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[PHP 5.2.6 posix_access() (posix ext) safe_mode bypass ]

Author: Maksymilian Arciemowicz (cXIb8O3)
SecurityReason.com
Date:
- - Written: 10.05.2008
- - Public: 17.06.2008

SecurityReason Research
SecurityAlert Id: 54

CVE: CVE-2008-2665
CWE: CWE-264
SecurityRisk: Low

Affected Software: PHP 5.2.6
Advisory URL: http://securityreason.com/achievement_securityalert/54
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.

posix_access ? Determine accessibility of a file

SYNOPSIS:

bool posix_access  ( string $file  [, int $mode  ] )

http://pl2.php.net/manual/pl/function.posix-access.php

!!! WARNING !!!
IT IS POSSIBLE TO EXPLOIT MORE FUNCTIONS WITH http: PREFIX. SECURITYREASON WILL NOT LIST ALL VULNERABLE FUNCTIONS

- --- 1. PHP 5.2.6 posix_access() safe_mode bypass ---
Let's see to posix_access() function

- ---
PHP_FUNCTION(posix_access)
{
  long mode = 0;
  int filename_len, ret;
  char *filename, *path;

  if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s|l", &filename, &filename_len, &mode) == FAILURE) {
    RETURN_FALSE;
  }

  path = expand_filepath(filename, NULL TSRMLS_CC);
  
  if (!path) {
    POSIX_G(last_error) = EIO;
    RETURN_FALSE;
  }

  if (php_check_open_basedir_ex(path, 0 TSRMLS_CC) ||
      (PG(safe_mode) && (!php_checkuid_ex(filename, NULL, CHECKUID_CHECK_FILE_AND_DIR, CHECKUID_NO_ERRORS)))) {
    efree(path);
    POSIX_G(last_error) = EPERM;
    RETURN_FALSE;
  }
  
  ret = access(path, mode);
  efree(path);

  if (ret) {
    POSIX_G(last_error) = errno;
    RETURN_FALSE;
  }

  RETURN_TRUE;
}
- ---

var_dump(posix_access("http://../../../etc/passwd"))==True
var_dump(posix_access("/etc/passwd"))==False

Why?

Because path = expand_filepath(filename, NULL TSRMLS_CC); will change "http://../../../etc/passwd" to path=/etc/passwd

(PG(safe_mode) && (!php_checkuid_ex(filename, NULL, CHECKUID_CHECK_FILE_AND_DIR, CHECKUID_NO_ERRORS))) will check realy path "http://../../../etc/passwd". http:// is using in php_checkuid_ex(), so safe_mode is bypassed.

!!! WARNING !!!
IT IS POSSIBLE TO EXPLOIT MORE FUNCTIONS WITH http: PREFIX. SECURITYREASON WILL NOT LIST ALL VULNERABLE FUNCTIONS

- --- 2. How to Fix ---
Do not use safe_mode as a main safety

- --- 3. Greets ---
sp3x Infospec schain p_e_a Chujwamwdupe

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