The Tool Desk
Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →The Corridor room looks like a tiny web puzzle: a page showing a hallway and 12 doors. The intended solution is not to click every door at random. Inspect the HTML source, compare the hexadecimal-looking values used in the door links, recognize that they are MD5 hashes of consecutive integers, and test the missing value for 0 in the existing URL format. In Athulya Biju’s September 8, 2026 walkthrough, that request opened the flag page.
This is a purpose-built TryHackMe training machine, not evidence that an unrelated production website is vulnerable. The useful lesson is broader than the room: an encoded or hashed object reference can still be predictable, and predictability is not an authorization check.
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What the Corridor room is testing
TryHackMe presents Corridor as a deployable machine built around a possible insecure direct object reference (IDOR). The official room asks, “Can you escape the Corridor?” and specifically directs learners to examine URL endpoints and hexadecimal-looking values. You can use the browser-based AttackBox or connect through the VPN, then start the lab machine and note its target IP address.
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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →| Clue | What it tells you | What it does not tell you |
|---|---|---|
| Port 80 responds | A web service is available over HTTP. | It does not identify the vulnerable parameter or endpoint. |
| A corridor image with 12 doors | The visible interface is intentionally sparse. | The number of pictures is not necessarily the number of valid object IDs. |
| Long hexadecimal URL values | The application may be transforming simple identifiers. | Hexadecimal text alone does not prove that the values are MD5. |
| Values matching consecutive numbers | The reference-generation rule is predictable. | It does not establish authorization on any real service. |
The room rewards changing your viewpoint. The rendered page shows a visual puzzle; the source and URLs show how the application represents each destination.
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Preparation and scope
- Start the Corridor machine in TryHackMe and wait until it reports that the target is ready.
- Use the supplied target IP, either from the AttackBox or from your own machine connected to the room’s VPN.
- Keep all testing inside the assigned lab address. Do not apply these techniques to a website or account without explicit permission.
- Record the original URL for at least one working door before changing anything. That gives you a known-good format for later tests.
The walkthrough reports behavior on this deliberately vulnerable exercise. It does not claim that every application using hashes in URLs has an IDOR, or that MD5 itself is an access-control mechanism.
Step 1: Confirm the web service with Nmap
Athulya Biju’s walkthrough begins with a basic Nmap scan and finds HTTP on port 80. A service-and-default-script scan is enough for this first pass:
nmap -sC -sV TARGET_IPReplace TARGET_IP with the address shown by the room. The important result is a web server listening on port 80. If Nmap reports the port as closed, verify that the machine has finished booting and that you are scanning the correct address. If you are using your own computer, check the VPN connection; if you are using AttackBox, launch the browser from that environment.
Open the target in a browser with http:// and the target address. The page is intentionally minimal. You see a corridor and doors rather than a conventional application with menus, account pages, or explanatory text.
Step 2: Inspect the page instead of relying on the picture
Right-click the page and choose the browser’s source-view option, or open developer tools and inspect the document. The room uses an HTML image map. In the markup, several <area> elements define clickable regions over the corridor image. Each area has a link whose destination includes a long hexadecimal-looking value.
This is the first important distinction:
- The image tells you where a clickable region appears.
- The
hrefvalue tells you what the server receives when you click it. - The source reveals links that may be easy to miss when you only use the visual interface.
Copy a few of the link values into a text editor. Preserve their case, length, and surrounding URL structure. Do not start by changing random characters. You are looking for a relationship between several valid links.
Step 3: Identify the hexadecimal values as MD5 hashes
The values in the links look like 32 hexadecimal characters, which is consistent with an MD5 digest. A hash-identification service such as CrackStation can identify a sample value and, for the reported sample, returns the original integer 1. The result is not a password-cracking exercise; it is a clue that the application may be calculating a digest from a small number.
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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesYou can verify the hypothesis locally without submitting anything outside the lab. For example:
printf '1' | md5sum
printf '2' | md5sum
printf '3' | md5sumOn systems where md5sum is unavailable, Python provides the same check:
python3 -c "import hashlib; print(hashlib.md5(b'1').hexdigest())"
python3 -c "import hashlib; print(hashlib.md5(b'2').hexdigest())"Compare the resulting 32-character strings with the values copied from the HTML. The walkthrough reports that the set corresponds to the integers 1 through 13. That sequence is the decisive observation: the URLs are not random opaque tokens, even though they look that way at first glance.
Why the sequence matters more than the door count
The page displays 12 doors, but the discovered values cover 13 consecutive integers. Do not assume that the visible count defines the complete identifier space. A user interface can omit an entry, reserve a special value, or simply fail to show every route the server accepts. In this room, the extra value becomes the lead for the final request.
