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[Security] Fix CodeQL alert #29: Uncontrolled data used in path expression #97

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[Security] Fix CodeQL alert #29: Uncontrolled data used in path expression #97

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6 changes: 5 additions & 1 deletion vulnerable_path_traversal.py
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Original file line number Diff line number Diff line change
Expand Up @@ -14,7 +14,11 @@ def download_file():
def read_file():
file_name = request.args.get('filename', 'default.txt')

with open(file_name, 'r') as f:
base_dir = os.path.realpath('/var/www/files/')
safe_path = os.path.realpath(os.path.join(base_dir, file_name))
if not safe_path.startswith(base_dir):
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@devin-ai-integration devin-ai-integration Bot Mar 26, 2026

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🔴 Path traversal bypass via startswith check without trailing separator

os.path.realpath('/var/www/files/') strips the trailing slash, so base_dir is /var/www/files. The check safe_path.startswith(base_dir) can be bypassed by accessing sibling directories whose names share the same prefix (e.g., /var/www/files_secret/). For example, filename=../files_secret/passwd resolves to /var/www/files_secret/passwd, which passes the startswith('/var/www/files') check, allowing reads outside the intended directory.

Example bypass

GET /read?filename=../files_secret/sensitive.txt

  • base_dir = /var/www/files
  • safe_path = os.path.realpath('/var/www/files/../files_secret/sensitive.txt') = /var/www/files_secret/sensitive.txt
  • '/var/www/files_secret/sensitive.txt'.startswith('/var/www/files') → True → access granted
Suggested change
if not safe_path.startswith(base_dir):
if not safe_path.startswith(base_dir + os.sep):
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@cursor cursor Bot Mar 26, 2026

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Path traversal fix bypassable via sibling directory prefix

High Severity

os.path.realpath strips the trailing slash from '/var/www/files/', so base_dir becomes '/var/www/files'. The startswith check then incorrectly permits access to sibling directories with a common prefix (e.g., /var/www/files_secret/), because '/var/www/files_secret/foo'.startswith('/var/www/files') is True. The check needs to compare against base_dir + os.sep to ensure the resolved path is actually within the intended directory.

Fix in Cursor Fix in Web

return 'Access denied', 403
with open(safe_path, 'r') as f:
content = f.read()

return content
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