［漏洞预警］CGI接口漏洞PHP,Go,Python等均受影响

httpoxy is a set of vulnerabilities that affect application code running in CGI, or CGI-like environments. It comes down to a simple namespace conflict:

• RFC 3875 (CGI) puts the HTTP Proxy header from a request into the environment variables as HTTP_PROXY

• HTTP_PROXY is a popular environment variable used to configure an outgoing proxy

This leads to a remotely exploitable vulnerability. If you’re running PHP or CGI, you should block the Proxy header now. Here’s how.

httpoxy is a vulnerability for server-side web applications. If you’re not deploying code, you don’t need to worry.

If a vulnerable HTTP client makes an outgoing HTTP connection, while running in a server-side CGI application, an attacker may be able to:

• Proxy the outgoing HTTP requests made by the web application

• Direct the server to open outgoing connections to an address and port of their choosing

• Tie up server resources by forcing the vulnerable software to use a malicious proxy

httpoxy is extremely easy to exploit in basic form. And we expect security researchers to be able to scan for it quickly. Luckily, if you read on and find you are affected, easy mitigations are available.

What Is Affected

A few things are necessary to be vulnerable:

• Code running under a CGI-like context, where HTTP_PROXY becomes a real or emulated environment variable

• An HTTP client that trusts HTTP_PROXY, and configures it as the proxy

• That client, used within a request handler, making an HTTP (as opposed to HTTPS) request

For example, the confirmed cases we’ve found so far:

But obviously there may be languages we haven’t considered yet. CGI is a common standard, andHTTP_PROXY seems to be becoming more popular over time. Take the below as a sample of the most commonly affected scenarios:

PHP

• Whether you are vulnerable depends on your specific application code and PHP libraries, but the problem seems fairly widespread

• Just using one of the vulnerable libraries, while processing a user’s request, is exploitable.

• If you’re using a vulnerable library, this vulnerability will affect any version of PHP

• It even affects alternative PHP runtimes such as HHVM deployed under FastCGI

• It is present in Guzzle, Artax, and probably many, many libraries

• Guzzle versions after 4.0.0rc2 are vulnerable, Guzzle 3 and below is not.

• Another example is in Composer’s StreamContextBuilder utility class

So, for example, if you are using a Drupal module that uses Guzzle 6 and makes an outgoing HTTP request (for example, to check a weather API), you are vulnerable to the request that plugin makes being “httpoxied”.

Python

• Python code must be deployed under CGI to be vulnerable. Usually, that’ll mean the vulnerable code will use a CGI handler like wsgiref.handlers.CGIHandler

• This is not considered a normal way of deploying Python webapps (most people are using WSGI or FastCGI, both of which are not affected), so vulnerable Python applications will probably be much rarer than vulnerable PHP applications.

• wsgi, for example, is not vulnerable, because os.environ is not polluted by CGI data

• Vulnerable versions of the requests library will trust and use os.environ['HTTP_PROXY'], without checking if CGI is in use

Go

• Go code must be deployed under CGI to be vulnerable. Usually, that’ll mean the vulnerable code uses the net/http/cgi package.

• As with Python, this is not considered a usual way of deploying Go as a web application, so this vulnerability should be relatively rare

• Go’s net/http/fcgi package, by comparison, does not set actual environment variables, so it is not vulnerable

• Vulnerable versions of net/http will trust and use HTTP_PROXY for outgoing requests, without checking if CGI is in use

Immediate Mitigation

The best immediate mitigation is to block Proxy request headers as early as possible, and before they hit your application. This is easy and safe.

• It’s safe because the Proxy header is undefined by the IETF, and isn’t listed on the IANA’s registry of message headers. This means there is no standard use for the header at all; not even a provisional use-case.

• Standards-compliant HTTP clients and servers will never read or send this header.

• You can either strip the header or completely block requests attempting to use it.

• You should try to do your mitigation as early and as far upstream as you can.

