ASN.1, or Abstract Syntax Notation One, is a standard language for describing structured data such as certificates, keys, and protocol messages. It defines what fields exist and how they relate, while separate encodings such as DER or BER define how that data is turned into bytes on the wire or in files.
In cybersecurity, ASN.1 matters because many security and telecom systems depend on it, so parser bugs can become security bugs. Attackers often send malformed ASN.1 objects to trigger crashes, memory corruption, or logic errors in libraries that decode them. Defenders care because these parsers may process untrusted input before authentication or policy checks finish. Careful validation, strict encoding rules, and timely library updates help reduce the risk.