CVE-2017-7522 in OpenVPN
Summary
by MITRE
OpenVPN versions before 2.4.3 and before 2.3.17 are vulnerable to denial-of-service by authenticated remote attacker via sending a certificate with an embedded NULL character.
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Analysis
by VulDB Data Team • 12/29/2020
The vulnerability identified as CVE-2017-7522 represents a critical denial-of-service weakness in OpenVPN software versions prior to 2.4.3 and 2.3.17. This flaw specifically targets the certificate validation process within the OpenVPN implementation, creating a potential attack vector for authenticated remote adversaries who can manipulate certificate data to trigger system instability. The vulnerability stems from insufficient input validation during certificate processing, where the software fails to properly handle certificates containing embedded NULL characters that are typically used to terminate strings in many programming contexts.
The technical exploitation of this vulnerability occurs when an authenticated attacker crafts a malicious certificate that contains embedded NULL characters within its data structure. During the certificate validation process, the OpenVPN server or client application processes this malformed certificate without proper sanitization, leading to unpredictable behavior that can cause the application to crash or become unresponsive. This type of vulnerability falls under the CWE-129 weakness category, which specifically addresses improper validation of array indices and buffer overflows that can lead to system instability and denial-of-service conditions. The flaw demonstrates a classic buffer handling issue where string termination characters are not properly managed during certificate parsing operations.
From an operational impact perspective, this vulnerability poses significant risks to organizations relying on OpenVPN for secure remote access solutions. An authenticated attacker who gains access to the VPN infrastructure can deliberately send malformed certificates to disrupt services, potentially affecting multiple users simultaneously if the vulnerability exists in the central authentication server. The attack requires minimal privileges since the attacker only needs authentication credentials to the VPN system, making it particularly dangerous in environments where VPN access is widely distributed. This vulnerability directly maps to ATT&CK technique T1499.004, which covers network denial-of-service attacks, and specifically targets the service availability aspect of the CIA triad.
The mitigation strategy for CVE-2017-7522 involves immediate patching of affected OpenVPN installations to versions 2.4.3 or 2.3.17 and later, which contain proper input validation mechanisms for certificate processing. Organizations should also implement certificate monitoring and validation procedures that automatically detect and reject certificates containing suspicious NULL character sequences. Network segmentation and access controls can help limit the potential impact of such attacks by restricting direct access to VPN servers. Additionally, implementing proper logging and alerting mechanisms around certificate validation events can help detect exploitation attempts. The fix implemented in the patched versions typically includes enhanced input sanitization routines that properly handle string termination and character validation during certificate parsing, preventing the NULL character injection from causing memory corruption or unexpected application behavior. Security teams should also consider implementing certificate authority validation policies that enforce stricter certificate format requirements and regularly audit certificate issuance processes to prevent malicious certificate deployment.