• Do it “at the edge”, where HTTP requests first enter your system.

• That way, you can fix lots of vulnerable software at once.

• Everything behind a reverse proxy or application firewall that strips the Proxy header is safe!

How you block a Proxy header depends on the specifics of your setup. The earliest convenient place to block the header might be at a web application firewall device, or directly on the webserver running Apache or Nginx. Here are a few of the more common mitigations:

Nginx/FastCGI

Use this to block the header from being passed on to PHP-FPM, PHP-PM etc.

fastcgi_param HTTP_PROXY "";

In FastCGI configurations, PHP is vulnerable (but many other languages that use Nginx FastCGI are not).

Apache

For specific Apache coverage (and details for other Apache software projects like Tomcat), we strongly recommend you read the Apache Software Foundation’s official advisory on the matter. The very basic mitigation information you’ll find below is covered in much greater depth there.

If you’re using Apache HTTP Server with mod_cgi, languages like Go and Python may be vulnerable (the HTTP_PROXY env var is “real”). And mod_php is affected due to the nature of PHP. If you are using mod_headers, you can unset the Proxy header before further processing with this directive:

RequestHeader unset Proxy early

If you are using mod_security, you can use a SecRule to deny traffic with a Proxy header. Here’s an example, vary the action to taste, and make sure SecRuleEngine is on. The 1000005 ID has been assigned to this issue.

SecRule &REQUEST_HEADERS:Proxy "@gt 0" "id:1000005,log,deny,msg:'httpoxy denied'"

Finally, if you’re using Apache Traffic Server, it’s not affected, but you can use it to strip the Proxy header; very helpful for any services sitting behind it. Again, see the ASF’s guidance.

HAProxy

This will strip the header off requests:

http-request del-header Proxy

Varnish

For Varnish, the following should unset the header. Add it to the pre-existing vcl_recv section:

sub vcl_recv {     [...]     unset req.http.proxy;     [...] }

Microsoft IIS with PHP or a CGI framework

httpoxy does not affect any Microsoft Web Frameworks, e.g. not ASP.NET nor Active Server Pages. But if you have installed PHP or any other third party framework on top of IIS, we recommend applying mitigation steps to protect from httpoxy attacks.

Update your apphost.config with the following rule:

<system.webServer>     <rewrite>         <rules>             <rule name="Erase HTTP_PROXY" patternSyntax="Wildcard">                 <match url="*.*" />                 <serverVariables>                     <set name="HTTP_PROXY" value="" />                 </serverVariables>                 <action type="None" />             </rule>         </rules>     </rewrite></system.webServer>

Other CGI software and applications

Please let us know of other places where httpoxy is found. We’d be happy to help you communicate fixes for your platform, server or library if you are affected. Contact [email protected] or @httpoxyto let us know. Or make a PR against the httpoxy-org repo.

Ineffective fixes in PHP

Userland PHP fixes don’t work. Don’t bother:

• Using unset($_SERVER['HTTP_PROXY']) does not affect the value returned from getenv(), so is not an effective mitigation

• Using putenv('HTTP_PROXY=') does not work either (to be precise: it only works if that value is coming from an actual environment variable rather than a header – so, it cannot be used for mitigation)

Prevention

Summary

• If you can avoid it, do not deploy into environments where the CGI data is merged into the actual environment variables

• Use and expect CGI_HTTP_PROXY to set the proxy for a CGI application’s internal requests, if necessary

• You can still support HTTP_PROXY, but you must assert that CGI is not in use

• In PHP, check PHP_SAPI == 'cli'

• Otherwise, a simple check is to not trust HTTP_PROXY if REQUEST_METHOD is also set. RFC 3875 seems to require this meta-variable:

  The REQUEST_METHOD meta-variable MUST be set to the method which should be used by the script to process the request

Don’t Trust HTTP_PROXY Under CGI

To put it plainly: there is no way to trust the value of an HTTP_ env var in a CGI environment. They cannot be distinguished from request headers according to the specification. So, any usage ofHTTP_PROXY in a CGI context is suspicious.

