CVE-2015-4243 in IOS
Summary
by MITRE
The PPPoE establishment implementation in Cisco IOS XE 3.5.0S on ASR 1000 devices allows remote attackers to cause a denial of service (device reload) by sending malformed PPPoE Active Discovery Request (PADR) packets on the local network, aka Bug ID CSCty94202.
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Analysis
by VulDB Data Team • 05/23/2022
The vulnerability described in CVE-2015-4243 represents a critical denial of service flaw within Cisco IOS XE software running on ASR 1000 series routers. This issue specifically targets the Point-to-Point Protocol over Ethernet (PPPoE) implementation during the Active Discovery phase, where the device fails to properly validate incoming PADR packets. The flaw enables remote attackers to exploit the system by transmitting malformed PPPoE packets directly on the local network segment, causing the affected router to crash and subsequently reload its operating system. This type of vulnerability falls under the category of input validation failures and can be classified as CWE-20, which represents "Improper Input Validation" in the Common Weakness Enumeration framework. The attack vector is particularly concerning because it requires minimal privileges and can be executed from within the local network, making it accessible to both internal and external threat actors who can gain network access.
The technical implementation of this vulnerability stems from insufficient packet validation mechanisms within the PPPoE Active Discovery Request processing code. When the ASR 1000 device receives malformed PADR packets, the parsing logic fails to properly handle unexpected data structures or malformed packet headers, leading to an unhandled exception that triggers the device reboot sequence. This behavior aligns with ATT&CK technique T1499.001, which covers "Endpoint Denial of Service" through the exploitation of software vulnerabilities. The device's failure to gracefully handle malformed input demonstrates a lack of proper error handling and defensive programming practices in the network protocol stack implementation. The impact is particularly severe as the device reload process can disrupt network connectivity for all services relying on that router, potentially affecting multiple network segments and causing cascading failures throughout the infrastructure.
The operational impact of this vulnerability extends beyond simple service disruption to encompass potential business continuity issues and network reliability concerns. Network administrators may experience unexpected downtime during critical operations, and the automatic reload behavior can prevent proper logging and forensic analysis of the attack. The vulnerability affects Cisco IOS XE 3.5.0S specifically, indicating that this was likely a targeted issue introduced in a particular software release or patch level. Organizations running affected equipment face the risk of repeated denial of service attacks that could be used to continuously disrupt network services, making this a particularly dangerous vulnerability for mission-critical infrastructure. The attack requires minimal technical expertise to execute, which increases the likelihood of exploitation by threat actors seeking to cause maximum disruption with minimal effort.
Mitigation strategies for CVE-2015-4243 should focus on both immediate protective measures and long-term remediation approaches. Cisco has released software updates and patches addressing this vulnerability, which should be deployed immediately on affected systems. Network segmentation and access control measures can help limit the attack surface by restricting which devices can send packets to the vulnerable router interfaces. Implementing rate limiting on PPPoE packet processing and monitoring for anomalous PADR packet patterns can provide early detection capabilities. The network security team should also consider deploying intrusion detection systems capable of identifying malformed PPPoE traffic patterns. Additionally, implementing proper network access controls and authentication mechanisms can prevent unauthorized devices from accessing the local network segments where vulnerable routers are located. Regular vulnerability assessments and penetration testing should be conducted to identify similar weaknesses in other network components, as this vulnerability demonstrates the importance of robust input validation in network protocol implementations. Organizations should also maintain comprehensive incident response procedures that account for device reload scenarios and ensure rapid recovery capabilities.