CVE-2017-3857 in IOS
Summary
by MITRE
A vulnerability in the Layer 2 Tunneling Protocol (L2TP) parsing function of Cisco IOS (12.0 through 12.4 and 15.0 through 15.6) and Cisco IOS XE (3.1 through 3.18) could allow an unauthenticated, remote attacker to cause an affected device to reload. The vulnerability is due to insufficient validation of L2TP packets. An attacker could exploit this vulnerability by sending a crafted L2TP packet to an affected device. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a denial of service (DoS) condition. This vulnerability affects Cisco devices that are running a vulnerable release of Cisco IOS or Cisco IOS XE Software if the L2TP feature is enabled for the device and the device is configured as an L2TP Version 2 (L2TPv2) or L2TP Version 3 (L2TPv3) endpoint. By default, the L2TP feature is not enabled. Cisco Bug IDs: CSCuy82078.
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Analysis
by VulDB Data Team • 08/28/2024
The vulnerability described in CVE-2017-3857 represents a critical denial of service weakness within Cisco's Layer 2 Tunneling Protocol implementation across multiple IOS and IOS XE software versions. This flaw exists in the parsing function responsible for processing L2TP packets, specifically affecting versions ranging from 12.0 through 12.4 and 15.0 through 15.6 for IOS, along with 3.1 through 3.18 for IOS XE. The vulnerability stems from inadequate input validation mechanisms that fail to properly examine the structure and content of incoming L2TP packets before processing them. This insufficient validation creates an exploitable condition where malformed or specially crafted packets can trigger unexpected behavior in the affected network infrastructure.
The technical exploitation of this vulnerability occurs through the transmission of specifically designed L2TP packets to targeted Cisco devices that have L2TP functionality enabled. When these malformed packets reach the affected device, the inadequate parsing logic fails to properly handle the unexpected packet structures, leading to a system crash or restart condition. The attack requires no authentication credentials and can be executed remotely, making it particularly dangerous as it allows adversaries to disrupt network services without requiring access to the device itself. The vulnerability specifically impacts devices configured as L2TPv2 or L2TPv3 endpoints, which are commonly used for secure remote access and virtual private network connections. The default configuration of Cisco devices does not enable L2TP functionality, but when administrators do enable it for legitimate business purposes, the system becomes susceptible to this attack vector.
The operational impact of this vulnerability extends beyond simple service disruption, as it can effectively render network infrastructure unusable for legitimate users and administrators. When an affected device experiences a reload due to this vulnerability, all network services dependent on that device become unavailable until the system recovers, potentially causing cascading failures throughout interconnected network segments. The vulnerability affects critical network infrastructure components that often serve as gateways for remote access, VPN connections, and enterprise network segmentation, making the potential disruption severe for organizations relying on these services. Network administrators may face significant challenges in identifying and mitigating this vulnerability, as the attack can occur silently without generating obvious alerts or logs, potentially allowing continued exploitation until the issue is detected and addressed.
Mitigation strategies for CVE-2017-3857 should prioritize immediate implementation of network segmentation and access control measures to prevent unauthorized access to potentially vulnerable devices. Organizations should disable L2TP functionality on devices where it is not required, as this represents the most effective immediate defense against exploitation. Cisco has released patches and software updates addressing this vulnerability, which should be deployed across all affected systems following vendor guidance and best practices for software maintenance. Network monitoring should be enhanced to detect unusual packet patterns that might indicate attempted exploitation, particularly focusing on L2TP traffic analysis. The vulnerability aligns with CWE-129, which addresses insufficient input validation, and maps to ATT&CK technique T1499.004 for network denial of service attacks, highlighting the importance of both defensive measures and incident response protocols. Regular vulnerability assessments and security audits should include verification of L2TP configurations to ensure that only necessary services remain enabled and properly secured.