Proxy signature schemes allow an original signer to delegate signing authority to a proxy signer, supporting flexible authentication in distributed and resource-constrained environments such as cloud computing, blockchain, and Internet of Things (IoT) networks. Although many constructions have been proposed, a large number have later been broken due to flawed design assumptions, weak delegation binding, or the absence of rigorous provable security. This paper surveys proxy signature schemes with a focus on their security weaknesses. {It presents} a taxonomy of existing constructions by cryptographic primitive, delegation mechanism, and proof framework, {and then consolidates} known cryptanal... More >