SybilGuard is a novel decentralized protocol designed to defend against sybil attacks in peer-to-peer and other distributed systems. Sybil attacks occur when a malicious user creates multiple fake identities to gain control over a large fraction of the system's nodes, thereby undermining the system's integrity. SybilGuard leverages the social network among user identities, where edges represent human-established trust relationships. Malicious users can create many identities but few trust relationships, leading to a disproportionately small "cut" in the graph between sybil nodes and honest nodes. SybilGuard exploits this property to bound the number of identities a malicious user can create. The protocol uses random walks in the social network to ensure that the number of attack edges (edges between sybil and honest nodes) is limited. SybilGuard ensures that the number of attack edges is independent of the number of sybil identities and is limited by the number of trust relation pairs between malicious users and honest users. The protocol guarantees that an honest node accepts, and is accepted by, most other honest nodes with high probability, and that an honest node only accepts a bounded number of sybil nodes. SybilGuard partitions the accepted nodes into equivalence groups, ensuring that only a certain number of those groups contain sybil nodes. The protocol also ensures that the number and size of sybil groups are bounded. SybilGuard is completely decentralized, and all functionalities are with respect to a given node. The protocol uses random routes and route intersection to determine whether a node should be accepted. The effectiveness of SybilGuard is shown both analytically and experimentally. The protocol is designed to be robust against sybil attacks and is effective in limiting the number of sybil nodes in the system.SybilGuard is a novel decentralized protocol designed to defend against sybil attacks in peer-to-peer and other distributed systems. Sybil attacks occur when a malicious user creates multiple fake identities to gain control over a large fraction of the system's nodes, thereby undermining the system's integrity. SybilGuard leverages the social network among user identities, where edges represent human-established trust relationships. Malicious users can create many identities but few trust relationships, leading to a disproportionately small "cut" in the graph between sybil nodes and honest nodes. SybilGuard exploits this property to bound the number of identities a malicious user can create. The protocol uses random walks in the social network to ensure that the number of attack edges (edges between sybil and honest nodes) is limited. SybilGuard ensures that the number of attack edges is independent of the number of sybil identities and is limited by the number of trust relation pairs between malicious users and honest users. The protocol guarantees that an honest node accepts, and is accepted by, most other honest nodes with high probability, and that an honest node only accepts a bounded number of sybil nodes. SybilGuard partitions the accepted nodes into equivalence groups, ensuring that only a certain number of those groups contain sybil nodes. The protocol also ensures that the number and size of sybil groups are bounded. SybilGuard is completely decentralized, and all functionalities are with respect to a given node. The protocol uses random routes and route intersection to determine whether a node should be accepted. The effectiveness of SybilGuard is shown both analytically and experimentally. The protocol is designed to be robust against sybil attacks and is effective in limiting the number of sybil nodes in the system.