Published:
08 September 1999
Protect yourself against future threats.
-----BEGIN PGP SIGNED MESSAGE----- =========================================================================== AUSCERT External Security Bulletin Redistribution ESB-1999.134 -- FreeBSD-SA-99:01 BSD File Flags and Programming Techniques 09 September 1999 =========================================================================== The FreeBSD Security Team has released the following advisory concerning a vulnerability involving file flags that may prevent device file ownership changes employed by some privileged programs. This vulnerability may allow local users to maintain ownership of devices used by other users. This may be exploited to gain increased privileges including root by snooping on login sessions. - ---------------------------BEGIN INCLUDED TEXT------------------ - -----BEGIN PGP SIGNED MESSAGE----- ============================================================================= 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. ============================================================================= - -----BEGIN PGP SIGNATURE----- Version: 2.6.3ia Charset: noconv Comment: Processed by Mailcrypt 3.4, an Emacs/PGP interface iQCVAwUBN9CAHFUuHi5z0oilAQEJPwP/XhzCOs4ipJkZIPWlSDvsvPLcJWXzb3HK Fs8gLV3CPnW7YdSpveosI3hBY9WNCVAFx9WkM5+n+FBSRfbRzFJkkblN85ZCz7pI +RXg6Sv5vuzy6SRxMRK2vu1FXuwZevVQaMq4ANUXpdo5MyUE8rMGb9PLWdxOxdf5 s6zlG0oFyvI= =CqoX - -----END PGP SIGNATURE----- - ---------------------------END INCLUDED TEXT-------------------- This security bulletin is provided as a service to AusCERT's members. As AusCERT did not write the document quoted above, AusCERT has had no control over its content. The decision to use any or all of this information is the responsibility of each user or organisation, and should be done so in accordance with site policies and procedures. 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