Directory Traversal Affecting @fastify/http-proxy package, versions >=11.6.0 <11.6.3

@fastify/http-proxy is a proxy http requests, for Fastify

Affected versions of this package are vulnerable to Directory Traversal via validateDestination() in the proxy path normalization logic. The function normalizes literal backslashes before percent-decoding the request path, so a path containing percent-encoded backslash dot-segments (%5C..%5C) passes the dot-segment check intact and is only decoded into backslash and double-dot sequences after validation has completed. An unauthenticated remote attacker can send a request such as GET /pub/%5C..%5Csecret to escape the configured rewritePrefix boundary, causing the forwarded request to reach sibling resources on the upstream that the prefix was intended to restrict. The impact is limited to confidentiality, as the attacker can retrieve upstream resources outside the intended prefix.

Note: This is only exploitable when the upstream server decodes %5C and treats the resulting backslash as a path separator (for example, a Windows host serving static files).

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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