Linux mode bits and directory traversal

# Linux mode bits and directory traversal

Evaluate a fictional unprivileged user against owner/group/other mode bits and explain why readable files may be unreachable through a directory.

This is an original, fictional, offline exercise for an introductory security learner. It is independent practice, not copied coursework or a claim of professional experience. Completion and competence remain unassessed.

## Before you start

Allow **30-45 minutes**: 5 minutes reading, 10 inspecting and predicting, 10 checking and explaining, and 5-20 for a changed or independent case. You need Python 3.10+ (standard library only), a terminal and a text editor. Comfort opening files, reading a small table and basic evidence notes is enough; programming, administrator rights, a Linux installation and cloud accounts are unnecessary. Previous introductory network or Wireshark practice is useful but not required.

> **Vocabulary:** Owner/group/other: mutually selected permission classes. Octal: base eight, used for the sum 4+2+1. Directory search: permission to resolve names through a directory. ACL: additional access-control entries outside this model. Mode: the ordinary permission bits in this exercise.

## Evidence and exact rules

The case JSON is the single authoritative input. Fixtures are guided.json, changed.json and challenge.json. All data is synthetic; artifacts is empty to avoid competing editable copies. Edit only a prepared run copy. The analyser is a pure calculation and never executes evidence text.

data contains actor name and complete group list, nodes with canonical absolute path/type/owner/group/three-digit octal mode, and requests with ID/path/operation. Supply / and every ancestor directory. Only ordinary file/directory nodes and non-root actors are supported. Special bits, links and missing ancestors are rejected. Nothing reads or changes your operating system.

Select owner bits when the actor owns the node, else group bits when its group is in the actor list, else other bits. Never fall through to a more permissive class. Read=4, write=2, execute/search=1. Every ancestor directory requires search. Target read-file needs 4, write-file 2, execute-file 1, list-dir 4, search-dir 1 and create-entry 3 (write plus search). All required bits and ancestor searches must pass for allowed-by-mode; otherwise denied-by-mode. list-dir means listing names only, not reading file contents or metadata.

## Run the exercise

Open a terminal in this lab folder. Use your Python command (`python`, or `py -3` if that is how Python is installed). Commands below use `python` consistently.

```text
python lab.py cases
python lab.py prepare --scenario guided --out run/attempt-01
python lab.py show --case run/attempt-01/case.json
```

Read the evidence. In run/attempt-01/answers.json, fill each decision with an exact label from the rules; add an explanation and evidence references such as record IDs or configuration line numbers. In notes.txt, separate observation, interpretation, uncertainty, next check and help used. Save your prediction before opening the answer.

```text
python lab.py analyse --case run/attempt-01/case.json
python lab.py check --case run/attempt-01/case.json --answers run/attempt-01/answers.json
python lab.py prepare --scenario changed --out run/attempt-02
python lab.py prepare --scenario challenge --out run/attempt-03
python -m unittest -v test_lab.py
python lab.py verify
```

For changed/challenge use the corresponding attempt folder in show/analyse/check. prepare refuses an existing output folder. --out is relative to this lab and must be below run/; explicit input paths are relative to your terminal directory. analyse/show default to the guided fixture if --case is omitted. Exit 0 means command success or decision match; 1 means differences/incomplete written fields; 2 means invalid input or a filesystem problem. The checker verifies decision fields and the presence of writing, not the quality of reasoning. verify checks shipped examples, not learner mastery.

## Deliverable and limits

Keep case.json, answers.json and notes.txt together in the ignored run folder. Deliver a short handover: the question, your conclusion, two precise evidence references, one alternative explanation or limitation, and the next evidence you would request. State whether you used the worked example and whether you completed a fresh case unaided. Do not describe an automated match as assessment or professional experience.

