less through 653 allows OS command execution via a newline character in the name of a file, because quoting is mishandled in filename.c. Exploitation typically requires use with attacker-controlled file names, such as the files extracted from an untrusted archive. Exploitation also requires the LESSOPEN environment variable, but this is set by default in many common cases.
The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes code syntax before inserting the input into an executable resource, such as a library, configuration file, or template.