From ylo@ssh.fi Tue Aug 13 09:17:13 1996 Received: from muuri.ssh.fi (ssh.fi [194.100.44.97]) by suburbia.net (8.7.4/Proff-950810) with ESMTP id JAA03253 for ; Tue, 13 Aug 1996 09:17:01 +1000 Received: from pilari.ssh.fi (pilari.ssh.fi [192.168.2.1]) by muuri.ssh.fi (8.7.5/8.7.3) with ESMTP id CAA24039; Tue, 13 Aug 1996 02:16:33 +0300 (EET DST) Received: (from ylo@localhost) by pilari.ssh.fi (8.7.5/8.7.3) id CAA27742; Tue, 13 Aug 1996 02:16:33 +0300 (EET DST) Date: Tue, 13 Aug 1996 02:16:33 +0300 (EET DST) Message-Id: <199608122316.CAA27742@pilari.ssh.fi> From: Tatu Ylonen To: ssh@clinet.fi, best-of-security@suburbia.net, ssh-announce@clinet.fi CC: f-secure-ssh-support@datafellows.com, ttsalo@ssh.fi Subject: fix for ssh security problem (users with writable home directories) Organization: SSH Communications Security, Finland -----BEGIN PGP SIGNED MESSAGE----- Matt Power reported a few days ago a security problem with SSH when you have users that have writable home directories. Basically, if a user has a writable home directory, any local user can create an authorized_keys file in that directory, and log in as that user. It turns out that some operating systems (at least Debian Linux and SunOS) come with such directories in the standard distribution (for users nobody and uucp). The patch below adds a check for the modes and ownership for RSA authentication. It checks - user's home directory - user's $HOME/.ssh directory - user's $HOME/.ssh/authorized_keys file This closes the above security problem. The patch will be included in future releases. Tatu - -- F-Secure SSH Development http://www.datafellows.com/, http://www.ssh.fi/ (Note, since this message is PGP-signed, you'll need to remove the extra '- ' from the beginning of some lines below.) *** ../ssh-1.2.14.orig/ssh.h Thu Jun 6 16:47:01 1996 - --- ssh.h Mon Aug 12 18:47:26 1996 *************** *** 341,347 **** /* Performs the RSA authentication dialog with the client. This returns 0 if the client could not be authenticated, and 1 if authentication was successful. This may exit if there is a serious protocol violation. */ ! int auth_rsa(struct passwd *pw, MP_INT *client_n, RandomState *state); /* Parses an RSA key (number of bits, e, n) from a string. Moves the pointer over the key. Skips any whitespace at the beginning and at end. */ - --- 341,348 ---- /* Performs the RSA authentication dialog with the client. This returns 0 if the client could not be authenticated, and 1 if authentication was successful. This may exit if there is a serious protocol violation. */ ! int auth_rsa(struct passwd *pw, MP_INT *client_n, RandomState *state, ! int strict_modes); /* Parses an RSA key (number of bits, e, n) from a string. Moves the pointer over the key. Skips any whitespace at the beginning and at end. */ *** ../ssh-1.2.14.orig/sshd.c Thu Jun 6 16:47:07 1996 - --- sshd.c Mon Aug 12 18:51:28 1996 *************** *** 1423,1429 **** MP_INT n; mpz_init(&n); packet_get_mp_int(&n); ! if (auth_rsa(pw, &n, &sensitive_data.random_state)) { /* Successful authentication. */ mpz_clear(&n); - --- 1440,1447 ---- MP_INT n; mpz_init(&n); packet_get_mp_int(&n); ! if (auth_rsa(pw, &n, &sensitive_data.random_state, ! options.strict_modes)) { /* Successful authentication. */ mpz_clear(&n); *** ../ssh-1.2.14.orig/userfile.c Thu Jun 6 16:47:06 1996 - --- userfile.c Mon Aug 12 18:23:56 1996 *************** *** 