Merge branch 'master' into develop

8 files changed, 513 insertions, 51 deletions

diff --git a/CHANGES.md b/CHANGES.md
index 9f70f74839..5f625dad1d 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,26 @@
+Synapse 1.47.1 (2021-11-23)
+===========================
+
+This release fixes a security issue in the media store, affecting all prior releases of Synapse. Server administrators are encouraged to update Synapse as soon as possible. We are not aware of these vulnerabilities being exploited in the wild.
+
+Server administrators who are unable to update Synapse may use the workarounds described in the linked GitHub Security Advisory below.
+
+Security advisory
+-----------------
+
+The following issue is fixed in 1.47.1.
+
+- **[GHSA-3hfw-x7gx-437c](https://github.com/matrix-org/synapse/security/advisories/GHSA-3hfw-x7gx-437c) / [CVE-2021-41281](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-41281): Path traversal when downloading remote media.**
+
+  Synapse instances with the media repository enabled can be tricked into downloading a file from a remote server into an arbitrary directory, potentially outside the media store directory.
+
+  The last two directories and file name of the path are chosen randomly by Synapse and cannot be controlled by an attacker, which limits the impact.
+
+  Homeservers with the media repository disabled are unaffected. Homeservers configured with a federation whitelist are also unaffected.
+
+  Fixed by [91f2bd090](https://github.com/matrix-org/synapse/commit/91f2bd090).
+
+
 Synapse 1.47.0 (2021-11-17)
 ===========================
 
diff --git a/debian/changelog b/debian/changelog
index ba75d0b251..35c9063388 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,3 +1,9 @@
+matrix-synapse-py3 (1.47.1) stable; urgency=medium
+
+  * New synapse release 1.47.1.
+
+ -- Synapse Packaging team <packages@matrix.org>  Fri, 19 Nov 2021 13:44:32 +0000
+
 matrix-synapse-py3 (1.47.0) stable; urgency=medium
 
   * New synapse release 1.47.0.
diff --git a/synapse/__init__.py b/synapse/__init__.py
index aa964afb5e..48ac38aec6 100644
--- a/synapse/__init__.py
+++ b/synapse/__init__.py
@@ -47,7 +47,7 @@ try:
 except ImportError:
     pass
 
-__version__ = "1.47.0"
+__version__ = "1.47.1"
 
 if bool(os.environ.get("SYNAPSE_TEST_PATCH_LOG_CONTEXTS", False)):
     # We import here so that we don't have to install a bunch of deps when
diff --git a/synapse/rest/media/v1/_base.py b/synapse/rest/media/v1/_base.py
index 014fa893d6..9b40fd8a6c 100644
--- a/synapse/rest/media/v1/_base.py
+++ b/synapse/rest/media/v1/_base.py
@@ -29,7 +29,7 @@ from synapse.api.errors import Codes, SynapseError, cs_error
 from synapse.http.server import finish_request, respond_with_json
 from synapse.http.site import SynapseRequest
 from synapse.logging.context import make_deferred_yieldable
-from synapse.util.stringutils import is_ascii
+from synapse.util.stringutils import is_ascii, parse_and_validate_server_name
 
 logger = logging.getLogger(__name__)
 
@@ -51,6 +51,19 @@ TEXT_CONTENT_TYPES = [
 
 
 def parse_media_id(request: Request) -> Tuple[str, str, Optional[str]]:
+    """Parses the server name, media ID and optional file name from the request URI
+
+    Also performs some rough validation on the server name.
+
+    Args:
+        request: The `Request`.
+
+    Returns:
+        A tuple containing the parsed server name, media ID and optional file name.
+
+    Raises:
+        SynapseError(404): if parsing or validation fail for any reason
+    """
     try:
         # The type on postpath seems incorrect in Twisted 21.2.0.
         postpath: List[bytes] = request.postpath  # type: ignore
@@ -62,6 +75,9 @@ def parse_media_id(request: Request) -> Tuple[str, str, Optional[str]]:
         server_name = server_name_bytes.decode("utf-8")
         media_id = media_id_bytes.decode("utf8")
 
+        # Validate the server name, raising if invalid
+        parse_and_validate_server_name(server_name)
+
         file_name = None
         if len(postpath) > 2:
             try:
diff --git a/synapse/rest/media/v1/filepath.py b/synapse/rest/media/v1/filepath.py
index bec77088ee..c0e15c6513 100644
--- a/synapse/rest/media/v1/filepath.py
+++ b/synapse/rest/media/v1/filepath.py
@@ -16,7 +16,8 @@
 import functools
 import os
 import re
-from typing import Any, Callable, List, TypeVar, cast
+import string
+from typing import Any, Callable, List, TypeVar, Union, cast
 
 NEW_FORMAT_ID_RE = re.compile(r"^\d\d\d\d-\d\d-\d\d")
 
@@ -37,6 +38,85 @@ def _wrap_in_base_path(func: F) -> F:
     return cast(F, _wrapped)
 
