CVE-2009-0635 in IOS
Summary
by MITRE
Memory leak in the Cisco Tunneling Control Protocol (cTCP) encapsulation feature in Cisco IOS 12.4, when an Easy VPN (aka EZVPN) server is enabled, allows remote attackers to cause a denial of service (memory consumption and device crash) via a sequence of TCP packets.
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Analysis
by VulDB Data Team • 05/03/2025
The vulnerability described in CVE-2009-0635 represents a critical memory management flaw within Cisco IOS 12.4 software that specifically affects devices configured with Easy VPN server functionality. This issue manifests through the Cisco Tunneling Control Protocol implementation, which is a proprietary encapsulation mechanism used to establish secure communications between remote clients and corporate networks. The vulnerability resides in how the system processes TCP packets when the cTCP feature is enabled, creating a condition where memory allocation occurs without proper subsequent deallocation, leading to progressive memory exhaustion.
The technical exploitation of this vulnerability occurs when remote attackers send a carefully crafted sequence of TCP packets to a vulnerable Cisco device that has Easy VPN server enabled. The flaw exists in the cTCP processing logic where the system fails to properly release memory allocated during the packet handling process, particularly when processing tunnel establishment and maintenance communications. This memory leak accumulates over time and eventually consumes all available system memory, causing the device to become unresponsive and ultimately crash. The vulnerability is particularly dangerous because it can be triggered remotely without authentication requirements, making it a significant threat to network availability.
From an operational impact perspective, this vulnerability directly violates the availability principles of the CIA triad and can be classified under CWE-401 as a failure to release memory resources. The attack vector allows for a straightforward denial of service scenario where network administrators face potential service interruptions that could affect business continuity. The device crash resulting from this memory exhaustion can lead to complete network outages for remote users attempting to establish connections through the Easy VPN service, particularly impacting organizations that rely heavily on remote access capabilities. This vulnerability also aligns with ATT&CK technique T1499.004 for network denial of service attacks, as it specifically targets network availability through resource exhaustion.
Organizations affected by this vulnerability should implement immediate mitigations including applying the relevant Cisco IOS software patches that address the memory leak in the cTCP implementation. Network administrators should also consider disabling the Easy VPN server functionality on affected devices until patches are applied, or implementing access controls that limit exposure to trusted networks only. Monitoring systems should be configured to detect unusual memory consumption patterns that might indicate exploitation attempts, and regular vulnerability assessments should be conducted to identify other potential memory management issues within network infrastructure. The remediation process should also include proper network segmentation to limit the impact scope and ensure that critical network services remain operational during patch deployment activities.