If you need to configure the proxy of a CGI application via an environment variable, use a variable name that will never conflict with request headers. That is: one that does not begin with HTTP_. We strongly recommend you go for CGI_HTTP_PROXY. (As seen in Ruby and libwww-perl’s mitigations for this issue.)

PHP

CLI-only code may safely trust $_SERVER['HTTP_PROXY'] or getenv('HTTP_PROXY'). But bear in mind that code written for the CLI context often ends up running in a SAPI eventually, particularly utility or library code. And, with open source code, that might not even be your doing. So, if you are going to rely on HTTP_PROXY at all, you should guard that code with a check of the PHP_SAPIconstant.

Network Configuration as Prevention

A defense-in-depth strategy that can combat httpoxy (and entire classes of other security problems) is to severely restrict the outgoing requests your web application can make to an absolute minimum. For example, if a web application is firewalled in such a way that it cannot make outgoing HTTP requests, an attacker will not be able to receive the “misproxied” requests (because the web application is prevented from connecting to the attacker).

HTTPS

And, of course, another defense-in-depth strategy that works is to use HTTPS for internal requests, not just for securing your site’s connections to the outside world. Those aren’t affected byHTTP_PROXY.

How It Works

Using PHP as an example, because it is illustrative. PHP has a method called getenv()1.

There is a common vulnerability in many PHP libraries and applications, introduced by confusinggetenv for a method that only returns environment variables. In fact, getenv() is closer to the$_SERVER superglobal: it contains both environment variables and user-controlled data.

Specifically, when PHP is running under a CGI-like server, the HTTP request headers (data supplied by the client) are merged into the $_SERVER superglobal under keys beginning with HTTP_. This is the same information that getenv reads from.

When a user sends a request with a Proxy header, the header appears to the PHP application asgetenv('HTTP_PROXY'). Some common PHP libraries have been trusting this value, even when run in a CGI/SAPI environment.

Reading and trusting $_SERVER['HTTP_PROXY'] is exactly the same vulnerability, but tends to happen much less often (perhaps because of getenv’s name, perhaps because the semantics of the$_SERVER superglobal are better understood among the community).

Minimal example code

Note that these examples require deployment into a vulnerable environment before there is actually a vulnerability (e.g. php-fpm, or Apache’s ScriptAlias)

$client = new GuzzleHttpClient();$client->get('http://api.internal/?secret=foo')

from wsgiref.handlers import CGIHandlerdef application(environ, start_response):     requests.get("http://api.internal/?secret=foo")CGIHandler().run(application)

cgi.Serve(     http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {         res, _ := http.Get("http://api.internal/?secret=foo")         // [...]

More complete PoC repos (using Docker, and testing with an actual listener for the proxied request) have been prepared under the httpoxy Github organization.

Why It Happened

Under the CGI spec, headers are provided mixed into the environment variables. (These are formally known as “Protocol-Specific Meta-Variables”2). That’s just the way the spec works, not a failure or bug.

The goal of the code, in most of the vulnerabilities, is to find the correct proxy to use, when auto-configuring a client for the internal HTTP request made shortly after. This task in Ruby could be completed by the find_proxy method of URI::Generic, which notes:

http_proxy and HTTP_PROXY are treated specially under the CGI environment, becauseHTTP_PROXY may be set by Proxy: header. So HTTP_PROXY is not used. http_proxy is not used too if the variable is case insensitive. CGI_HTTP_PROXY can be used instead.

Other instances of the same vulnerability are present in other languages. For example, when using Go’s net/http/cgi module, and deploying as a CGI application. This indicates the vulnerability is a standard danger in CGI environments.

