PHP 5.2.0 session.save_path safe_mode and open_basedir bypass

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[PHP 5.2.0 session.save_path safe_mode and open_basedir bypass]

Author: Maksymilian Arciemowicz (SecurityReason)
Date:

• • Written: 02.10.2006
• • Public: 08.12.2006
    SecurityAlert Id: 43
    CVE: CVE-2006-6383
    SecurityRisk: High
    Affected Software: PHP 5.2.0
    Advisory URL: http://securityreason.com/achievement_securityalert/43
    Vendor: http://www.php.net

• — 0.Description —
  PHP is an HTML-embedded scripting language. Much of its syntax is borrowed from C, Java and Perl with a couple of
  unique PHP-specific features thrown in. The goal of the language is to allow web developers to write dynamically
  generated pages quickly.

A nice introduction to PHP by Stig Sather Bakken can be found at http://www.zend.com/zend/art/intro.php on the Zend
website. Also, much of the PHP Conference Material is freely available.

Session support in PHP consists of a way to preserve certain data across subsequent accesses. This enables you to
build more customized applications and increase the appeal of your web site.

A visitor accessing your web site is assigned a unique id, the so-called session id. This is either stored in a cookie
on the user side or is propagated in the URL.

session.save_path defines the argument which is passed to the save handler. If you choose the default files handler,
this is the path where the files are created. Defaults to /tmp. See also session_save_path().

There is an optional N argument to this directive that determines the number of directory levels your session files
will be spread around in. For example, setting to '5;/tmp' may end up creating a session file and location like
/tmp/4/b/1/e/3/sess_4b1e384ad74619bd212e236e52a5a174If . In order to use N you must create all of these directories
before use. A small shell script exists in ext/session to do this, it's called mod_files.sh. Also note that if N is
used and greater than 0 then automatic garbage collection will not be performed, see a copy of php.ini for further
information. Also, if you use N, be sure to surround session.save_path in "quotes" because the separator (;) is also
used for comments in php.ini.

• — 1. session.save_path safe mode and open basedir bypass —
  session.save_path can be set in ini_set(), session_save_path() function. In session.save_path there must be path where
  you will save yours tmp file. But syntax for session.save_path can be:

[/PATH]

OR

[N;/PATH]

N - can be a string.

EXAMPLES:

1. session_save_path("/DIR/WHERE/YOU/HAVE/ACCESS")
2. session_save_path("5;/DIR/WHERE/YOU/HAVE/ACCESS")

and

3. session_save_path("/DIR/WHERE/YOU/DONT/HAVE/ACCESS\0;/DIR/WHERE/YOU/HAVE/ACCESS")

• -1477-1493— Code from PHP520 ext/session/session.c [START]
  PHP_FUNCTION(session_save_path)
  {
  zval **p_name;
  int ac = ZEND_NUM_ARGS();
  char *old;

    if (ac < 0 || ac > 1 || zend_get_parameters_ex(ac, &p_name) == FAILURE)
            WRONG_PARAM_COUNT;
    
    old = estrdup(PS(save_path));
  
    if (ac == 1) {
            convert_to_string_ex(p_name);
            zend_alter_ini_entry("session.save_path", sizeof("session.save_path"), Z_STRVAL_PP(p_name),
  

Z_STRLEN_PP(p_name), PHP_INI_USER, PHP_INI_STAGE_RUNTIME);
}

    RETVAL_STRING(old, 0);

}

• -1477-1493— Code from PHP520 ext/session/session.c [END]

Values are set to hash_memory (but before that, safe_mode and open_basedir check this value).
And if you are starting session (for example session_start()), that value from session.save_path is checked by
function PS_OPEN_FUNC(files).

• -242-300— Code from PHP520 ext/session/mod_files.c [START]
  PS_OPEN_FUNC(files)
  {
  ps_files *data;
  const char *p, *last;
  const char *argv[3];
  int argc = 0;
  size_t dirdepth = 0;
  int filemode = 0600;

    if (*save_path == '\0') {
            /* if save path is an empty string, determine the temporary dir */
            save_path = php_get_temporary_directory();
    }
    
    /* split up input parameter */
    last = save_path;
    p = strchr(save_path, ';');
    while (p) {
            argv[argc++] = last;
            last = ++p;
            p = strchr(p, ';');
            if (argc > 1) break;
    }
    argv[argc++] = last;
  
    if (argc > 1) {
            errno = 0;
            dirdepth = (size_t) strtol(argv[0], NULL, 10);
            if (errno == ERANGE) {
                    php_error(E_WARNING, 
                                    "The first parameter in session.save_path is invalid");
                    return FAILURE;
            }
    }
    
    if (argc > 2) {
            errno = 0;
            filemode = strtol(argv[1], NULL, 8);
            if (errno == ERANGE || filemode < 0 || filemode > 07777) {
                    php_error(E_WARNING, 
                                    "The second parameter in session.save_path is invalid");
                    return FAILURE;
            }
    }
    save_path = argv[argc - 1];
  
    data = emalloc(sizeof(*data));
    memset(data, 0, sizeof(*data));
    
    data->fd = -1;
    data->dirdepth = dirdepth;
    data->filemode = filemode;
    data->basedir_len = strlen(save_path);
    data->basedir = estrndup(save_path, data->basedir_len);
    
    PS_SET_MOD_DATA(data);
    
    return SUCCESS;
  

}

• -242-300— Code from PHP520 ext/session/mod_files.c [END]

Because in session.save_path there is a NULL byte before ";", strchr() doesn't see ";" and path is
/DIR/WHERE/YOU/DONT/HAVE/ACCESS.

Problem exists because safe_mode and open_basedir check what is after ;. And it is needed to set correct path after
";".

• — 2. How to fix —
  http://cvs.php.net/viewcvs.cgi/php-src/NEWS

• — 3. Greets —

For: sp3x
and
l5x, p_e_a, lorddav, pi3

• — 4. Contact —
  Author: SecurityReason.Com [ Maksymilian Arciemowicz ( cXIb8O3 ) ]
  Email: cxib [at] securityreason [dot] com
  GPG: http://securityreason.com/key/Arciemowicz.Maksymilian.gpg

Regards
SecurityReason

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