-----BEGIN PGP SIGNED MESSAGE----- __________________________________________________________ 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. _____________________________________________________________________________ CIAC, the Computer Incident Advisory Capability, is the computer security incident response team for the U.S. Department of Energy (DOE) and the emergency backup response team for the National Institutes of Health (NIH). CIAC is located at the Lawrence Livermore National Laboratory in Livermore, California. CIAC is also a founding member of FIRST, the Forum of Incident Response and Security Teams, a global organization established to foster cooperation and coordination among computer security teams worldwide. 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