History of httpoxy

This bug was first discovered over 15 years ago. The timeline goes something like:

• March 2001

• The issue is discovered in libwww-perl and fixed. Reported by Randal L. Schwartz. 3

• April 2001

• The issue is discovered in curl, and fixed there too (albeit probably not for Windows). Reported by Cris Bailiff. 4

• July 2012

• In implementing HTTP_PROXY for Net::HTTP, the Ruby team notice and avoid the potential issue. Nice work Akira Tanaka! 5

• November 2013

• The issue is mentioned on the nginx mailing list. The user humbly points out the issue: “unless I’m missing something, which is very possible”. No, Jonathan Matthews, you were exactly right! 6

• February 2015

• The issue is mentioned on the Apache httpd-dev mailing list. Spotted by Stefan Fritsch. 7

• July 2016

• Scott Geary, an engineer at Vend, found an instance of the bug in the wild. The Vend security team found the vulnerability was still exploitable in PHP, and present in many modern languages and libraries. We started to disclose to security response teams.

So, the bug was lying dormant for years, like a latent infection: pox. We imagine that many people may have found the issue over the years, but never investigated its scope in other languages and libraries. If you’ve found a historical discussion of interest that we’ve missed, let us know.

CVEs

httpoxy has a number of CVEs assigned. These cover the cases where

• a language or CGI implementation makes the Proxy header available in such a way that the application cannot tell whether it is a real environment variable, or

• an application trusts the value of the HTTP_PROXY environment variable by default in a CGI environment (but only where that application should have been able to tell it came from a request)

The assigned CVEs so far:

• CVE-2016-5385: PHP

• CVE-2016-5386: Go

• CVE-2016-5387: Apache HTTP Server

• CVE-2016-5388: Apache Tomcat

• CVE-2016-1000109: HHVM

• CVE-2016-1000110: Python

We suspect there may be more CVEs coming for httpoxy, as less common software is checked over. If you want to get a CVE assigned for an httpoxy issue, there are a couple of options:

• For open source code projects, you can use the Distributed Weakness Filing Project (DWF). They have a simple way to report (public) issues using the form at iwantacve.org

• For closed source code projects, you can talk to MITRE, or one of their participating CNAs/vendors/coordinators.

Thanks and Further Coverage

We’ll be linking to official announcements from affected teams here, as they become available.

• The CERT vulnerability note - VU#797896

• The Apache Software Foundation advisory

• Drupal advisory

• Cloudflare sites protected from httpoxy

Over the past two weeks, the Vend security team worked to disclose the issue responsibly to as many affected parties as we could. We’d like to thank the members of:

• The Red Hat Product Security team, who provided extremely useful advice and access to their experience disclosing widespread vulnerabilities - if you’re sitting on a big or complicated disclosure, they’re a great resource to reach out to for help.

• The language and implementation teams, who kept to disclosure norms and provided lively discussion.

There’s an extra page with some meta-discussion on the whole named disclosure thing and contact details. The content on this page is licensed as CC0 (TL;DR: use what you like, no permission/attribution necessary).

I’ve put together some more opinionated notes on httpoxy on my Medium account.

Regards,
Dominic Scheirlinck and the httpoxy disclosure team

Page updated at 2016-07-18 15:15 UTC

References

1. The PHP documentation manual page for getenv ↩

2. RFC 3875 4.1.18: Protocol-Specific Meta-Variables ↩

3. The fix applied correctly handles cases with case-insensitive environment variables. libwww-perl-5.51 announcement ↩

4. The fix applied to Curl does not correctly handle cases with case-insensitive environment variables - it specifically mentions the fix would not be enough for “NT” (Windows). The commit itself carries the prescient message “since it might become a security problem.” ↩

5. The mitigation in Ruby, like that for libwww-perl, correctly handles case-insensitive environment variables. ↩

6. The nginx mailing list even had a PHP-specific explanation. ↩

7. Discussed in reference to CGI specifically. ↩

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