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.

close_altfile in filename.c in less before 606 omits shell_quote calls for LESSCLOSE.

In GNU Less before 609, crafted data can result in "less -R" not filtering ANSI escape sequences sent to the terminal.