SNMPv3 auth/priv passwords can contain any combination of alphanumeric characters.

Including certain special characters yields varying results. One support site mentions that the following characters should be avoided in the auth/priv passwords:

, / : " ' |

Another site says, "There's an issue with the '@' character since it's used by Cisco switches/routers to query VLAN info."

This may also vary based on the version of firmware in use. Some characters may work under certain firmware versions.