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Security in depth · Unit 22 · Lesson 4 of 44
Directories control names and traversal
Separate listing a directory from resolving a file within it.
Helpful before thisLinux privilege escalation
After this lesson you can
- Distinguish directory read/search rights from permissions on file contents.
One idea. One situation. One reasoned decision.
How it works
Directory permissions describe operations on a namespace. Read permits listing names; search, represented by execute, permits traversing or looking up entries. Write relates to changing entries, normally together with search permission. File contents have their own permissions. This distinction explains why protecting only a file’s write bit may not adequately protect its location or replacement. Evaluate the containing directories as well as the file.
Read the supplied record
Assume ordinary discretionary access checks, no privileged override, no existing open handle, and no alternate path to the file.
| Resource | Reviewer’s rights |
|---|---|
| Project directory | Read, without search |
| A known report inside it | Read |
The reviewer may obtain the directory’s names, but opening the report through this pathname still requires search permission on the directory. Knowing the filename does not supply that permission. The file’s own read grant answers a later, separate question.
For maintenance, also separate editing file contents from adding, renaming, or removing directory entries. Directory write and search are relevant to entry changes; sticky-directory rules and other controls may impose further restrictions. Do not infer permission to perform every directory operation from a single successful file read.
A useful review identifies each required operation along the path and states which evidence supports it.
The key distinction: Control of directory entries and control of file contents are separate.
Check yourself
No timer. No penalties. Read the explanation and try again whenever you like.
This lesson’s questions have changed. Your reading progress is saved; review the updated questions.
-
The reviewer can list a directory but lacks search permission on it. A file inside grants the reviewer read access. With no overrides or alternate access path, can the reviewer open that pathname?
Show the answer
Correct answer: No: pathname resolution still needs directory search permission. The file-level read grant is only one required condition. The specified path cannot be resolved through this directory.
Try it
- WriteExplain why a reviewer might see a filename yet fail to open that file. Name the directory operation and file operation separately, using the fictional record.