CVE-2018-16550 in TeamViewer
Summary
by MITRE
TeamViewer 10.x through 13.x allows remote attackers to bypass the brute-force authentication protection mechanism by skipping the "Cancel" step, which makes it easier to determine the correct value of the default 4-digit PIN.
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Analysis
by VulDB Data Team • 02/05/2026
TeamViewer versions 10 through 13 contain a critical security flaw that undermines the application's built-in brute-force protection mechanisms. This vulnerability stems from an implementation error in the authentication flow where attackers can bypass the standard protection measures by simply skipping the cancellation step during PIN entry. The flaw specifically targets the default 4-digit PIN verification process, which is designed to prevent automated attacks through rate limiting and account lockout mechanisms. When users proceed directly to PIN entry without completing the cancellation sequence, the system fails to properly track authentication attempts, effectively nullifying the security controls intended to protect against dictionary attacks.
The technical nature of this vulnerability aligns with CWE-307, which addresses improper restriction of repeated activities, and represents a significant weakness in the authentication protocol design. Attackers can exploit this by rapidly cycling through PIN attempts without triggering the intended protection mechanisms, making it considerably easier to perform successful brute-force attacks against default PIN values. This flaw operates at the application layer and requires no special privileges or advanced technical skills to exploit, making it particularly dangerous in environments where TeamViewer is used for remote access management.
The operational impact of this vulnerability extends beyond simple credential guessing, as it fundamentally compromises the security model of the authentication system. Organizations using TeamViewer within their infrastructure face increased risk of unauthorized access, especially when default PINs are used or when users select weak PIN values. The vulnerability affects the core security principle of least privilege, as attackers can systematically test PIN combinations without facing the intended rate limiting or account lockout protections. This creates a pathway for persistent attackers to gain unauthorized access to systems, potentially leading to full network compromise, data exfiltration, or lateral movement within the affected environment.
Security professionals should implement immediate mitigations including mandatory PIN changes for all TeamViewer installations, enforcement of complex PIN policies, and deployment of network monitoring to detect unusual authentication patterns. Organizations should also consider implementing additional authentication layers such as two-factor authentication or hardware tokens to compensate for the weakened PIN protection. The vulnerability demonstrates the importance of proper authentication flow implementation and highlights the need for comprehensive security testing of authentication mechanisms. According to ATT&CK framework category T1110, this vulnerability falls under credential access techniques that leverage weak authentication controls. System administrators should also consider network segmentation and access control policies to limit the potential impact of successful exploitation, as the vulnerability primarily affects authentication rather than system exploitation. Regular security audits should be conducted to identify similar implementation flaws in other authentication systems, as this type of vulnerability can manifest in various software applications that rely on similar protection mechanisms.