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8lgm-19.txt

8lgm-19.txt
Posted Sep 23, 1999

8lgm-19.txt

SHA-256 | fa9ab2f48471d34726eb841ac7562adef8204df8a3949aef925f2504cc436ccd

8lgm-19.txt

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===========================================================================
[8lgm]-Advisory-19.UNIX.SunOS-kernel.1-Jun-1994

SYSTEM CALL:

link(2)

KNOWN VULNERABLE OS:

SunOS 4.1.*

DESCRIPTION:

The synopsis of the link(2) system call is:

int link(path1, path2)
char *path1, *path2;

Under SunOS 4.1.*, link(2) will incorrectly follow symbolic links
for path2.

IMPACT:

Programs using the link(2) system call where path2 is located in a
publically writable directory, can potentially be used to gain root
access (e.g. the advisory:-

[8lgm]-Advisory-15.UNIX.mail3.28-Nov-1994

is based on binmail using this vulnerability in link(2).)

REPEAT BY:

An example exploit for the [8lgm]-Advisory-15.UNIX.mail3.28-Nov-1994
advisory is available from the 8lgm fileserver, as of now. To
obtain this program, send mail to 8lgm-fileserver@8lgm.org, with a
line in the body of the message containing:-

SEND suln.c

DISCUSSION:

A secure link(2) system call can allow path1 to be a symbolic link.
However, allowing path2 to be a symbolic link can potentially cause
security problems.

Consider a program, creating hard links in a publically writable
directory, as a privileged uid. The program has no way of creating
a hard link in a secure manner (ie attempting to write code to
provide a workaround would be non-atomic, and therefore open to race
conditions. To use hard links in the situation described would
require using the chroot(2) system call, producing a non-elegant
fix).

WORKAROUND:

The link(2) system call is used almost exclusively for file locking.
Using the open(2) system call, it is possible to write a secure,
file locking mechanism.

Sample locking code using open(2), and not link(2), can be seen in
CERT Advisory

CA-95:02.binmail.vulnerabilities

The code contained in this advisory is a replacement for binmail,
and is recommended for use.

FIX:

Contact vendor for fix.

-----------------------------------------------------------------------

FEEDBACK AND CONTACT INFORMATION:

majordomo@8lgm.org (Mailing list requests - try 'help'
for details)

8lgm@8lgm.org (Everything else)

8LGM FILESERVER:

All [8LGM] advisories may be obtained via the [8LGM] fileserver.
For details, 'echo help | mail 8lgm-fileserver@8lgm.org'

8LGM WWW SERVER:

[8LGM]'s web server can be reached at http://www.8lgm.org.
This contains details of all 8LGM advisories and other useful
information.
===========================================================================

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