A recently disclosed vulnerability in OpenBSD’s networking stack has revealed a serious flaw that allows attackers to bypass Password Authentication Protocol (PAP) authentication under specific conditions. The weakness exists in the sppp_pap_input() function of the sppp(4) subsystem, responsible for handling synchronous PPP connections commonly used in PPPoE deployments. Researchers determined that the authentication logic has been defective since its introduction in 1999. During credential verification, the code relied on attacker-supplied username and password length fields when calling bcmp(), rather than validating them against the expected credential lengths. Because zero-length values were not rejected, an attacker could submit empty credentials and still satisfy the authentication checks, resulting in unauthorized access. This flaw effectively permits complete authentication bypass without requiring valid usernames or passwords, posing a significant security risk to affected systems. The same design issue also created a secondary memory-safety concern. If a malicious PAP request specified lengths larger than the actual stored credentials, the comparison routine could read beyond the intended memory boundaries. This kernel heap overread condition may expose adjacent memory contents and became more dangerous after a 2009 code change that replaced fixed-size buffers with dynamically allocated storage. Since the vulnerability is reachable through the PPPoE data path, exploitation does not require prior authentication. A rogue PPPoE server operating within the same broadcast domain can take advantage of the flaw to impersonate a trusted service and attract client connections. In a demonstrated proof-of-concept attack, researchers successfully completed PPPoE negotiation, transmitted PAP messages containing empty credentials, and established fully functional sessions. The OpenBSD client accepted the authentication request, obtained network configuration, and exchanged traffic through the attacker-controlled endpoint. Developers addressed the issue by introducing strict length validation before credential comparison, mirroring protections already present in the CHAP implementation. The vulnerability was privately reported on June 12, 2026, and patched within days.
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