Wireless weak-link sensor networks (abbreviated as “weak networks”) are widely deployed in extreme environments such as underground, deep sea, and pipelines. Affected by physical characteristics including strong attenuation, high noise, and intermittent connectivity, their communication links persistently suffer from high packet loss, asymmetry, low bandwidth, and frequent disconnections. As a result, the security mechanisms of traditional wireless sensor networks struggle to remain effective in such environments. External attackers can exploit the vulnerabilities of weak network links to carry out identity forgery, man-in-the-middle attacks, and replay attacks, while compromised internal nodes may undermine network stability through impersonation, selective forwarding, and collaborative attacks. Therefore, constructing a security mechanism tailored to the characteristics of weak networks is a core issue in ensuring their usability.To address the above challenges, this paper proposed a security framework for weak network environments. For external access security, a disconnection-robust authenticated key agreement protocol (D-ADH) is designed. It significantly reduced interaction overhead by using fixed negotiation public keys and a single secure broadcast, and employed a lightweight request-retransmission mechanism to improve the success rate of key negotiation in high packet loss environments. For internal malicious node detection, a dynamic trust management mechanism based on Type-2 fuzzy logic (FDTM) was proposed. It integrated multiple sources of trust evidence, including communication success rate, data consistency, and traffic behavior, and introduced environment-aware and trend-based update methods, enabling trust inference to effectively distinguish anomalies caused by weak network noise from real malicious behaviors. The proposed security framework can simultaneously achieve stable external authentication capabilities and high-precision internal malicious node identification under extreme link conditions, providing a deployable, scalable, and highly robust security solution for weak networks.