CVE-2021-4235: YAML Go package vulnerable to denial of service

The vulnerability stems from uncontrolled alias resolution in YAML parsing. The commit diff shows critical counters (decodeCount, aliasCount, aliasDepth) were added to unmarshal() and alias() functions to prevent abuse. The vulnerable versions lacked these checks, allowing: 1) Decoder.Decode to process malicious documents through public API, 2) unmarshal() to recursively resolve nodes without tracking, and 3) alias() to chase references indefinitely. The GHSA/CVE specifically implicates these core parsing functions when handling aliases.