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[Security] Fix CodeQL alert #28: Uncontrolled data used in path expression #96

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[Security] Fix CodeQL alert #28: Uncontrolled data used in path expression #96

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5 changes: 4 additions & 1 deletion vulnerable_path_traversal.py
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,10 @@
def download_file():
filename = request.args.get('file')

file_path = os.path.join('/var/www/uploads/', filename)
base_dir = '/var/www/uploads/'
file_path = os.path.realpath(os.path.join(base_dir, filename))
if not file_path.startswith(os.path.realpath(base_dir)):
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@devin-ai-integration devin-ai-integration Bot Mar 26, 2026

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🔴 Path traversal fix is bypassable due to missing trailing separator in startswith check

The startswith check on line 12 is vulnerable to a directory-prefix bypass. os.path.realpath('/var/www/uploads/') strips the trailing slash, returning /var/www/uploads. An attacker can supply filename=../uploads_evil/secret.txt, which resolves to /var/www/uploads_evil/secret.txt. This passes the startswith('/var/www/uploads') check because the string /var/www/uploads_evil/... does start with /var/www/uploads. The fix must append os.sep to the resolved base directory to ensure only true children of the base directory are matched.

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if not file_path.startswith(os.path.realpath(base_dir)):
if not file_path.startswith(os.path.realpath(base_dir) + os.sep):
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@cursor cursor Bot Mar 26, 2026

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Path traversal check bypassable via sibling directory names

High Severity

The startswith check is bypassable because os.path.realpath(base_dir) strips the trailing slash from /var/www/uploads/, producing /var/www/uploads. A sibling directory like /var/www/uploads_evil/ would also match startswith('/var/www/uploads'), allowing access to files outside the intended directory. The check needs to compare against the resolved base path with a trailing os.sep appended.

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return 'Access denied', 403
return send_file(file_path)

@app.route('/read')
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