When using the LD_PRELOAD environmental variable in SUID or SGID applications, glibc does not verify that preloaded libraries in /etc/ld.so.cache are also SUID/SGID, which could allow a local user to overwrite arbitrary files by loading a library from /lib or /usr/lib.
kdesu program in KDE2 (KDE before 2.2.0-6) does not properly verify the owner of a UNIX socket that is used to send a password, which allows local users to steal passwords and gain privileges.