DelegatedRole.istargetinpathpattern 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):

python 1183 @staticmethod 1184 def istargetinpathpattern(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 targetparts = targetpath.split("/") 1189 patternparts = pathpattern.split("/") 1190 if len(targetparts) != len(patternparts): 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(targetparts, patternparts, 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):

python 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