CVE-2002-1336 in TightVNC
Summary
by MITRE
TightVNC before 1.2.6 generates the same challenge string for multiple connections, which allows remote attackers to bypass VNC authentication by sniffing the challenge and response of other users.
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Analysis
by VulDB Data Team • 09/06/2025
The vulnerability identified as CVE-2002-1336 affects TightVNC versions prior to 1.2.6 and represents a significant authentication flaw that undermines the security of remote desktop connections. This issue stems from the predictable nature of challenge strings generated during the VNC authentication process, creating a scenario where attackers can exploit previously captured authentication data to gain unauthorized access to systems. The vulnerability specifically targets the VNC protocol's challenge-response authentication mechanism, which is designed to prevent unauthorized connections by requiring proper authentication credentials.
The technical flaw manifests in the cryptographic implementation of TightVNC's authentication system where the same challenge string is generated for multiple concurrent connections instead of using unique, random challenge values for each authentication attempt. This predictable behavior violates fundamental security principles for authentication protocols, as it allows attackers to capture a valid challenge-response pair during one authentication session and reuse that information to authenticate as the legitimate user in subsequent connection attempts. The vulnerability operates at the protocol level, specifically within the VNC authentication handshake process where the server sends a challenge string to the client, and the client responds with a hash of the password combined with the challenge.
The operational impact of this vulnerability extends beyond simple unauthorized access, creating a persistent security risk that can be exploited by network-based attackers who have the capability to monitor network traffic. Attackers can perform passive network sniffing to capture the challenge-response pairs exchanged during legitimate authentication sessions, then leverage this captured information to authenticate themselves without knowing the actual password. This type of attack falls under the category of credential replay attacks and represents a serious weakness in the authentication system's design. The vulnerability is particularly dangerous in environments where VNC servers are accessible over untrusted networks, as it allows attackers to bypass authentication mechanisms entirely and gain full access to target systems.
The security implications of this vulnerability align with CWE-310, which addresses cryptographic weaknesses in authentication protocols, and can be mapped to ATT&CK technique T1075 for legitimate credentials, as attackers can leverage captured authentication data to establish persistent access. Organizations using vulnerable versions of TightVNC face significant risks including unauthorized system access, data theft, and potential lateral movement within networks. The vulnerability demonstrates a failure in implementing proper randomization mechanisms for cryptographic challenges, which is a fundamental requirement for secure authentication protocols. Mitigation efforts should focus on upgrading to TightVNC version 1.2.6 or later, which implements proper challenge string generation, along with network segmentation and monitoring to detect potential credential replay attacks. Additionally, organizations should consider implementing additional authentication layers such as VPNs or SSH tunneling to protect VNC connections from passive network monitoring attacks.