Directory Traversal Affecting github.com/labstack/echo/v4/middleware package, versions <4.16.0

github.com/labstack/echo/v4/middleware is a middleware package for echo.

Affected versions of this package are vulnerable to Directory Traversal via the StaticDirectoryHandler and Static middleware in Echo’s static file serving. An unauthenticated attacker can read files protected by route-level middleware by requesting a public static path containing encoded .. segments that normalize to a protected file path. This occurs when the static filesystem includes those files, because path cleaning serves them through the public route without invoking the protected route’s middleware; with UseEscapedPathForMatching=true, encoded separators can also be normalized before the static handler checks the wildcard, allowing a similar bypass.

Notes

• The encoded-dot-dot bypass works with the default router configuration; UseEscapedPathForMatching=true is only needed for the separate encoded-separator variant.

Workarounds

• Put a reverse proxy in front of the application that normalizes request paths by removing dot segments and merging slashes, preventing traversal paths from reaching the static handler.
• Keep protected files outside the directory served by the static filesystem, preventing them from being exposed through the public static route.

A Directory Traversal attack (also known as path traversal) aims to access files and directories that are stored outside the intended folder. By manipulating files with "dot-dot-slash (../)" sequences and its variations, or by using absolute file paths, it may be possible to access arbitrary files and directories stored on file system, including application source code, configuration, and other critical system files.

Directory Traversal vulnerabilities can be generally divided into two types:

• Information Disclosure: Allows the attacker to gain information about the folder structure or read the contents of sensitive files on the system.

st is a module for serving static files on web pages, and contains a vulnerability of this type. In our example, we will serve files from the public route.

If an attacker requests the following URL from our server, it will in turn leak the sensitive private key of the root user.

curl http://localhost:8080/public/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/root/.ssh/id_rsa

Note %2e is the URL encoded version of . (dot).

• Writing arbitrary files: Allows the attacker to create or replace existing files. This type of vulnerability is also known as Zip-Slip.

One way to achieve this is by using a malicious zip archive that holds path traversal filenames. When each filename in the zip archive gets concatenated to the target extraction folder, without validation, the final path ends up outside of the target folder. If an executable or a configuration file is overwritten with a file containing malicious code, the problem can turn into an arbitrary code execution issue quite easily.

The following is an example of a zip archive with one benign file and one malicious file. Extracting the malicious file will result in traversing out of the target folder, ending up in /root/.ssh/ overwriting the authorized_keys file:

2018-04-15 22:04:29 .....           19           19  good.txt
2018-04-15 22:04:42 .....           20           20  ../../../../../../root/.ssh/authorized_keys

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