In plain words
Think of a file cabinet where you keep private papers. If someone can trick the file clerk into showing them a paper from a different drawer than they are allowed to see, that is kind of like what this bug does. It is a problem with the Apache HTTP Server, which is a type of web server¹ that helps websites work.
What's broken
The bug is in a part of the server called mod_userdir², which helps the server find and show the right user's files. The problem happens when the server is set up in a certain way, using something called an absolute non-wildcard UserDir directive³. This makes the server get confused about which files it should show, and it might show files it should not, like private papers in the wrong drawer.
How it actually works
Here is how it actually works: when the server gets a request for a file, it uses the mod_userdir module to figure out where that file is. But because of the bug, if the request is written in a special way, using something like '/./' (a single dot directory)⁴, the server gets tricked into looking in the wrong place. This is because the server does not correctly understand the '/./' part, which is like a shortcut to the current directory⁵. So, it might show a file from a different user's directory, which is private and should not be shown.
Proof of concept
There is a public description of this bug, but it is not clear from the source if there is a proof-of-concept (PoC)⁶ that shows exactly how it works. A PoC would be like a demonstration that proves the bug is real and can be used to get to private files. If there was a public PoC, it would mean that the bug is not just theoretical, but something that can actually be used by attackers.
Is it real
It is not clear from the source if this bug is being actively used by attackers. The bug is considered low severity, and it only affects servers that are set up in a certain way. This means that not all Apache HTTP Servers are vulnerable, only the ones that use the mod_userdir module with an absolute non-wildcard UserDir directive.
What to do
To fix this bug, the Apache HTTP Server needs to be updated to a version that does not have this problem. The exact fix will depend on the specific version of the server and how it is set up. Users should check the official Apache documentation for instructions on how to update their server.
Terms
- ¹ web server
- , a program that helps websites work by handling requests from users
- ² mod_userdir
- , a part of the Apache HTTP Server that helps it find and show the right user's files
- ³ UserDir directive
- , a way of telling the server which files to show, using a specific format
- ⁴ /./
- , a special way of writing a file path that can trick the server into looking in the wrong place
- ⁵ current directory
- , a shortcut to the current directory, like a symbol that means 'start looking here'
- ⁶ proof-of-concept
- , a demonstration that proves a bug is real and can be used to attack a system