Owner bits take precedence over group, then other; there is no fallback to a more permissive class. Ancestor directories need search (execute) permission. This evaluates ordinary mode bits for an unprivileged synthetic actor only. ACLs, SELinux, capabilities, special bits, symlinks and mount restrictions are not modelled. No host permissions were inspected or changed.

Use WORKED-EXAMPLE.md after making a first prediction, and DRILLS.md for progressively less support. Do not use these rules as an operational security verdict or modify a real host as part of this exercise.

## Primary references

The following documentation supports the underlying concepts; the tiny schema and explicit teaching rules above are specific to this lab. Links were checked during authoring on 23 October 2026. Running the lab needs no internet connection.

- [Linux path resolution manual](https://man7.org/linux/man-pages/man7/path_resolution.7.html)
- [Linux inode and mode manual](https://man7.org/linux/man-pages/man7/inode.7.html)

# Worked example: Linux mode bits and directory traversal

Try the guided case before reading this page. Use `python lab.py prepare --scenario guided --out run/worked-01` if you need a fresh ignored copy.

learner belongs to students. / is 755, so other search is allowed. /srv is owned by custodian with group students and mode 750: learner uses group 5, which includes search. /srv/notes.txt is 640 with group students: learner uses group 4, allowing read but not write. /srv/drop is 733 with group staff: learner uses other 3. This permits search and creating an entry, but not listing names (read=4 is absent). The file result depends on both the selected target class and every ancestor search check.

The reference decision fields for guided are:

```json
{
  "read-notes": "allowed-by-mode",
  "write-notes": "denied-by-mode",
  "list-drop": "denied-by-mode",
  "search-drop": "allowed-by-mode",
  "create-drop": "allowed-by-mode"
}
```

Now open `python lab.py analyse --case run/worked-01/case.json` and compare observations, not only labels. Record one difference between your first prediction and the reference. If there was no difference, explain the rule that produced each decision rather than copying the label.

Handover prompt: explain what this evidence establishes, cite its exact IDs or lines, give a plausible alternative or remaining uncertainty, and name the smallest next check. Record any help used. A correct label by itself is not evidence of independent understanding.

# Drills: Linux mode bits and directory traversal

## Guided practice

Prepare guided in a fresh run folder, read the README rules, and make a prediction for every decision ID. Use WORKED-EXAMPLE.md only after saving that prediction. Correct your explanation in a different paragraph so the original reasoning remains visible.

## Prompts only

Close the worked example and use these questions to reconsider the evidence:

- Why can other=6 fail to help the owner of a file with owner=0?
- What does directory execute mean that file execute does not?

## Independent attempt

Prepare challenge in a fresh folder. Keep reference outputs, worked example and analyse closed until you have saved answers.json plus a five-sentence handover in notes.txt. Include the observation, interpretation, an alternative, uncertainty and next check. Use the README for syntax if needed and record that help. Afterwards run analyse/check and have a person inspect the explanation. Independent completion is unassessed unless someone actually reviews it.

## Changed case

Prepare changed. Predict which conclusions change and which do not. Explain the changed evidence before checking:

- Why does changing the actor to alex change the selected class on notes.txt?
- Why do group/other permissions not provide fallback for this owner?

Small edit drill: In a copied guided case, change /srv mode from 750 to 740. Predict every affected request before running analysis. Do not run chmod on any real file.

## Teach-back

Walk through ancestor search and selected target bits for a file read, then contrast listing names with resolving a known filename. Use exact record IDs from your attempt. A partner should ask one counterexample rather than accepting a memorised label. Record the question and your response.

## Later recall

After 24-72 hours, reopen only your own notes and a fresh case. Write the central rule from memory, identify one boundary or missing-evidence condition and predict the result. Then check the documentation and record what you forgot. Do not count rereading an answer as an independent attempt.

## Handover

Keep the copied case, completed worksheet and notes together. State the scope, conclusion, two evidence references, unresolved uncertainty and requested next check. Include help used and whether someone assessed the reasoning. Do not claim experience beyond this fictional exercise.