CVE-2016-1000032 in TGCaptcha2
Summary
by MITRE
TGCaptcha2 version 0.3.0 is vulnerable to a replay attack due to a missing nonce allowing attackers to use a single solved CAPTCHA multiple times.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 05/18/2019
The vulnerability identified in TGCaptcha2 version 0.3.0 represents a critical security flaw that undermines the fundamental purpose of CAPTCHA systems designed to distinguish human users from automated bots. This issue manifests as a replay attack vulnerability where the system fails to implement proper nonce mechanisms, allowing malicious actors to reuse previously solved CAPTCHA challenges across multiple authentication attempts. The absence of unique identifiers or time-bound tokens creates an environment where a single valid CAPTCHA solution can be exploited repeatedly, effectively nullifying the security controls intended to prevent automated abuse.
From a technical perspective, the flaw stems from the implementation of the CAPTCHA validation process lacking cryptographic nonce generation and validation mechanisms. A nonce is a number used once that ensures each CAPTCHA challenge is unique and time-bound, preventing attackers from simply replaying successful solutions. The vulnerability directly maps to CWE-310, which categorizes cryptographic issues related to the improper use of random numbers, and more specifically to CWE-347, which addresses the improper validation of certificates and cryptographic signatures. This weakness creates a persistent security gap where the authentication system becomes vulnerable to automated attack vectors that exploit the predictable nature of the validation process.
The operational impact of this vulnerability extends beyond simple authentication bypasses, creating significant risks for applications relying on TGCaptcha2 for security protection. Attackers can leverage this weakness to perform automated account takeovers, spam registration, and denial-of-service attacks against legitimate users. The vulnerability enables malicious actors to consume system resources inefficiently while maintaining persistent access to protected services, potentially leading to data breaches, unauthorized transactions, and service disruption. Organizations utilizing this component face increased risk of automated abuse that can overwhelm system resources and compromise user account integrity.
Mitigation strategies for this vulnerability require immediate implementation of proper nonce mechanisms within the CAPTCHA validation system. The solution involves generating unique, time-bound tokens for each CAPTCHA challenge and implementing strict validation procedures that prevent reuse of previously solved challenges. Security measures should include cryptographic random number generation for nonce creation, timeout mechanisms for challenge validity, and comprehensive logging of CAPTCHA usage patterns. Organizations should also consider implementing rate limiting and behavioral analysis to detect and prevent automated attack patterns. The remediation aligns with ATT&CK technique T1110.003, which addresses credential stuffing attacks, and requires adherence to security best practices outlined in NIST SP 800-63B for authentication system design. Regular security assessments and code reviews should be implemented to ensure proper nonce implementation and prevent similar vulnerabilities in future releases.