nano-id is a unique string ID generator for Rust. Affected versions of the nano-id crate incorrectly generated IDs using a reduced character set in the `nano_id::base62` and `nano_id::base58` functions. Specifically, the `base62` function used a character set of 32 symbols instead of the intended 62 symbols, and the `base58` function used a character set of 16 symbols instead of the intended 58 symbols. Additionally, the `nano_id::gen` macro is also affected when a custom character set that is not a power of 2 in size is specified. It should be noted that `nano_id::base64` is not affected by this vulnerability. This can result in a significant reduction in entropy, making the generated IDs predictable and vulnerable to brute-force attacks when the IDs are used in security-sensitive contexts such as session tokens or unique identifiers. The vulnerability is fixed in 0.4.0.