A flaw was found in the way the libuser library handled the /etc/passwd file. A local attacker could use an application compiled against libuser (for example, userhelper) to manipulate the /etc/passwd file, which could result in a denial of service or possibly allow the attacker to escalate their privileges to root.
Find out more about CVE-2015-3246 from the
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This issue affects the versions of libuser as shipped with Red Hat Enterprise Linux 5. Red Hat Enterprise Linux 5 is now in Production 3 Phase of the support and maintenance life cycle. This vulnerability has been rated as having Important security impact and is not currently planned to be addressed in future updates. For additional information, refer to the Red Hat Enterprise Linux Life Cycle: https://access.redhat.com/support/policy/updates/errata/.
Add pam_warn and pam_deny rules to /etc/pam.d/chfn and /etc/pam.d/chsh to prevent non-root users from using this functionality. With these edits, the files should contain:
auth sufficient pam_rootok.so
auth required pam_warn.so
auth required pam_deny.so
auth include system-auth
account include system-auth
password include system-auth
session include system-auth
After these changes, attempts by unprivileged users to use chfn and chsh (and the respective functionality in the userhelper program) will fail, and will be logged (by default in /var/log/secure).