Multiple Security Vulnerabilities in Fcron
 
iDEFENSE Security Advisory 11.15.04
www.idefense.com/application/poi/display?id=157&type=vulnerabilities
November 15, 2004
 
I. BACKGROUND
 
Fcron is a periodical command scheduler which aims at replacing Vixie
Cron, and implements most of its functionalities. More information about
Fcron is available from http://fcron.free.fr/description.php.
 
II. DESCRIPTION
 
Multiple vulnerabilities have been found in Fcron.
 
**** ISSUE 1 - File contents disclosure ****
 
Local exploitation of a design error vulnerability in the fcronsighup=20
component of Fcron may allow users to view the contents of root owned=20
files.
 
The vulnerability specifically exists within the set user id (setuid)=20
root program fcronsighup. When the filename of a root owned file is=20
passed as an argument to this program, it attempts to parse the file as=20
a configuration file. Any lines in the file that are not parsable will=20
be output as error messages. The following example demonstrates how an=20
attacker can abuse this vulnerability to glean sensitive information=20
from the /etc/shadow password file:
 
bash$ fcronsighup /etc/shadow
14:33:09 Unknown var name at line
root:<password-hash>:12475:0:99999:7::: : line ignored
 
**** ISSUE 2 - Configuration Bypass Vulnerability ****
 
Local exploitation of a design error vulnerability in the fcronsighup=20
component of Fcron may allow users to bypass access restrictions.
 
The problem specifically exists in the checking performed by the
fcronsighup utility on the file passed as a configuration file. It
checks if the file is root owned, and not writable by any other users.
 
When a setuid process is run by an ordinary user the /proc filesystem
pseudofiles associated with it are owned by root and the contents of the
 
"cmdline" and "evironment" files are controllable by the user.
 
By pointing the fcronsighup configuration file to a /proc entry owned by
 
root, such as /proc/self/cmdline or /proc/self/environ, it is possible=20
for a user to supply their own configuration settings.
 
**** ISSUE 3 - File Removal and Empty File Creation Vulnerability****
 
Local exploitation of a design error vulnerability in the fcronsighup=20
component of Fcron may allow users to remove arbitrary files or create=20
arbitrary empty files.
 
The vulnerability specifically exists in the fcronsighup utility which=20
does signaling of the running fcron daemon. Fcronsighup creates a file=20
named in part from a value read from configuration file. This file is=20
created using open() with O_RDWR|O_CREAT and 0644 parameters while=20
running with full root privileges. After some time has passed the file=20
is removed. The filename string is generated by the following code:
 
snprintf(sigfile, sizeof(sigfile), "%s/fcrontab.sig", fcrontabs);
 
By padding the front of the filename with a large number of slash=20
symbols ("/") it is possible to create or remove a file in an arbitrary=20
location. For example: to create the file /tmp/owned, the configuration
option which sets the value for "fcrontabs" can be set to contain
(sizeof(sigfile)-strlen("/tmp/owned")) "/" characters, followed by the
string "/tmp/owned". The code will attempt to append the string
"/fcrontab.sig" to this string, but the limitation imposed on it by the
call to snprintf() will cause it to fail. When the filename is resolved,
the extra "/"s in the filename are ignored, resulting in an absolute
reference to the file /tmp/owned.
 
**** ISSUE 4 -  Information Disclosure Vulnerability ****
 
Local exploitation of a design error vulnerability in the fcrontab=20
component of Fcron may allow users to view the contents of fcron.allow=20
and fcron.deny.
 
The problem specifically exists because Fcron leaks the file descriptors
 
of the opened files /etc/fcron.allow and /etc/fcron.deny to the invoked=20
editor. The default permissions on these files do not allow them to be=20
read by unprivileged users:
 
-rw-r----- 1 root fcron 253 Jul 29 12:45 /etc/fcron.allow
-rw-r----- 1 root fcron 255 Jul 29 12:45 /etc/fcron.deny
 
An attacker can exploit this vulnerability by setting the EDITOR=20
environment variable to a program which outputs the contents of the open
 
file descriptor; descriptor 3 to view the contents of fcron.allow and=20
descriptor 4 to view the contents of fcron.deny.
 
III. ANALYSIS
 
Local users can bypass configuration settings, remove arbitrary files,=20
create files with root permissions, read the contents of root owned=20
files and send a SIGHUP to any process, potentially killing it. These=20
actions may allow them to perform a denial of service or potentially=20
elevate their privileges.
 
IV. DETECTION
 
iDEFENSE has confirmed that Fcron versions 2.0.1 and 2.9.4 are
vulnerable. It is suspected that earlier versions are also affected.
 
V. WORKAROUND
 
Consider changing the permissions on the fcronsighup binary to only=20
allow trusted users access. Make the binary only executable by users=20
in the 'trusted' group by performing the following commands as root:
 
# chown root:trusted /usr/bin/fcronsighup
# chmod 4110 /usr/bin/fcronsighup
 
Also consider performing the same operation on the fcrontab binary to
prevent exploitation of Issue 4.
 
VI. VENDOR RESPONSE
 
The following releases are available to address these vulnerabilities:
 
2.0.2 : stable branch
   http://fcron.free.fr/archives/fcron-2.0.2.src.tar.gz (France)
   or
   ftp://ftp.seul.org/pub/fcron/fcron-2.0.2.src.tar.gz (USA)
 
2.9.5.1 : dev branch
   http://fcron.free.fr/archives/fcron-2.9.5.1.src.tar.gz  (France)
   or
   ftp://ftp.seul.org/pub/fcron/fcron-2.9.5.1.src.tar.gz (USA)
 
VII. CVE INFORMATION
 
The Common Vulnerabilities and Exposures (CVE) project has assigned the
following names to these issues:
 
ISSUE 1 - File contents disclosure
CAN-2004-1030
 
ISSUE 2 - Configuration Bypass Vulnerability
CAN-2004-1031
 
ISSUE 3 - File Removal and Empty File Creation Vulnerability
CAN-2004-1032
 
ISSUE 4 -  Information Disclosure Vulnerability
CAN-2004-1033
 
These are candidates for inclusion in the CVE list
(http://cve.mitre.org), which standardizes names for security problems.
 
VIII. DISCLOSURE TIMELINE
 
10/21/2004  Initial vendor notification
10/21/2004  Initial vendor response
11/15/2004  Coordinated public disclosure
 
IX. CREDIT
 
Karol Wiesek is credited with discovering these vulnerabilities.
 
Get paid for vulnerability research
http://www.idefense.com/poi/teams/vcp.jsp
 
X. LEGAL NOTICES
 
Copyright (c) 2004 iDEFENSE, Inc.
 
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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
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