Mono provides the necessary software to develop and run .NET client and server applications.
Sebastian Krahmer of the SuSE Security Team discovered that the System.CodeDom.Compiler classes of Mono create temporary files with insecure permissions.
A local attacker could create links in the temporary file directory, pointing to a valid file somewhere on the filesystem. When an affected class is called, this could result in the file being overwritten with the rights of the user running the script.
There is no known workaround at this time.
All Mono users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=dev-lang/mono-1.1.13.8.1"