llhttp is a high-performance HTTP parser used by Node.js to turn raw network bytes into structured requests and headers. It is the code that decides where a request starts, how headers are separated, and whether incoming data is valid enough to pass to the application.
This matters because parser bugs can become security bugs. If llhttp misreads malformed or edge-case HTTP input, attackers may trigger crashes, memory pressure, or request smuggling conditions where a proxy and backend disagree about request boundaries. Defenders care about llhttp updates because parser fixes often close off denial-of-service paths and harden boundary checks in the HTTP stack. In practice, keeping llhttp current helps Node.js services interpret traffic consistently and reduces the chance that ambiguous input can be turned into abuse.