KitCTFCTF 2022 - ein-pfund-mails

Cameron published on October 26, 2022

6 min, 1178 words

Problem Description

The ein-pfund-mails challenge is a baby rated challenge in the Misc category for KitCTFCTF 2022. The title ein pfund mails is actually German for "A pound of mail". Which makes sense since we are given an archive (mails.tar.gz) containing 3993 .eml files. We're told one of these leaked email files contains our flag, but we're unable to determine which file contains the correct flag.

Analysis

First off, let's take a look at one of the email files.

MIME-Version: 1.0
From: Martin Wagner <mawalu98@gmail.com>
Date: Thu, 16 Jun 2022 17:05:13 +0200
Message-ID: <CAA2ev=GHMZn2+6vgHv9kEQV5h4ZWcOapubMOJtg2BTM7VsgOAw@mail.gmail.com>
Subject: Flag
To: martin@mawalabs.de
Content-Type: multipart/alternative; boundary="000000000000a433c905e191f775"

--000000000000a433c905e191f775
Content-Type: text/plain; charset="UTF-8"

Hi,

die Flag ist KCTF{1d2fa1ed91a0310dad83242abc3f8a92}

LG
Martin

--000000000000a433c905e191f775
Content-Type: text/html; charset="UTF-8"

<div dir="ltr">Hi,<div><br></div><div>die Flag ist KCTF{1d2fa1ed91a0310dad83242abc3f8a92}</div><div><br></div><div>LG</div><div>Martin</div></div>

--000000000000a433c905e191f775--

Of note, are several signatures and a possible flag (KCTF{1d2fa1ed91a0310dad83242abc3f8a92}).

If we take a diff of the two different emails (e.g. diff fffc5.eml ffdef.eml), only the flag is changed.

110c110
< die Flag ist KCTF{1d2fa1ed91a0310dad83242abc3f8a92}
---
> die Flag ist KCTF{a3afe0043bf7736216bc7ace6efac886}
118c118
< <div dir="ltr">Hi,<div><br></div><div>die Flag ist KCTF{1d2fa1ed91a0310dad83242abc3f8a92}</div><div><br></div><div>LG</div><div>Martin</div></div>
---
> <div dir="ltr">Hi,<div><br></div><div>die Flag ist KCTF{a3afe0043bf7736216bc7ace6efac886}</div><div><br></div><div>LG</div><div>Martin</div></div>

This confirms what the prompt told us, so we can't identify the correct flag by any differences between the files. Instead we'll have to utilize the signatures provided in the emails to verify a valid .eml.

Solution

The first signature we see in the file is a DKIM-Signature. There is a useful explainer over on mailtrap[.]com, if you want to get into the nitty gritty, but the short of it is that DKIM is a common signature type that's used to verify the sender and content of an email have not been altered. That should be perfect to find the original, unaltered email in our ein pfund mails.

Luckily, there are several DKIM verification libraries available on PyPI. I chose check-dkim since it was at the top of the list for my search. In the case of check-dkim, it's actually a CLI script. So, after installing (pip install check-dkim), we can verify that it works for one of our .eml files.

$ check-dkim mail/fffc5.eml 
Error verifying DKIM
body hash mismatch (got b'SHy1AAdR/+J5fTOT5HqeEr23p+JmXnlXWdr1QxcFqcU=', expected b'Hc1fzmKy9aocJCtYl88l4HEWgiYgp/nBHaexg4xOWtk=')

Now let's write a simple bash script to verify every email file.

#!/bin/bash
# dkim_verify.sh
find "$1" -iname *.eml -type f -exec echo -ne "FILE: {} --- " \; -exec check-dkim {} \;

Execute the script to start checking.

./dkim_verify.sh ./mail/

Here is the output from our first few files.

FILE: mail/c9f01.eml --- Error verifying DKIM                                                                                                                                               
body hash mismatch (got b'KSjh/SWf9CoOfANlP1JwziULd7TJwo2jAXdS6WxxiXk=', expected b'Hc1fzmKy9aocJCtYl88l4HEWgiYgp/nBHaexg4xOWtk=')                                                          
FILE: mail/483c0.eml --- Error verifying DKIM                                                                                                                                               
body hash mismatch (got b'NPfXTjlgeHmYX3XL505Y9ZlDsWR/nKofQRnlX2Eg+Vk=', expected b'Hc1fzmKy9aocJCtYl88l4HEWgiYgp/nBHaexg4xOWtk=')
FILE: mail/e1262.eml --- Error verifying DKIM
body hash mismatch (got b'IwEUYGwkRHvyIUocHDTn1mq653UyllFukFAsC7/bewY=', expected b'Hc1fzmKy9aocJCtYl88l4HEWgiYgp/nBHaexg4xOWtk=')
FILE: mail/a9331.eml --- Error verifying DKIM
body hash mismatch (got b'2kwfDpgExCVsPaquMn2PMAqKf/UEuOQneY6YpVutQ+U=', expected b'Hc1fzmKy9aocJCtYl88l4HEWgiYgp/nBHaexg4xOWtk=')
FILE: mail/c65c1.eml --- Error verifying DKIM
body hash mismatch (got b'GGvQpfeY8Vzvfms3TWn1TsYW2Ws4CKhenm26CVZ7kCs=', expected b'Hc1fzmKy9aocJCtYl88l4HEWgiYgp/nBHaexg4xOWtk=')

After a few minutes we get a hit. Here is the output leading up to the valid .eml.

FILE: mail/3c586.eml --- Error verifying DKIM                                                                                                                                               
body hash mismatch (got b'DopvYFdcPVYCj2nGEr3Jdll+EK7xiVVk33K/6xRJp90=', expected b'Hc1fzmKy9aocJCtYl88l4HEWgiYgp/nBHaexg4xOWtk=')
FILE: mail/06570.eml --- Error verifying DKIM                                                                                                                                               
body hash mismatch (got b'uJ/mmb0346p2GMzAqGnz/6hn5G0cL/jiotY2dl2tBJk=', expected b'Hc1fzmKy9aocJCtYl88l4HEWgiYgp/nBHaexg4xOWtk=')
FILE: mail/438b5.eml --- DKIM verified successfully

Now we can check which flag is in 438b5.eml.

grep -oP "(KCTF{.*})" mail/438b5.eml 
KCTF{1f8e659e892f2b2a05a54b8448ccbff9}
KCTF{1f8e659e892f2b2a05a54b8448ccbff9}

Here we have our flag! KCTF{1f8e659e892f2b2a05a54b8448ccbff9}