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PHP SSRF

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SSRF PHP functions

Some function such as file_get_contents(), fopen(), file(), md5_file() accept URLs as input that they will follow making possible SSRF vulnerabilities if the use can control the data:

file_get_contents("http://127.0.0.1:8081");
fopen("http://127.0.0.1:8081", "r");
file("http://127.0.0.1:8081");
md5_file("http://127.0.0.1:8081");

Wordpress SSRF via DNS Rebinding

As explained in this blog post, even the Wordpress function wp_safe_remote_get is vulnerable to DNS rebinding, making it potentially vulnerable to SSRF attacks. The main validation it calls is wp_http_validate_url, which checks that the protocol is http:// or https:// and that the port is one of 80, 443, and 8080, but it's vulnerable to DNS rebinding.

Other vulnerable functions according to the post are:

  • wp_safe_remote_request()

  • wp_safe_remote_post()

  • wp_safe_remote_head()

  • WP_REST_URL_Details_Controller::get_remote_url()

  • download_url()

  • wp_remote_fopen()

  • WP_oEmbed::discover()

CRLF

Moreover, in some cases it might be even possible to send arbitrary headers via CRLF "vulnerabilities" in the previous functions:

# The following will create a header called from with value Hi and 
# an extra header "Injected: I HAVE IT"
ini_set("from", "Hi\r\nInjected: I HAVE IT");
file_get_contents("http://127.0.0.1:8081");

GET / HTTP/1.1
From: Hi
Injected: I HAVE IT
Host: 127.0.0.1:8081
Connection: close

# Any of the previously mentioned functions will send those headers

For more info about that CRLF vuln, check this bug https://bugs.php.net/bug.php?id=81680&edit=1

Note that these function might have other methods to set arbitrary headers in requests, like:

$url = "";

$options = array(
  'http'=>array(
    'method'=>"GET",
    'header'=>"Accept-language: en\r\n" .
              "Cookie: foo=bar\r\n" .  // check function.stream-context-create on php.net
              "User-Agent: Mozilla/5.0 (iPad; U; CPU OS 3_2 like Mac OS X; en-us) AppleWebKit/531.21.10 (KHTML, like Gecko) Version/4.0.4 Mobile/7B334b Safari/531.21.102011-10-16 20:23:10\r\n" // i.e. An iPad 
  )
);

$context = stream_context_create($options);
$file = file_get_contents($url, false, $context);

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