1019,1021 **** - --- 1019,1034 ---- return -1; } } + + int userfile_check_owner_permissions(struct passwd *pw, const char *path) + { + struct stat st; + if (userfile_stat(pw->pw_uid, path, &st) < 0) + return 0; + + if ((st.st_uid != 0 && st.st_uid != pw->pw_uid) || + (st.st_mode & 022) != 0) + return 0; + else + return 1; + } *** ../ssh-1.2.14.orig/userfile.h Thu Jun 6 16:47:01 1996 - --- userfile.h Mon Aug 12 18:24:02 1996 *************** *** 102,105 **** - --- 102,110 ---- /* Performs pclose() on the given uid. Returns <0 if an error occurs. */ int userfile_pclose(UserFile uf); + /* Check owner and permissions of a given file/directory. + Permissions ----w--w- must not exist and owner must be either + pw->pw_uid or root. Return value: 0 = not ok, 1 = ok */ + int userfile_check_owner_permissions(struct passwd *pw, const char *path); + #endif /* USERFILE_H */ *** ../ssh-1.2.14.orig/auth-rsa.c Thu Jun 6 16:47:06 1996 - --- auth-rsa.c Mon Aug 12 18:52:29 1996 *************** *** 143,149 **** 0 if the client could not be authenticated, and 1 if authentication was successful. This may exit if there is a serious protocol violation. */ ! int auth_rsa(struct passwd *pw, MP_INT *client_n, RandomState *state) { char line[8192]; int authenticated; - --- 143,150 ---- 0 if the client could not be authenticated, and 1 if authentication was successful. This may exit if there is a serious protocol violation. */ ! int auth_rsa(struct passwd *pw, MP_INT *client_n, RandomState *state, ! int strict_modes) { char line[8192]; int authenticated; *************** *** 153,161 **** unsigned long linenum = 0; struct stat st; ! /* Open the file containing the authorized keys. */ ! sprintf(line, "%.500s/%.100s", pw->pw_dir, SSH_USER_PERMITTED_KEYS); if (userfile_stat(pw->pw_uid, line, &st) < 0) return 0; - --- 154,191 ---- unsigned long linenum = 0; struct stat st; ! /* Check permissions & owner of user's home directory */ ! if (strict_modes && !userfile_check_owner_permissions(pw, pw->pw_dir)) ! { ! log("Rsa authentication refused for %.100s: bad modes for %.200s", ! pw->pw_name, pw->pw_dir); ! packet_send_debug("Bad file modes for %.200s", pw->pw_dir); ! return 0; ! } ! ! /* Check permissions & owner of user's .ssh directory */ ! sprintf(line, "%.500s/%.100s", pw->pw_dir, SSH_USER_DIR); ! ! if (strict_modes && !userfile_check_owner_permissions(pw, line)) ! { ! log("Rsa authentication refused for %.100s: bad modes for %.200s", ! pw->pw_name, line); ! packet_send_debug("Bad file modes for %.200s", line); ! return 0; ! } + /* Check permissions & owner of user's authorized keys file */ + sprintf(line, "%.500s/%.100s", pw->pw_dir, SSH_USER_PERMITTED_KEYS); + + if (strict_modes && !userfile_check_owner_permissions(pw, line)) + { + log("Rsa authentication refused for %.100s: bad modes for %.200s", + pw->pw_name, line); + packet_send_debug("Bad file modes for %.200s", line); + return 0; + } + + /* Open the file containing the authorized keys. */ if (userfile_stat(pw->pw_uid, line, &st) < 0) return 0; -----BEGIN PGP SIGNATURE----- Version: 2.6.3i Charset: latin1 Comment: Processed by Mailcrypt 3.4, an Emacs/PGP interface iQCVAwUBMg+6zakZxfGWH0o1AQHiMwP/TF+6dlo3pGWHBb8ogeKQEchjPZed6c1t c5/ME035p9VTJbScaiIc83ibUT86keoKrsJ/mdrFb7VM9d+iyFL7EK/bgf2FpkwH BagXolGhDNk77InIkPOuKL/C5Lo/Yi3l1KUnE58eoFrZJeK6X+7Y1rVCoZUFeJEA ZQ3KGz8goSM= =zfbk -----END PGP SIGNATURE-----