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__________________________________________________________
The U.S. Department of Energy
Computer Incident Advisory Capability
___ __ __ _ ___
/ | /_\ /
\___ __|__ / \ \___
__________________________________________________________
INFORMATION BULLETIN
FreeBSD File Flags and Man-In-The-Middle Attack
September 7, 1999 17:00 GMT Number J-066
_____________________________________________________________________________
PROBLEM: A local user can execute a man-in-the-middle attack should
certain programs fail when subsequent users sign on.
PLATFORM: All systems running FreeBSD 3.2 (and earlier) or
FreeBSD-Current before 1999/08/02.
DAMAGE: A local user can snoop and alter all text that the other user
(including root) writes. Results of this include the ability
to execute commands as the user, and stealing the user's
password (and anything else the user writes over the
connection, including passwords for other machines).
SOLUTION: Apply the available patches. There are no immediate or
temporary workarounds.
_____________________________________________________________________________
VULNERABILITY The risk is MEDIUM because the FreeBSD Advisory did not
ASSESSMENT: indicate that this vulnerability had been discussed in public
forums.
_____________________________________________________________________________
[ Start FreeBSD Advisory ]
=============================================================================
FreeBSD-SA-99:01 Security Advisory
FreeBSD, Inc.
Topic: BSD File Flags and Programming Techniques
Category: core
Module: kernel
Announced: 1999-09-04
Affects: FreeBSD 3.2 (and earlier)
FreeBSD-current before the correction date.
Corrected: FreeBSD-3.3 RELEASE
FreeBSD-current as of 1999/08/02
FreeBSD-3.2-stable as of 1999/08/02
FreeBSD-2.2.8-stable as of 1999/08/04
FreeBSD only: NO
Patches: ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-99:01/
I. Background
BSD 4.4 added various flags to files in the file system. These flags
control various aspects of which operations are permitted on those
files. Historically, root has been been able to do all of these
operations so many programs that knew they were running as root didn't
check to make sure that these operations succeeded.
II. Problem Description
A user can set flags and mode on the device which they logged into.
Since a bug in login and other similar programs causes the normal
chown to fail, this first user will own the terminal of any login.
III. Impact
Local users can execute a man-in-the-middle attack against any other
user (including root) when the other users logs in. This give them
the ability to snoop and alter all text that the user writes. Results
of this include the ability to execute commands as the user, and
stealing the user's password (and anything else the users writes over
the connection, including passwords for other machines).
IV. Workaround
None.
V. Solution
FreeBSD-current
Index: kern/vfs_syscalls.c
===================================================================
RCS file: /home/imp/FreeBSD/CVS/src/sys/kern/vfs_syscalls.c,v
retrieving revision 1.125
retrieving revision 1.128
diff -u -r1.125 -r1.128
--- vfs_syscalls.c 1999/07/29 17:02:56 1.125
+++ vfs_syscalls.c 1999/08/04 04:52:18 1.128
@@ -1892,13 +1892,23 @@
int error;
struct vattr vattr;
+ /*
+ * Prevent non-root users from setting flags on devices. When
+ * a device is reused, users can retain ownership of the device
+ * if they are allowed to set flags and programs assume that
+ * chown can't fail when done as root.
+ */
+ if ((vp->v_type == VCHR || vp->v_type == VBLK) &&
+ ((error = suser_xxx(p->p_ucred, p, PRISON_ROOT)) != 0))
+ return (error);
+
VOP_LEASE(vp, p, p->p_ucred, LEASE_WRITE);
vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, p);
VATTR_NULL(&vattr);
vattr.va_flags = flags;
error = VOP_SETATTR(vp, &vattr, p->p_ucred, p);
VOP_UNLOCK(vp, 0, p);
- return error;
+ return (error);
}
/*
FreeBSD-3.2-stable
Index: kern/vfs_syscalls.c
===================================================================
RCS file: /home/imp/FreeBSD/CVS/src/sys/kern/vfs_syscalls.c,v
retrieving revision 1.112.2.3
retrieving revision 1.112.2.5
diff -u -r1.112.2.3 -r1.112.2.5
--- vfs_syscalls.c 1999/07/30 01:07:23 1.112.2.3
+++ vfs_syscalls.c 1999/08/11 21:39:50 1.112.2.5
@@ -1839,13 +1839,23 @@
int error;
struct vattr vattr;
+ /*
+ * Prevent non-root users from setting flags on devices. When
+ * a device is reused, users can retain ownership of the device
+ * if they are allowed to set flags and programs assume that
+ * chown can't fail when done as root.
+ */
+ if ((vp->v_type == VCHR || vp->v_type == VBLK) &&
+ ((error = suser(p->p_ucred, &p->p_acflag)) != 0))
+ return (error);
+
VOP_LEASE(vp, p, p->p_ucred, LEASE_WRITE);
vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, p);
VATTR_NULL(&vattr);
vattr.va_flags = flags;
error = VOP_SETATTR(vp, &vattr, p->p_ucred, p);
VOP_UNLOCK(vp, 0, p);
- return error;
+ return (error);
}
/*
FreeBSD 2.2.8-stable:
Index: kern/vfs_syscalls.c
===================================================================
RCS file: /home/imp/FreeBSD/CVS/src/sys/kern/vfs_syscalls.c,v
retrieving revision 1.51.2.7
retrieving revision 1.51.2.8
diff -u -r1.51.2.7 -r1.51.2.8
--- vfs_syscalls.c 1998/07/03 03:50:31 1.51.2.7
+++ vfs_syscalls.c 1999/08/04 18:58:56 1.51.2.8
@@ -1439,6 +1439,17 @@
if (error)
return (error);
vp = nd.ni_vp;
+ if ((error = VOP_GETATTR(vp, &vattr, p->p_ucred, p)))
+ return (error);
+ /*
+ * Prevent non-root users from setting flags on devices. When
+ * a device is reused, users can retain ownership of the device
+ * if they are allowed to set flags and programs assume that
+ * chown can't fail when done as root.
+ */
+ if ((vp->v_type == VCHR || vp->v_type == VBLK) &&
+ ((error = suser(p->p_ucred, &p->p_acflag)) != 0))
+ return (error);
LEASE_CHECK(vp, p, p->p_ucred, LEASE_WRITE);
VOP_LOCK(vp);
VATTR_NULL(&vattr);
VI. Credits
Theo de Raadt came up with the firewalling solution presented here.
lumpy@blue.9mm.com brought this problem to light.
=============================================================================
FreeBSD, Inc.
Web Site: http://www.freebsd.org/
Confidential contacts: security-officer@freebsd.org
Security notifications: security-notifications@freebsd.org
Security public discussion: freebsd-security@freebsd.org
PGP Key: ftp://ftp.freebsd.org/pub/FreeBSD/CERT/public_key.asc
Notice: Any patches in this document may not apply cleanly due to
modifications caused by digital signature or mailer software.
Please reference the URL listed at the top of this document
for original copies of all patches if necessary.
=============================================================================
[ End FreeBSD Advisory ]
_____________________________________________________________________________
CIAC wishes to acknowledge the contributions of FreeBSD, Inc. for the
information contained in this bulletin.
_____________________________________________________________________________
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