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The intended solution to TryHackMe’s Corridor room is to look past the 12-door picture, inspect the HTML source, and compare the hexadecimal-looking URL values. The values decode as MD5 hashes of the integers 1 through 13. Once that pattern is established, generating the MD5 hash for 0 and placing it in the authorized room URL opens the flag page. The flag is intentionally not reproduced here so you can finish the room yourself.
This walkthrough follows Athulya Biju’s September 8, 2026 account of the room and the current TryHackMe instructions. It explains the useful observation, the unproductive SHA-384 detour, and the security lesson about predictable object references without treating a purpose-built lab as evidence of a flaw in an unrelated production service.
What the Corridor room is testing
Corridor is a small, beginner-oriented web CTF built around an insecure direct object reference (IDOR) idea. TryHackMe’s room prompt asks, “Can you escape the Corridor?” and tells learners to examine URL endpoints and hexadecimal-looking values. The visual interface is deliberately sparse: a corridor image presents 12 doors, but the important information is not printed on the page. It is in the way the links are constructed.
The room is a deployable training machine. If you are not using the VPN, the official instructions say to start an AttackBox; in either case, start the lab machine and use the address it assigns. Perform every request only against that room instance and stop when the exercise is complete.
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What you need
| Requirement | Purpose |
|---|---|
| TryHackMe Corridor room | Provides the authorized target and the flag page. |
| AttackBox or a VPN-connected system | Lets your browser and scanning tools reach the room network. |
| A browser with “View source” or developer tools | Reveals the image-map links hidden behind the corridor picture. |
| Nmap | Confirms which service is exposed on the target. |
An MD5 calculator, md5sum, Python, or an equivalent local tool |
Generates and checks the short numeric hashes. |
Step 1: Start the machine and confirm the web service
Copy the target IP shown by TryHackMe, then run a basic scan from the AttackBox or your VPN-connected host:
nmap -sV -Pn <TARGET_IP>In the reported walkthrough, the scan found port 80 running HTTP. A version scan is useful because it confirms that the room is presenting a web service rather than leaving you guessing which protocol to test. If the host appears down, -Pn tells Nmap to scan without relying on a ping response; that is helpful in lab networks where ICMP is filtered.
Open the HTTP address in your browser. Use the exact scheme and port displayed by the room. Do not scan arbitrary Internet addresses while following this exercise: the point is to understand the room’s implementation, not to discover unrelated systems.
Step 2: Look at the page, then look at its source
The first page gives very little away. You see a corridor and 12 doors, but no obvious username, form, search box, or explanatory text. Clicking doors is a reasonable first check, yet the important clue appears when you inspect the document itself.
View the HTML
- In the browser, right-click the page and choose View page source, or open developer tools and select the Elements panel.
- Search for
<map,<area,href, or the image filename. An HTML image map associates clickable regions of one image with separate links. - Record the destination values without changing them. They may look like long hexadecimal strings rather than readable door numbers.
- Compare several destinations side by side. The repeated format and the fact that each link differs in a controlled way are more informative than the artwork.
The source inspection changes the question from “Which door looks promising?” to “How does this page identify a door?” That shift is the central move in the room.
Step 3: Recognize the MD5 pattern
According to the walkthrough, checking one of the hexadecimal-looking values with CrackStation identified it as an MD5 hash and returned the integer 1. Repeating the check for the other values revealed hashes corresponding to the numbers 1 through 13. The visible door count is therefore not the whole data set: the source contains a sequence whose numeric range extends beyond what the picture suggests.
| Observation | What it tells you | What it does not tell you |
|---|---|---|
| Every destination has the same 32-character hexadecimal shape. | The page is probably transforming a short value rather than using a descriptive path. | It does not prove the transformation is secure or that the value is random. |
One value resolves to 1 when checked as MD5. |
MD5 is a plausible encoding for the link identifiers. | It does not authorize access by itself. |
| The remaining values resolve to integers 1–13. | The identifiers form a predictable sequence. | It does not mean every hash-based URL on the Internet is vulnerable. |
| The page visibly shows 12 doors. | The interface is only a partial representation of the underlying links. | The picture alone cannot establish the full identifier range. |
Verify locally instead of guessing
You can reproduce the numeric checks locally. On a Linux AttackBox, the following command prints the MD5 digest for the character 1 without adding a newline:
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printf 1 | md5sumUse the same form for other integers. Python provides an equivalent check:
python3 -c "import hashlib; print(hashlib.md5(b'1').hexdigest())"The byte string matters. Hashing the text 1 and hashing a file containing 1 followed by a newline produce different results. If your output does not match a source value, first check for an accidental newline, spaces, quotation marks, or a different character encoding.
For this room, the useful hypothesis is not merely “these strings are hashes.” It is “the URL identifier is the MD5 of a small integer, and the sequence is predictable.” Test that hypothesis against multiple links before attempting the next step.
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Step 4: Test the missing value in the room URL
Once the sequence is clear, generate the MD5 digest of 0 and substitute it for the existing identifier in the room’s URL. The walkthrough reports that this request opened the page containing the flag.
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printf 0 | md5sum, or use a local hash tool. - Copy the complete digest, excluding any trailing spaces or command output formatting.
