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libuser vulnerabilities

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It was discovered that the libuser library contains two vulnerabilities which, in combination, allow unprivileged local users to gain root privileges.

libuser is a library that provides read and write access to files like /etc/passwd, which constitute the system user and group database. On Red Hat Enterprise Linux it is a central system component.

What is being disclosed today?

Qualys reported two vulnerabilities:

It turns out that these vulnerabilities can be exploited by an unprivileged local user to gain root privileges on an affected system. However, due to the way libuser works, only users who have accounts already listed in /etc/passwd can exploit this vulnerability, and the user needs to supply the account password as part of the attack. These requirements mean that exploitation by accounts listed only in LDAP (or some other NSS data source) or by system accounts without a valid password is not possible.

Further analysis showed that the first vulnerability, CVE-2015-3245, is also due to a missing check in libuser.

Which system components are affected by these vulnerabilities?

libuser is a library, which means that in order to exploit it, a program which employs it must be used. Ideally, such a program has the following properties:

  1. It uses libuser.
  2. It is SUID-root.
  3. It allows putting almost arbitrary content into /etc/passwd.

Without the third item, exploitation may still be possible, but it will be much more difficult. If the program is not SUID-root, a user will not have unlimited attempts to exploit the race condition.

A survey of programs processing /etc/passwd and related files presents this picture:

  • passwd is SUID-root, but it uses PAM to change the password, which has custom code to modify /etc/passwd not affected by the race condition. The account locking functionality in passwd does use libuser, but it is restricted to root.
  • chfn and chsh from util-linux are SUID-root and use libuser to change /etc/passwd (the latter depending on how util-linux was compiled) but they have fairly strict filters controlling what users can put into these files.
  • lpasswd, lchfn, lchsh and related utilities from libuser are not SUID-root.
  • userhelper in the usermode package has all three qualifications: libuser-based, SUID-root, and lack of filters.

This is why userhelper is a plausible target for exploitation, and other programs such as passwd and chfn are not.

How can these vulnerabilities be addressed?

System administrators can apply updates from your operating system vendor. Details of affected Red Hat products and security advisories are available on the knowledge base article on the Red Hat Customer Portal.

This security update will change libuser to apply additional checks to the values written to the user and group files (so that injecting newlines is no longer possible), and replaces the locking and file update code to follow the same procedures as the rest of the system. The first change is sufficient to prevent newline injection with userhelper as well, which means that only libuser needs to be updated.

If software updates are not available or cannot be applied, it is possible to block access to the vulnerable functionality with a PAM configuration change. System administrators can edit the files /etc/pam.d/chfn and /etc/pam.d/chsh and block access to non-root users by using pam_warn (for logging) and pam_deny:

#%PAM-1.0
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

This will prevent users from changing their login shells and their GECOS field. userhelper identifies itself to PAM as “chfn”, which means this change is effective for this program as well.

Acknowledgements

Red Hat would like to thank Qualys for reporting these vulnerabilities.