sharutils is a collection of tools to deal with shar archives.
Joey Hess has discovered that the program unshar, which is a part of sharutils, creates temporary files in a world-writable directory with predictable names.
A local attacker could create symbolic links in the temporary files directory, pointing to a valid file somewhere on the filesystem. When unshar is executed, this would result in the file being overwritten with the rights of the user running the utility, which could be the root user.
There is no known workaround at this time.
All sharutils users should upgrade to the latest version:
# emerge --sync # emerge --ask --oneshot --verbose ">=app-arch/sharutils-4.2.1-r11"