<!DOCTYPE qhelp PUBLIC

"-//Semmle//qhelp//EN"

"qhelp.dtd">

<qhelp>

<overview>

<p>

Accessing files using paths constructed from user-controlled data can allow an attacker to access

unexpected resources. This can result in sensitive information being revealed or deleted, or an

attacker being able to influence behavior by modifying unexpected files.

</p>

<p>

Paths that are naively constructed from data controlled by a user may be absolute paths,

or may contain unexpected special characters such as "..". Such a path could point anywhere

on the file system.

</p>

</overview>

<recommendation>

<p>

Validate user input before using it to construct a file path.

</p>

<p>

Common validation methods include checking that the normalized path is relative and

does not contain any ".." components, or checking that the path is contained within a safe folder. The method you should use depends on how the path is used in the application, and whether the path should be a single path component.

</p>

<p>

If the path should be a single path component (such as a file name), you can check for the

existence of any path separators ("/" or "\"), or ".." sequences in the input, and reject

the input if any are found.

</p>

<p>

Note that removing "../" sequences is <i>not</i> sufficient, since the input could still

contain a path separator followed by "..". For example, the input ".../...//" would still

result in the string "../" if only "../" sequences are removed.

</p>

<p>

Finally, the simplest (but most restrictive) option is to use an allow list of safe patterns

and make sure that the user input matches one of these patterns.

</p>

</recommendation>

<example>

<p>

In the first example, a file name is read from an HTTP request and then used to access a file.

However, a malicious user could enter a file name which is an absolute path, such as

"/etc/passwd".

</p>

<p>

In the second example, it appears that the user is restricted to opening a file within the

<code>"user"</code> home directory. However, a malicious user could enter a file name containing

special characters. For example, the string <code>"../../etc/passwd"</code> will result in the code

reading the file located at "/home/user/../../etc/passwd", which is the system's

password file. This file would then be sent back to the user, giving them access to password

information.

</p>

<sample src="TaintedPath.go" />

<p>

If the input should only be a file name, you can check that it doesn't contain any

path separators or ".." sequences.

</p>

<sample src="TaintedPathGood.go" />

<p>

Note that this approach is only suitable if the input is expected to be a single file name.

</p>

<p>

If the input can be a path with multiple components, you can make it safe by verifying

that the path is within a specific directory that is considered safe.

You can do this by resolving the input with respect to that directory, and then checking

that the resulting path is still within it.

</p>

<sample src="TaintedPathGood2.go" />

<p>

Note that <code>/home/user</code> is just an example, you should replace it with the actual

safe directory in your application. Also, while in this example the path of the safe

directory is absolute, this may not always be the case, and you may need to resolve it

first before checking the input.

</p>

</example>

<references>

<li>OWASP: <a href="https://owasp.org/www-community/attacks/Path_Traversal">Path Traversal</a>.</li>

</references>

</qhelp>