CVE-2024-28820 in Three Rings Auth-LDAP Plugin
Summary
by MITRE • 06/27/2024
Buffer overflow in the extract_openvpn_cr function in openvpn-cr.c in openvpn-auth-ldap (aka the Three Rings Auth-LDAP plugin for OpenVPN) 2.0.4 allows attackers with a valid LDAP username and who can control the challenge/response password field to pass a string with more than 14 colons into this field and cause a buffer overflow.
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Analysis
by VulDB Data Team • 10/25/2024
The vulnerability identified as CVE-2024-28820 represents a critical buffer overflow flaw within the openvpn-auth-ldap plugin version 2.0.4, specifically within the extract_openvpn_cr function located in the openvpn-cr.c source file. This plugin serves as an authentication module for OpenVPN implementations that rely on LDAP directory services for user verification. The flaw arises from inadequate input validation when processing challenge/response password fields during the authentication process, creating a pathway for malicious exploitation that could compromise the integrity and availability of the authentication system.
The technical implementation of this vulnerability stems from the function's failure to properly bounds-check input data when parsing the challenge/response field, which is typically used in multi-factor authentication scenarios. When an authenticated user submits a password string containing more than 14 colons, the function processes this input without sufficient validation, leading to a classic buffer overflow condition where data exceeds the allocated memory buffer. This specific threshold of 14 colons appears to be directly related to how the function parses and splits the input string, likely using colon delimiters to separate authentication components. The vulnerability manifests as a stack-based buffer overflow that can potentially be exploited to execute arbitrary code or cause denial of service conditions within the OpenVPN authentication process.
The operational impact of this vulnerability extends beyond simple authentication failures, as it creates opportunities for attackers to gain unauthorized access to systems protected by OpenVPN configurations using this plugin. An attacker with valid LDAP credentials can leverage this flaw to manipulate the challenge/response mechanism, potentially leading to privilege escalation, authentication bypass, or complete system compromise. The vulnerability is particularly concerning because it requires minimal prerequisites - only valid LDAP authentication credentials and the ability to control the password field - making it accessible to threat actors with basic authentication access. This weakness directly aligns with attack patterns documented in the MITRE ATT&CK framework under credential access and privilege escalation techniques, specifically targeting the authentication process as a critical attack surface.
Mitigation strategies for CVE-2024-28820 should prioritize immediate patching of the openvpn-auth-ldap plugin to version 2.0.5 or later, which contains the necessary input validation fixes. Organizations should implement network segmentation and access controls to limit exposure of systems utilizing this plugin, particularly in environments where LDAP authentication is critical. Additionally, monitoring should be enhanced to detect unusual authentication patterns or malformed challenge/response inputs that could indicate exploitation attempts. The vulnerability demonstrates the importance of proper input validation and bounds checking in authentication modules, aligning with CWE-121, which addresses stack-based buffer overflow conditions. Security teams should also consider implementing intrusion detection systems that can identify potential exploitation attempts through anomalous input patterns, as this vulnerability could serve as a stepping stone for more sophisticated attacks targeting the broader OpenVPN infrastructure or underlying network resources.