## Introduction
What we have is a server running a Python script that allows to clone a gitrepository and run several commands on it. The target repository is cloned intothe `/tmp/<encoded_repo_url>` folder. After cloning, the following commands areavailable:
1. List files in repository (cd & ls)2. Show file in repository (cat)3. Check for updates (git fetch)4. Pull updates (unimplemented)5. Exit
## Command number 2
As with many challenges, the flag resides in the `/flag` file. Command number 2caught my attention since it prints the contents of a file of choice, and if I'mable to make it print the contents of the flag file, I win. Of course, it's notthat simple - the command verifies that the target file resides inside therepository folder. Below is the command implementation:
```python_REPO_DIR = "/tmp/<encoded_repo_url>"
# ...
def show_file():  filepath = input(">>> Path of the file to display: ")  real_filepath = os.path.realpath(os.path.join(_REPO_DIR, filepath))  if _REPO_DIR != os.path.commonpath((_REPO_DIR, real_filepath)):    print("Hacker detected!")    return  result = subprocess.run(["cat", real_filepath], capture_output=True)  if result.returncode != 0:    print("Error while retrieving file content.")    return  print(result.stdout.decode())```
The author considered the possibility that the target file could be a symboliclink, and used `os.path.realpath` to resolve symlinks before verifying that thetarget file is inside the repository folder. If I could only fool`os.path.realpath` and make it return a symlink without resolving it...
## A failed attempt: Invalid UTF-8 characters
Paths in Linux [may contain invalid UTF-8characters](https://unix.stackexchange.com/questions/667652/can-a-file-path-be-invalid-utf-8).For example, try `mkdir ''$'\334''berraschung'`. I hoped that it would confusePython which by default uses UTF-8 for strings, but it worked correctly until Iaccessed the string with the path. Looks like path operations worked correctly,but trying to access the resulting string raised the `UnicodeDecodeError`exception.
## Analyzing `os.path.realpath`
Next thing I did was looking at the Python implementation of `os.path.realpath`.The implementation uses the `lstat` function to check whether a file is asymlink. If `lstat` fails, Python assumes that the file isn't a symlink, so Ichecked what could cause `lstat` to fail. One of the possible error conditionsis `ENAMETOOLONG` which is returned when the path is too long. I didn't findother error conditions that looked relevant, so I focused on long paths.
Note: Python 3.10 adds the `strict` parameter to `os.path.realpath`, which is`False` by default. When `strict` is set to `True`, `os.path.realpath` fails if`lstat` returns an error. My solution to the challenge wouldn't work with thisparameter set to `True`.
## Abusing long paths
Typically Linux has a maximum filename length of 255 characters, and paths thatare passed to syscalls can't exceed 4095 characters([reference](https://unix.stackexchange.com/questions/596653/nested-directory-depth-limit-in-ext4)).
So I tried creating a symlink in my repository with a long path, like this:
> `/tmp/repo/<many_characters>/lnk`
The idea was that the path would be longer than 4095 characters, `lstat` wouldfail and `os.path.realpath` would return the path as-is. That worked, but `cat`failed with the same path for the same reason - it was too long - so I had tothink of a better idea. Since `os.path.realpath`, upon the failure of `lstat`,keeps traversing the path, and since I needed a shorter path, I tried adding`..` directories to shorten the result:
> `/tmp/repo/<many_characters>/lnk/../../..`
It indeed shortened the path, but I needed the result to be both a symlink and apath that's not too long, and if it would land on a symlink, it would follow it.Catch 22.
Finally, I found the following solution: If the link was traversed in the past,Python assumes it's a symlink loop and gives up. So I made sure to start with asymlink that would make Python think there's a loop and return it:
> `/tmp/repo/flag` → points to `<many_characters>/lnk/../../flag`
> `/tmp/repo/<many_characters>/lnk` → points to `/`
In this case, `os.path.realpath('/tmp/repo/flag')` tries to resolve`/tmp/repo/<many_characters>/lnk` with `lstat` which fails with `ENAMETOOLONG`,and keeps on traversing the path, finally getting back to `/tmp/repo/flag` andreturning it. Then, `/tmp/repo/flag` is passed to `cat` which resolves`<many_characters>/lnk` (note that this time it's shorter than 4095 characterssince it's a relative path) to `/`, and finally traversing to `/flag` andprinting it.
## Solution script
Here's a script implementing the solution:
```pythonimport urllib.parse
def encode_repo_url(repo_url):    return urllib.parse.quote(repo_url, safe='').replace('.', '%2e')
print('1. Create the repo:')print('mkdir -p 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-s / 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-s 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flag')
print('2. Commit and push the repo to GitHub or similar')
print('3. Use the following for the challenge:')
# Fill in your repo details.prefix = 'https://github.com'user = 'm417z'repo = 'legit'
url1 = f'{prefix}/{user}/'url2 = f'/../{repo}'url1_enc = encode_repo_url(url1)url2_enc = encode_repo_url(url2)
# Make sure the repo folder name is 255 characters long for everything to align# nicely.assert(len(url1_enc + url2_enc) < 255)url = url1 + '_'*(255 - len(url1_enc + url2_enc)) + url2
print(f'Clone the repo: {url}')print(f'Read the file: flag')```