executed from any location with network access to the affected device, making it particularly dangerous in environments where phone systems are directly exposed to external networks. Organizations may experience extended downtime during peak business hours, potentially resulting in significant financial losses and operational disruption. The vulnerability's impact extends beyond individual device compromise as it can affect larger communication infrastructures where multiple phones are deployed, potentially cascading into broader network availability issues. According to ATT&CK framework methodology, this vulnerability maps to T1499.004, which covers network denial of service attacks, and T1595.001, covering network device compromise through protocol manipulation.
Mitigation strategies for CVE-2015-4226 should focus on immediate firmware updates from Cisco, specifically targeting the patched versions that address the RTP packet handling deficiencies. Organizations should implement network segmentation to isolate affected phones from external networks, deploying firewalls and access control lists to restrict RTP traffic to trusted sources only. Network monitoring solutions should be configured to detect unusual RTP packet patterns that might indicate exploitation attempts, enabling proactive threat detection. Device administrators should consider disabling the packet-storing feature if it is not essential for business operations, though this may impact functionality such as call recording or voicemail services. Regular vulnerability assessments and penetration testing should be conducted to identify similar weaknesses in other networked communication devices, particularly those running older firmware versions. Additionally, implementing intrusion detection systems specifically tuned to recognize malformed RTP packet signatures can provide early warning of attempted exploitation, allowing for rapid response and mitigation actions to prevent service disruption.