CVE-2021-32061: Path Traversal via dump of malicious bucket

[sa7mon]

tl;dr

In version 2.0.1 and older of S3Scanner, if a user attempts to dump the contents of a bucket which contains objects with special characters in their keys, those characters can be used to save the files outside of the folder specified with --dump-dir.

Thanks

I'd like to give a huge thanks to a security researcher named RyotaK for reporting this issue to me. They provided a detailed explanation and helped walk me through the steps to reproduce. Very excellent experience.

The Bug

The issue is what's commonly known as a "path traversal" vulnerability. In this case, though, it's the ability to save files outside the intended area as opposed to reading files.

For example: this bucket was created by RyotaK for demonstration purposes.

See how the object keys contain ../ ? When S3Scanner goes to download that file, it concatenates the dump directory and this key to form the file path the file should get downloaded to. So if a user ran the following command:

s3scanner dump --dump-dir /projects/s3scanner/buckets/ --bucket s3scanner-test-bucket 

they would end up with a file called test.txt in /projects/s3scanner/ which is one level up from where they wanted it. By adding a bunch of these characters together (../../../../../) an attacker could craft a malicious object key which would place their file anywhere on the S3Scanner user's system.

You may be surprised (like I was) to learn that AWS allows such characters in object keys. Their documentation explains that while you can do this, there are limitations:

...

In addition, be aware of the following prefix limitations:

• Objects with a prefix of "./" must uploaded or downloaded with the AWS Command Line Interface (AWS CLI), AWS SDKs, or REST API. You cannot use the Amazon S3 console.

• Objects with a prefix of "../" cannot be uploaded using the AWS Command Line Interface (AWS CLI) or Amazon S3 console.

In the "real world" the chances of an S3Scanner user encountering a bucket with these "malicious" keys is very low - especially considering I was not able to create such a bucket of my own. This threat poses a fairly low risk since the difficulty of pulling off the attack is a bit high and would only happen if a bad actor was purposely targeting users of this tool.

RyotaK has engaged MITRE who has reserved CVE-2021-32061 for this vulnerability. The CVE will be updated after this advisory has been posted.

Remediation

The good news is that I have already pushed the fix for this issue (#121). If an object to be downloaded has a key that would land outside of the --dump-dir, the file won't be downloaded. The user will get a message that looks like this:

I'll be drafting the v2.0.2 release to push out this update across the git repo, PyPi package, and Docker image.