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.