A signer is the device or system that uses private keys to approve a transaction. In crypto custody, the signer may be a hardware wallet, a secure module, or a multisig controller. Its job is not to store value itself, but to produce a valid cryptographic authorization without exposing the private key.
Signers matter because whoever controls the signer can move funds. If the device is tampered with before setup, or if its firmware, supply chain, or recovery process is compromised, an attacker may gain the ability to approve transactions or steal keys. Defenders reduce this risk by verifying device provenance, checking firmware integrity, using fresh seeds on trusted signers, and separating approvals across multiple devices in multisig designs. In practice, a suspicious signer should be treated as untrusted, even if the wallet interface looks normal.