CVE-1999-0160 in IOS
Summary
by MITRE
some classic cisco ios devices have a vulnerability in the ppp chap authentication to establish unauthorized ppp connections.
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Analysis
by VulDB Data Team • 04/17/2026
The vulnerability identified as CVE-1999-0160 represents a critical weakness in Cisco IOS software affecting numerous network devices from the late 1990s era. This flaw specifically targets the Point-to-Point Protocol Challenge Handshake Authentication Protocol implementation within Cisco routers and switches, creating a pathway for unauthorized network access through manipulated authentication processes. The vulnerability stems from insufficient validation of authentication parameters during the CHAP negotiation phase, allowing attackers to bypass legitimate authentication mechanisms and establish unauthorized PPP connections to network resources.
This technical weakness manifests in the improper handling of CHAP challenge-response sequences where Cisco IOS devices fail to adequately verify the authenticity of authentication messages. The flaw exists in the protocol implementation logic that processes authentication requests, particularly when dealing with malformed or manipulated challenge packets. Attackers can exploit this vulnerability by crafting specific PPP CHAP packets that appear legitimate to the target device but contain modified authentication data, effectively enabling them to gain network access without proper credentials. The vulnerability operates at the network layer protocol level, specifically within the PPP authentication framework, making it particularly dangerous as it can be exploited from remote locations without physical access to the network infrastructure.
The operational impact of CVE-1999-0160 extends beyond simple unauthorized access, as it can lead to complete network compromise and unauthorized data exfiltration. Network administrators face significant risks including potential man-in-the-middle attacks, unauthorized network traversal, and access to sensitive internal resources. The vulnerability affects a broad range of Cisco IOS versions from the 11.x and 12.x series, making it widespread across enterprise and service provider networks. Organizations relying on legacy Cisco equipment for remote access, dial-up connections, or VPN services face the highest risk exposure. The attack vector typically involves network-based exploitation requiring minimal privileges and can be executed from external network positions, making detection and prevention challenging.
Mitigation strategies for this vulnerability involve immediate software patching and configuration hardening measures. Cisco released IOS software updates addressing this specific weakness, requiring network administrators to upgrade their device firmware to versions containing the necessary security fixes. Additionally, implementing network access controls such as ACLs to restrict PPP CHAP traffic, disabling unnecessary PPP services, and employing stronger authentication methods like EAP or MS-CHAPv2 can provide layered protection. The vulnerability aligns with CWE-284, which addresses improper access control in authentication mechanisms, and maps to ATT&CK technique T1078 for valid accounts and T1566 for credential harvesting. Organizations should conduct comprehensive vulnerability assessments to identify affected devices and implement network segmentation to limit potential exploitation impact. Regular security monitoring and intrusion detection system rules specifically targeting malformed PPP CHAP packets can help detect exploitation attempts and provide early warning of potential attacks.