Unpatched Apache reverse proxy flaw allows access to internal network

Security researcher reveals how to bypass older patch for an Apache reverse proxy vulnerability

A yet-to-be-patched flaw discovered in the Apache HTTP server allows attackers to access protected resources on the internal network if some rewrite rules are not defined properly.

The vulnerability affects Apache installations that operate in reverse proxy mode, a type of configuration used for load balancing, caching and other operations that involve the distribution of resources over multiple servers.

In order to set up Apache HTTPD to run as a reverse proxy, server administrators use specialized modules like mod_proxy and mod_rewrite.

Security researchers from Qualys warn that if certain rules are not configured correctly, attackers can trick servers into performing unauthorized requests to access internal resources.

The problem isn't new and a vulnerability that allowed similar attacks was addressed back in October. However, while reviewing the patch for it, Qualys researcher Prutha Parikh realized that it can be bypassed due to a bug in the procedure for URI (Uniform Resource Identifier) scheme stripping. The scheme is the URI part that comes before the colon ":" character, such as http, ftp or file.

One relatively common rewrite and proxying rule is "^(.*) http://internal_host$1", which redirects the request to the machine internal_host. However, if this is used and the server receives, for example, a request for "host::port" (with two colons), the "host:" part is stripped and the rest is appended to http://internal_host in order to forward it internally.

The problem is that in this case, the remaining part is ":port", therefore transforming the forwarded request into http://internal_host:port, an unintended behavior that can result in the exposure of a protected resource.

In order to mitigate the problem server administrators should add a forward slash before $1 in the rewrite rule, the correct form being "^(.*) http://internal_host/$1", Parikh said.

The Apache developers are aware of the problem and are currently discussing the best method of fixing it. One possibility would be to strengthen the previous patch in the server code in order to reject such requests, however, there's no certainty that other bypass methods won't be discovered.

"We could try improve that fix, but I think it would be simpler to change the translate_name hooks in mod_proxy and mod_rewrite to enforce the requirement in the 'right' place," said Red Hat senior software engineer Joe Orton on the Apache dev mailing list. Orton proposed a patch that is currently being reviewed by the other developers.

Join the newsletter!

Or

Sign up to gain exclusive access to email subscriptions, event invitations, competitions, giveaways, and much more.

Membership is free, and your security and privacy remain protected. View our privacy policy before signing up.

Error: Please check your email address.

Tags internetservicessoftwareWeb serverspatchesHostedExploits / vulnerabilitiesApache Foundation

Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.
Lucian Constantin

Lucian Constantin

IDG News Service
Show Comments

Brand Post

Most Popular Reviews

Latest Articles

Resources

PCW Evaluation Team

Tom Pope

Dynabook Portégé X30L-G

Ultimately this laptop has achieved everything I would hope for in a laptop for work, while fitting that into a form factor and weight that is remarkable.

Tom Sellers

MSI P65

This smart laptop was enjoyable to use and great to work on – creating content was super simple.

Lolita Wang

MSI GT76

It really doesn’t get more “gaming laptop” than this.

Jack Jeffries

MSI GS75

As the Maserati or BMW of laptops, it would fit perfectly in the hands of a professional needing firepower under the hood, sophistication and class on the surface, and gaming prowess (sports mode if you will) in between.

Taylor Carr

MSI PS63

The MSI PS63 is an amazing laptop and I would definitely consider buying one in the future.

Christopher Low

Brother RJ-4230B

This small mobile printer is exactly what I need for invoicing and other jobs such as sending fellow tradesman details or step-by-step instructions that I can easily print off from my phone or the Web.

Featured Content

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?