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Step 4: Test the missing integer in the existing URL format
Generate the MD5 digest of the string 0:
printf '0' | md5sumor:
python3 -c "import hashlib; print(hashlib.md5(b'0').hexdigest())"Now take a known working door URL and replace only its hash component with the digest you just generated. Keep the protocol, host, path, and any surrounding syntax unchanged. The exact endpoint formatting is learned from the links in the room’s source; do not invent a new route when the application already gives you one.
- Click a normal door and copy its complete URL.
- Identify the 32-character value in that URL.
- Replace that value with the MD5 result for
0. - Load the modified URL while still connected to the Corridor machine.
- Confirm that the response is the room’s flag page, then submit the flag through the normal TryHackMe answer field.
Biju reports that this request opened the flag page. The flag is intentionally not reproduced here, so the room remains a hands-on exercise.
Why this qualifies as an IDOR teaching example
An IDOR occurs when an application exposes a reference to an object and fails to enforce whether the requester is allowed to access that object. The reference might be a database number, filename, account identifier, or transformed value. The key issue is authorization, not the visual appearance of the identifier.
In Corridor, the application appears to use an MD5 digest of a small integer in the URL. Because the underlying integers are consecutive and easy to test, a learner can derive another valid reference. Hashing changes the representation; it does not create permission boundaries. Anyone who can infer or enumerate the input values can calculate the corresponding digests.
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This room demonstrates the principle in a controlled setting. It does not show that MD5 URLs are automatically insecure, nor does it establish that a production service can be accessed merely by trying adjacent values. A real assessment would still require authorization, an understanding of the application’s object model, and evidence that the server omitted an authorization check.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.The SHA-384 stylesheet detour
The page also includes a Bootstrap stylesheet with a SHA-384 value in its integrity attribute. Biju tried to investigate that value with CyberChef’s Magic operation, but the attempt produced no useful result. This is a dead end for solving Corridor.
The integrity attribute serves a different purpose from the door links. Subresource Integrity lets a browser verify that a fetched stylesheet matches an expected cryptographic digest. It is a tamper-detection mechanism for the resource, not an application object identifier. Treating every hash-like string on a page as part of the challenge can waste time; first ask what the value is attached to and what function it serves.
Troubleshooting the walkthrough
Nmap finds no open port
- Confirm that the TryHackMe machine was started and has finished deploying.
- Check that the target IP has not changed after restarting the machine.
- If scanning from your own computer, reconnect to the room VPN.
- From AttackBox, make sure you are using the address shown by the room rather than a copied address from another session.
The page loads, but there are no obvious links
View the raw source, not only the Elements panel after scripts have modified the document. Search for <area, href, and 32-character hexadecimal strings. The clickable regions may be defined over one image, so they are easier to understand in markup than on the screen.
Your local hash does not match
- Hash the exact text, with no newline.
printf '1'is safer than a plainechoon systems that append a newline. - Use the decimal character itself: the digest for the string
1is different from the digest for a binary integer or the string01. - Check that you copied all 32 hexadecimal characters and did not include quotation marks or spaces.
- Compare several known door values before attempting the missing value.
The modified URL returns an error
Re-copy the complete working URL and change only the identifier. A malformed path, missing slash, altered case elsewhere in the address, or an expired machine can look like a failed hypothesis. Restarting the room may also change the target address, so update your browser and scan results together.
You reach a page but cannot submit an answer
Use the flag displayed by the active machine and submit it in the room’s answer field. Do not rely on a screenshot or a value copied from another learner’s session; lab instances and flags can be session-specific.
What defenders should take from Corridor
The defensive lesson is to authorize every object request on the server. An application should determine the authenticated principal, load the requested object, and verify that the principal is permitted to access it before returning data. Replacing a sequential ID with an MD5, Base64 string, or other reversible-looking transformation does not replace that check.
- Use server-side authorization for every read, update, and delete operation.
- Do not assume that an unguessable-looking URL is a permission decision.
- Prefer references that are difficult to enumerate, but treat that as defense in depth rather than authorization.
- Return consistent, carefully designed errors so unauthorized requests do not disclose whether an object exists.
- Log object-access decisions and test neighboring identifiers during authorized security testing.
- Include direct-object-reference tests in regression suites whenever a route accepts an object key from the client.
For learners, the practical habit is equally useful: compare related requests, inspect source, and ask what simple input may have produced an opaque value. For defenders, the same observation is a reminder that obscurity can hide an implementation detail without preventing access.
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Quick Recap
Walkthrough checklist
- Start the Corridor machine and note its target IP.
- Run
nmap -sC -sV TARGET_IPand confirm HTTP on port 80. - Open the page and inspect its source.
- Find the image-map
<area>links and copy several URL values. - Identify the 32-character values as MD5 digests by matching one to the integer
1. - Verify that the links represent integers 1 through 13.
- Calculate the MD5 digest of the string
0without a trailing newline. - Replace a working URL’s hash component with that digest.
- Load the modified URL and retrieve the flag from the resulting page.
- Submit the flag in TryHackMe while keeping all activity within the assigned lab.
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