Challenges: Corridor (TryHackMe)

This challenge presented a web application that at first glance appeared to be static, with a single background image and minimal interactive content. Enumeration using tools like Gobuster didn’t reveal much, but manually inspecting the page source uncovered an interesting HTML <img> with an associated <map> defining clickable areas, each linked to what looked like MD5 hashes. These hashes represented potential IDs, hinting that access to resources was linked to predictable or manipulable identifiers.

Hello Hacker!

TopTierConversions LTD is proud to announce its latest and greatest product launch: MD2PDF.

This easy-to-use utility converts markdown files to PDF and is totally secure! Right...?

Note: Please allow 3-5 minutes for the VM to boot up fully before attempting the challenge.

Answer the questions below

What is the flag?

nmap -sV IP_Address

gobuster dir -u IP_Address -w /usr/share/wordlists/dirb/common.txt

Enumerating with Gobuster was not of much help on this case:

Went ahead to check the site it had a background image and inspected the page to check the source code. It showed the img and an image-map which was interesting: it had unique identical md5 hash for alt text and title for each area within the map

<img src="/static/img/corridor.png" usemap="#image-map">
<map name="image-map">
    <area target="" alt="c4ca4238a0b923820dcc509a6f75849b" title="c4ca4238a0b923820dcc509a6f75849b" href="c4ca4238a0b923820dcc509a6f75849b" coords="257,893,258,332,325,351,325,860" shape="poly">
    <area target="" alt="c81e728d9d4c2f636f067f89cc14862c" title="c81e728d9d4c2f636f067f89cc14862c" href="c81e728d9d4c2f636f067f89cc14862c" coords="469,766,503,747,501,405,474,394" shape="poly">
    <area target="" alt="eccbc87e4b5ce2fe28308fd9f2a7baf3" title="eccbc87e4b5ce2fe28308fd9f2a7baf3" href="eccbc87e4b5ce2fe28308fd9f2a7baf3" coords="585,698,598,691,593,429,584,421" shape="poly">
    <area target="" alt="a87ff679a2f3e71d9181a67b7542122c" title="a87ff679a2f3e71d9181a67b7542122c" href="a87ff679a2f3e71d9181a67b7542122c" coords="650,658,644,437,658,652,655,437" shape="poly">
    <area target="" alt="e4da3b7fbbce2345d7772b0674a318d5" title="e4da3b7fbbce2345d7772b0674a318d5" href="e4da3b7fbbce2345d7772b0674a318d5" coords="692,637,690,455,695,628,695,467" shape="poly">
    <area target="" alt="1679091c5a880faf6fb5e6087eb1b2dc" title="1679091c5a880faf6fb5e6087eb1b2dc" href="1679091c5a880faf6fb5e6087eb1b2dc" coords="719,620,719,458,728,471,728,609" shape="poly">
    <area target="" alt="8f14e45fceea167a5a36dedd4bea2543" title="8f14e45fceea167a5a36dedd4bea2543" href="8f14e45fceea167a5a36dedd4bea2543" coords="857,612,933,610,936,456,852,455" shape="poly">
    <area target="" alt="c9f0f895fb98ab9159f51fd0297e236d" title="c9f0f895fb98ab9159f51fd0297e236d" href="c9f0f895fb98ab9159f51fd0297e236d" coords="1475,857,1473,354,1537,335,1541,901" shape="poly">
    <area target="" alt="45c48cce2e2d7fbdea1afc51c7c6ad26" title="45c48cce2e2d7fbdea1afc51c7c6ad26" href="45c48cce2e2d7fbdea1afc51c7c6ad26" coords="1324,766,1300,752,1303,401,1325,397" shape="poly">
    <area target="" alt="d3d9446802a44259755d38e6d163e820" title="d3d9446802a44259755d38e6d163e820" href="d3d9446802a44259755d38e6d163e820" coords="1202,695,1217,704,1222,423,1203,423" shape="poly">
    <area target="" alt="6512bd43d9caa6e02c990b0a82652dca" title="6512bd43d9caa6e02c990b0a82652dca" href="6512bd43d9caa6e02c990b0a82652dca" coords="1154,668,1146,661,1144,442,1157,442" shape="poly">
    <area target="" alt="c20ad4d76fe97759aa27a0c99bff6710" title="c20ad4d76fe97759aa27a0c99bff6710" href="c20ad4d76fe97759aa27a0c99bff6710" coords="1105,628,1116,633,1113,447,1102,447" shape="poly">
    <area target="" alt="c51ce410c124a10e0db5e4b97fc2af39" title="c51ce410c124a10e0db5e4b97fc2af39" href="c51ce410c124a10e0db5e4b97fc2af39" coords="1073,609,1081,620,1082,459,1073,463" shape="poly">
</map>

Next step is converting the MD5 hashes and this look like id of each area

c4ca4238a0b923820dcc509a6f75849b - 1
c81e728d9d4c2f636f067f89cc14862c → 2
a87ff679a2f3e71d9181a67b7542122c → 7
e4da3b7fbbce2345d7772b0674a318d5 → 5
1679091c5a880faf6fb5e6087eb1b2dc → 3
8f14e45fceea167a5a36dedd4bea2543 → 4
c9f0f895fb98ab9159f51fd0297e236d → 0
45c48cce2e2d7fbdea1afc51c7c6ad26 → 3
d3d9446802a44259755d38e6d163e820 → 6
6512bd43d9caa6e02c990b0a82652dca → 9
c20ad4d76fe97759aa27a0c99bff6710 → 12
c51ce410c124a10e0db5e4b97fc2af39 → 13

Tried adjusting the 12 hashes here http://IP_Address/hash:

the 12 hashes on the image map don’t work but trying a different id which is md5 hash equavilent to 0, the flag is revealed

At this point we know that this is an IDOR challenge and tried considering the next possible id like 0 and getting it’s id.

“Security through obscurity isn’t security – always enforce proper access controls, even on hidden or hashed IDs.”

cfcd208495d565ef66e7dff9f98764da → 0

Using this one reveals the flag

Lessons for Developers

Never Rely on Obfuscation
Encoding or hashing object identifiers (like MD5 of IDs) does not provide security. If access control checks are missing, attackers can still manipulate them.
Implement Proper Authorization
Always verify that the user is authorized to access the requested resource, regardless of how the identifier is presented.
Use Non-Predictable IDs
Consider using UUIDs or securely generated random identifiers combined with proper access control to make guessing object references harder.
Conduct Security Testing
Regularly test for IDOR vulnerabilities as part of secure code review and penetration testing, especially in legacy or static-looking applications.

By converting these MD5 hashes back to their original numeric values, it became clear the application was using obfuscated, but not securely protected, identifiers. Changing the identifier to the MD5 of 0 (cfcd208495d565ef66e7dff9f98764da) directly exposed the flag without any authorization checks. This is a classic example of an Insecure Direct Object Reference (IDOR), where sensitive data can be accessed simply by modifying an object reference in the request.