YAML versions before 1.27_001 for Perl allow a loaded perl/glob document to replace any package variable, which can lead to arbitrary code execution.

A perl/glob document names a package and a symbol, and supplies the value assigned to it. Nothing restricts the name, so the target can be @INC or YAML's own load options.

A perl/glob document that sets $YAML::LoadCode or $YAML::UseCode turns on code loading, which is off by default, for every later Load() in the process. A perl/code document is then passed to a string eval, so an attacker who supplies two documents to separate Load() calls in one process can execute arbitrary Perl code.