tuf has platform-dependent delegation path matching

DelegatedRole._is_target_in_pathpattern uses fnmatch.fnmatch to decide whether a given target path is authorized by a delegation's glob pattern.

Python's fnmatch.fnmatch calls os.path.normcase() on both arguments before matching. On POSIX hosts normcase is the identity function; on Windows hosts os.path resolves to ntpath, whose normcase lowercases its input and replaces / with \.

As a result, python-tuf's delegation path pattern matching is case-sensitive on Linux/macOS but case-INSENSITIVE on Windows. This makes the authorization decision for a target dependent on the host operating system of the client running the updater.

The result on Windows is a TUF specification violation in the python-tuf ngclient implementation.

Vulnerable code

tuf/api/_payload.py (HEAD 7ecb67d):

1183  @staticmethod
1184  def _is_target_in_pathpattern(targetpath: str, pathpattern: str) -> bool:
1185      """Determine whether ``targetpath`` matches the ``pathpattern``."""
1186      # We need to make sure that targetpath and pathpattern are pointing to
1187      # the same directory as fnmatch doesn't threat "/" as a special symbol.
1188      target_parts = targetpath.split("/")
1189      pattern_parts = pathpattern.split("/")
1190      if len(target_parts) != len(pattern_parts):
1191          return False
1192
1193      # Every part in the pathpattern could include a glob pattern, that's why
1194      # each of the target and pathpattern parts should match.
1195      for target, pattern in zip(target_parts, pattern_parts, strict=True):
1196          if not fnmatch.fnmatch(target, pattern):
1197              return False
1198      return True

fnmatch.fnmatch source (Python 3.12, unchanged in current mainline):

def fnmatch(name, pat):
    ...
    name = os.path.normcase(name)
    pat = os.path.normcase(pat)
    return fnmatchcase(name, pat)

Fix

Replace fnmatch.fnmatch with fnmatch.fnmatchcase, which is explicitly documented as "not applying case normalization", so it behaves identically across platforms.

Attack

1. A TUF repository with two path-based delegations whose patterns differ only in case — for example, Foo/* and foo/*.
2. The "attacker" delegation is listed BEFORE the "legit" delegation in the delegation order.
3. The client searches for foo/something: on Windows, it will find the "attacker" provided target "Foo/something".

Exploitability caveats

• The attack needs a repository configuration with case-colliding delegation path patterns. The attacker must control one of the delegated roles.
• Delegation ordering matters: the attacker-controlled role must be visited BEFORE the legit role in the pre-order walk.
• The client must run on Windows. No effect on Linux/macOS.

Credit

Reporter: Koda Reef @kodareef5 Advisory edits: Jussi Kukkonen @jku