CVE-2015-6403 in Small Business Phone
Summary
by MITRE
The TFTP implementation on Cisco Small Business SPA30x, SPA50x, SPA51x phones 7.5.7 improperly validates firmware-image file integrity, which allows local users to load a Trojan horse image by leveraging shell access, aka Bug ID CSCut67400.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 07/01/2022
The vulnerability CVE-2015-6403 affects Cisco Small Business SPA30x, SPA50x, and SPA51x IP phones running firmware version 7.5.7, representing a critical security flaw in the Trivial File Transfer Protocol implementation that enables unauthorized code execution. This vulnerability resides within the firmware update mechanism of these telephony devices, where the system fails to properly validate the integrity of firmware image files during the update process. The flaw specifically manifests when local users with shell access attempt to load malicious firmware images, effectively allowing them to install Trojan horse software that can persistently compromise the device. The vulnerability is categorized under CWE-20 as "Improper Input Validation" and represents a significant weakness in the device's security architecture that directly impacts the integrity of the firmware update process. The bug ID CSCut67400 identifies this specific issue within Cisco's internal tracking system, highlighting the organization's recognition of the severity and impact of the flaw.
The technical implementation of this vulnerability stems from insufficient validation mechanisms within the TFTP server component of the affected Cisco phones. When a firmware update is initiated, the system should verify the integrity of the downloaded image file through cryptographic checksums or digital signatures before proceeding with the installation. However, the vulnerable implementation lacks these critical validation steps, allowing attackers with local shell access to substitute legitimate firmware images with malicious ones. The TFTP protocol itself is inherently insecure, lacking authentication and encryption mechanisms, which compounds the risk when combined with the weak validation logic. Attackers can exploit this by first gaining shell access to the device, typically through legitimate administrative access or by leveraging other vulnerabilities, and then replacing the firmware image with a modified version that contains backdoor functionality or other malicious code. This attack vector aligns with ATT&CK technique T1059.001 for Command and Scripting Interpreter and T1068 for Exploitation for Privilege Escalation, as it requires local access but enables persistent system compromise.
The operational impact of CVE-2015-6403 extends far beyond simple device compromise, as these IP phones are integral components of business communication networks and often serve as entry points for broader network infiltration. When successfully exploited, the malicious firmware can provide attackers with persistent access to the network, allowing them to monitor voice communications, intercept sensitive information, or use the device as a pivot point for attacking other network resources. The vulnerability affects devices that are commonly deployed in small business environments where network security monitoring may be limited, making the exploitation more likely to go undetected. Organizations relying on these devices for critical communication infrastructure face potential data breaches, compliance violations, and operational disruptions. The impact is particularly severe given that these phones often lack robust security features and may be managed by non-security-savvy personnel who are less likely to implement proper network segmentation or monitoring. The vulnerability also undermines the trust model of the device's firmware update process, potentially affecting the integrity of the entire communication infrastructure that depends on these endpoints.
Mitigation strategies for CVE-2015-6403 should focus on immediate firmware updates from Cisco, which address the validation weakness in the TFTP implementation and restore proper integrity checking mechanisms. Organizations must ensure that all affected devices are updated to patched firmware versions that include proper cryptographic validation of firmware images. Network segmentation should be implemented to limit access to these devices, restricting shell access to only authorized administrative personnel and implementing strong access controls for device management interfaces. Additional security measures include monitoring network traffic for unusual TFTP activity, implementing network access control lists to restrict TFTP traffic, and conducting regular security audits of telephony infrastructure. The vulnerability highlights the importance of secure firmware update mechanisms and proper input validation in embedded systems, reinforcing the need for robust security practices in network equipment. Organizations should also implement comprehensive monitoring solutions that can detect unauthorized firmware modifications and maintain detailed logs of device configuration changes. The remediation process should include thorough testing of updated firmware to ensure compatibility with existing network configurations while maintaining the enhanced security controls that prevent similar vulnerabilities from reoccurring in the future.