- Use the browser’s current room URL as the template. Replace only the existing hexadecimal identifier with the digest for
0. - Send the request to the TryHackMe machine you started. Do not redirect the same experiment to a public website or another host.
- When the flag page appears, submit the flag in TryHackMe. The value is omitted here deliberately.
This is a controlled test of the pattern you observed, not a license to brute-force every possible path. You already have a concrete reason to test the predecessor value: the room exposes a short, sequential set of MD5-derived identifiers.
If the request does not work
- Confirm that the lab machine is still running and that you are using its current IP address.
- Check that you hashed the text
0, not0followed by a newline. - Compare the digest length and character case with the values already present. MD5 output is normally 32 hexadecimal characters; the server may nevertheless treat case or URL encoding differently.
- Replace only the identifier portion. Changing the path, port, protocol, or query string can send you to a different route.
- Refresh the source and copy a current link if the room has restarted. A new instance can have a different address even when the page content is the same.
The SHA-384 stylesheet clue was a dead end
While inspecting the page, the author also noticed a SHA-384 value in a Bootstrap stylesheet’s integrity attribute and tried CyberChef’s Magic operation. That investigation did not produce a useful result and is not part of the reported solution.
An integrity attribute serves a different purpose: it lets a browser verify that a fetched resource matches an expected cryptographic digest. It is not the identifier used by the corridor links. Treating every hexadecimal string in source as an equally promising lead can waste time. First connect a value to the behavior you are trying to explain—in this case, the URL destinations behind the image map.
Why this demonstrates an IDOR concept
An IDOR occurs when an application lets a requester access an object by changing a reference without enforcing that the requester is authorized to view that object. The reference might be a database number, filename, account identifier, or a transformed value in a URL.
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Hashing a predictable identifier changes its appearance, not the authorization decision. If a server accepts the MD5 of an integer and never checks whether that integer is allowed for the current requester, someone who can infer the construction may request another object. In Corridor, the room intentionally makes that mistake so learners can observe it safely.
That lesson has two separate parts:
- Discoverability: source code and related URLs reveal how the client constructs references.
- Authorization: the server must independently verify that the requester may access the referenced object.
A random-looking or encrypted-looking token can reduce casual guessing, but it is not a substitute for an authorization check. Conversely, seeing an MD5-shaped URL does not establish an IDOR in a real service. You need evidence that changing the reference crosses an authorization boundary, and you must obtain that evidence only with permission.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common sticking points in the room
“I only see 12 doors, so where does 13 come from?”
The artwork is not a complete inventory of the values used by the application. The walkthrough found hashes for numbers 1 through 13 in the source. Count the link destinations and inspect their targets instead of inferring the range from the illustration.
“My hash lookup says unknown”
Check whether you copied the full 32-character digest and whether the original value included a newline. A lookup service may also lack a particular digest even when it is a valid MD5. Generate the expected values locally and compare them directly.
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“The page source is minified or hard to read”
Use the browser’s search function for area and href, then use developer tools to inspect the image-map element. The formatting is irrelevant; the destination attributes are what matter.
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“The page returns a 404 after I edit the URL”
Make sure you preserved the route and changed only the identifier. Confirm that the machine has not reset and that you did not paste a digest with an extra newline. A 404 can indicate a malformed path rather than a failed hypothesis.
“Should I try every possible integer?”
No. The room gives you a narrow, evidence-based sequence. Test the single missing predecessor that the walkthrough identifies, then stop once you reach the flag page. Broad enumeration against systems you do not own or have explicit permission to test is inappropriate.
A repeatable method for similar authorized labs
Corridor is useful because the workflow generalizes without depending on a particular tool:
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- Separate appearance from implementation. If the interface is sparse, inspect source, scripts, forms, image maps, and network requests.
- Compare related references. Look for fixed lengths, character sets, prefixes, and changes that correlate with the object being selected.
- Identify the transformation. Test a known value with a local calculator or a permitted analysis tool; do not assume that every digest is an identifier.
- Form a narrow hypothesis. State exactly what you think the server is doing and which single request would confirm it.
- Validate inside scope. Use the lab’s own machine and document the request and response.
- Stop at the learning objective. Do not turn a training exercise into indiscriminate enumeration.
This process keeps reconnaissance, interpretation, and validation distinct. It also makes failed experiments useful: the stylesheet digest did not explain navigation, while the image-map values did.
What you should—and should not—take away
The room shows that a simple page can expose important assumptions through its HTML and URLs. It also shows why a transformed identifier is not the same thing as access control. Those are valid lessons within the deliberately vulnerable TryHackMe machine.
It would be incorrect to claim that this walkthrough proves a live production service is vulnerable, that all MD5-based URLs are exploitable, or that a source-code inspection alone establishes unauthorized access. In a real assessment, you would need written authorization, a defined target, and evidence that the server permits access to an object outside the tester’s rights.
Final takeaway
Start with the service scan, but do not stop at the picture. In Corridor, the decisive evidence is the image map in the page source: its URL values are MD5 hashes for a predictable integer sequence. Generating the hash for 0 and substituting it into the room’s existing URL reaches the flag page, while the SHA-384 stylesheet value is only a distraction. The deeper lesson is that obscuring an object reference cannot replace a server-side authorization check.
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