CVE-2015-5224 login-utils: file name collision due to incorrect mkstemp use

[Qualys Security Advisory <qsa () qualys com>]

Dear List,

A CVE-ID and a patch have just been issued for a vulnerability that we
discovered in util-linux (while investigating the libuser bugs):

CVE-2015-5224 login-utils: file name collision due to incorrect mkstemp use
https://github.com/karelzak/util-linux/commit/bde91c85bdc77975155058276f99d2e0f5eab5a9

If the chfn and chsh binaries (both setuid-root) from
util-linux/login-utils are compiled WITHOUT libuser support, they
eventually call mkostemp(localtmp, O_RDWR|O_CREAT|O_EXCL|O_CLOEXEC);
where localtmp is "/etc/%s.XXXXXX" and %s is __progname (i.e., argv[0]'s
basename).

An attacker could repeatedly execve chfn as "ld.so" until mkostemp()
creates "/etc/ld.so.preload" (after a few days, in our tests).  This
particular example doesn't actually work, because "preload" is 7 chars
and "XXXXXX" is 6 chars, but it seems there are other interesting
possibilities, like "/etc/rc.status" and "/etc/krb5.keytab".

The impact of this vulnerability is probably very limited: most Linux
distributions ship either the chfn/chsh binaries from the shadow-utils
(NOT the login-utils ones), or the login-utils ones but WITH libuser
support.

With best regards,

-- 
the Qualys Security Advisory team