fix: prevent trust bypass via encoded URL traversal

A template URL with percent-encoded parent-directory segments (e.g.
`%2e%2e`, `%2E%2E`, `%2e.`, `.%2e`) or backslashes (`%5c`, `%5C`) could
match a trusted URL prefix while HTTP servers and Git transports decode
the segments and ultimately fetch a repository outside the trusted
prefix. This allowed unsafe features from an untrusted repository to
run without explicit trust.

Trust matching now decodes percent-encoded characters in URL paths
before resolving dot segments, so the form used for the trust check
matches what the server actually resolves. The fix also neutralizes
encoded path separators (`%2f`) and folds backslashes (literal or `%5c`)
to forward slashes, since some servers treat them as path separators.

Local filesystem paths are unaffected: the OS does not decode `%2e%2e`,
and Git treats it as a literal directory name.

Author: sisp

copier/_settings.py

@@ -10,7 +10,7 @@
 from os.path import expanduser
 from pathlib import Path
 from typing import Any
-from urllib.parse import urlsplit, urlunsplit
+from urllib.parse import unquote, urlsplit, urlunsplit
 
 import yaml
 from platformdirs import user_config_path
@@ -154,8 +154,16 @@ def _normalize(url: str) -> str:
         url = expanduser(url)  # noqa: PTH111
     if "://" in url:
         parts = urlsplit(url)
-        path = posixpath.normpath(parts.path) if parts.path else parts.path
-        if parts.path.endswith("/") and not path.endswith("/"):
+        # Percent-decode before normalizing so that encoded dot segments (e.g.
+        # `%2e%2e`) and encoded separators (e.g. `%2f`) are collapsed by
+        # `posixpath.normpath`. Backslashes (literal or `%5c`) are folded to
+        # forward slashes because some servers and intermediaries treat them
+        # as path separators while `posixpath.normpath` does not. Otherwise
+        # the form used for the trust check could differ from what the
+        # HTTP/Git layer ultimately resolves, allowing a trust-prefix bypass.
+        decoded_path = unquote(parts.path).replace("\\", "/")
+        path = posixpath.normpath(decoded_path) if decoded_path else decoded_path
+        if decoded_path.endswith("/") and not path.endswith("/"):
             path += "/"
         return urlunsplit(
             (parts.scheme, parts.netloc, path, parts.query, parts.fragment)

tests/test_settings.py

@@ -169,6 +169,81 @@ def test_load_settings_with_invalid_data(
             {"https://github.com/user/../evil/repo.git"},
             True,
         ),
+        (
+            "https://github.com/user/%2e%2e/evil/repo.git",
+            {"https://github.com/user/"},
+            False,
+        ),
+        (
+            "https://github.com/user/%2E%2E/evil/repo.git",
+            {"https://github.com/user/"},
+            False,
+        ),
+        (
+            "https://github.com/user/%2e%2E/evil/repo.git",
+            {"https://github.com/user/"},
+            False,
+        ),
+        (
+            "https://github.com/user/%2e./evil/repo.git",
+            {"https://github.com/user/"},
+            False,
+        ),
+        (
+            "https://github.com/user/.%2e/evil/repo.git",
+            {"https://github.com/user/"},
+            False,
+        ),
+        (
+            "https://github.com/user%2f%2e%2e%2fevil/repo.git",
+            {"https://github.com/user/"},
+            False,
+        ),
+        (
+            "https://github.com/user/%2e%2e/evil/repo.git",
+            {"https://github.com/user/%2e%2e/evil/repo.git"},
+            True,
+        ),
+        (
+            "https://github.com/user/%2e%2e/evil/repo.git",
+            {"https://github.com/user/../evil/repo.git"},
+            True,
+        ),
+        (
+            "https://github.com/user/%2e%2e%5cevil/repo.git",
+            {"https://github.com/user/"},
+            False,
+        ),
+        (
+            "https://github.com/user/%2e%2e%5Cevil/repo.git",
+            {"https://github.com/user/"},
+            False,
+        ),
+        (
+            "https://github.com/user/..%5cevil/repo.git",
+            {"https://github.com/user/"},
+            False,
+        ),
+        (
+            "https://github.com/user/..\\evil/repo.git",
+            {"https://github.com/user/"},
+            False,
+        ),
+        (
+            "https://github.com/user/%2e%2e%5cevil/repo.git",
+            {"https://github.com/user/%2e%2e%5cevil/repo.git"},
+            True,
+        ),
+        (
+            "https://github.com/user/%2e%2e%5cevil/repo.git",
+            {"https://github.com/user/..\\evil/repo.git"},
+            True,
+        ),
+        (
+            "https://github.com/user/%2e%2e%5cevil/repo.git",
+            {"https://github.com/user/../evil/repo.git"},
+            True,
+        ),
         (f"{Path.home()}/template", [], False),
         (f"{Path.home()}/template", {f"{Path.home()}/template"}, True),
         (f"{Path.home()}/template", {"~/template"}, True),