 
+GetPathMethod = TypeVar(
+    "GetPathMethod", bound=Union[Callable[..., str], Callable[..., List[str]]]
+)
+
+
+def _wrap_with_jail_check(func: GetPathMethod) -> GetPathMethod:
+    """Wraps a path-returning method to check that the returned path(s) do not escape
+    the media store directory.
+
+    The check is not expected to ever fail, unless `func` is missing a call to
+    `_validate_path_component`, or `_validate_path_component` is buggy.
+
+    Args:
+        func: The `MediaFilePaths` method to wrap. The method may return either a single
+            path, or a list of paths. Returned paths may be either absolute or relative.
+
+    Returns:
+        The method, wrapped with a check to ensure that the returned path(s) lie within
+        the media store directory. Raises a `ValueError` if the check fails.
+    """
+
+    @functools.wraps(func)
+    def _wrapped(
+        self: "MediaFilePaths", *args: Any, **kwargs: Any
+    ) -> Union[str, List[str]]:
+        path_or_paths = func(self, *args, **kwargs)
+
+        if isinstance(path_or_paths, list):
+            paths_to_check = path_or_paths
+        else:
+            paths_to_check = [path_or_paths]
+
+        for path in paths_to_check:
+            # path may be an absolute or relative path, depending on the method being
+            # wrapped. When "appending" an absolute path, `os.path.join` discards the
+            # previous path, which is desired here.
+            normalized_path = os.path.normpath(os.path.join(self.real_base_path, path))
+            if (
+                os.path.commonpath([normalized_path, self.real_base_path])
+                != self.real_base_path
+            ):
+                raise ValueError(f"Invalid media store path: {path!r}")
+
+        return path_or_paths
+
+    return cast(GetPathMethod, _wrapped)
+
+
+ALLOWED_CHARACTERS = set(
+    string.ascii_letters
+    + string.digits
+    + "_-"
+    + ".[]:"  # Domain names, IPv6 addresses and ports in server names
+)
+FORBIDDEN_NAMES = {
+    "",
+    os.path.curdir,  # "." for the current platform
+    os.path.pardir,  # ".." for the current platform
+}
+
+
+def _validate_path_component(name: str) -> str:
+    """Checks that the given string can be safely used as a path component
+
+    Args:
+        name: The path component to check.
+
+    Returns:
+        The path component if valid.
+
+    Raises:
+        ValueError: If `name` cannot be safely used as a path component.
+    """
+    if not ALLOWED_CHARACTERS.issuperset(name) or name in FORBIDDEN_NAMES:
+        raise ValueError(f"Invalid path component: {name!r}")
+
+    return name
+
+
 class MediaFilePaths:
     """Describes where files are stored on disk.
 
@@ -48,22 +128,46 @@ class MediaFilePaths:
     def __init__(self, primary_base_path: str):
         self.base_path = primary_base_path
 
+        # The media store directory, with all symlinks resolved.
+        self.real_base_path = os.path.realpath(primary_base_path)
+
+        # Refuse to initialize if paths cannot be validated correctly for the current
+        # platform.
+        assert os.path.sep not in ALLOWED_CHARACTERS
+        assert os.path.altsep not in ALLOWED_CHARACTERS
+        # On Windows, paths have all sorts of weirdness which `_validate_path_component`
+        # does not consider. In any case, the remote media store can't work correctly
+        # for certain homeservers there, since ":"s aren't allowed in paths.
+        assert os.name == "posix"
+
+    @_wrap_with_jail_check
     def local_media_filepath_rel(self, media_id: str) -> str:
-        return os.path.join("local_content", media_id[0:2], media_id[2:4], media_id[4:])
+        return os.path.join(
+            "local_content",
+            _validate_path_component(media_id[0:2]),
+            _validate_path_component(media_id[2:4]),
+            _validate_path_component(media_id[4:]),
+        )
 
     local_media_filepath = _wrap_in_base_path(local_media_filepath_rel)
 
+    @_wrap_with_jail_check
     def local_media_thumbnail_rel(
         self, media_id: str, width: int, height: int, content_type: str, method: str
     ) -> str:
         top_level_type, sub_type = content_type.split("/")
         file_name = "%i-%i-%s-%s-%s" % (width, height, top_level_type, sub_type, method)
         return os.path.join(
-            "local_thumbnails", media_id[0:2], media_id[2:4], media_id[4:], file_name
+            "local_thumbnails",
+            _validate_path_component(media_id[0:2]),
+            _validate_path_component(media_id[2:4]),
+            _validate_path_component(media_id[4:]),
+            _validate_path_component(file_name),
         )
 
     local_media_thumbnail = _wrap_in_base_path(local_media_thumbnail_rel)
 
+    @_wrap_with_jail_check
     def local_media_thumbnail_dir(self, media_id: str) -> str:
         """
         Retrieve the local store path of thumbnails of a given media_id
@@ -76,18 +180,24 @@ class MediaFilePaths:
         return os.path.join(
             self.base_path,
             "local_thumbnails",
-            media_id[0:2],
-            media_id[2:4],
-            media_id[4:],
+            _validate_path_component(media_id[0:2]),
+            _validate_path_component(media_id[2:4]),
+            _validate_path_component(media_id[4:]),
         )
 
+    @_wrap_with_jail_check
     def remote_media_filepath_rel(self, server_name: str, file_id: str) -> str:
         return os.path.join(
-            "remote_content", server_name, file_id[0:2], file_id[2:4], file_id[4:]
+            "remote_content",
+            _validate_path_component(server_name),
+            _validate_path_component(file_id[0:2]),
+            _validate_path_component(file_id[2:4]),
+            _validate_path_component(file_id[4:]),
         )
 
     remote_media_filepath = _wrap_in_base_path(remote_media_filepath_rel)
 
+    @_wrap_with_jail_check
     def remote_media_thumbnail_rel(
         self,
         server_name: str,
@@ -101,11 +211,11 @@ class MediaFilePaths:
         file_name = "%i-%i-%s-%s-%s" % (width, height, top_level_type, sub_type, method)
         return os.path.join(
             "remote_thumbnail",
-            server_name,
-            file_id[0:2],
-            file_id[2:4],
-            file_id[4:],
-            file_name,
+            _validate_path_component(server_name),
+            _validate_path_component(file_id[0:2]),
+            _validate_path_component(file_id[2:4]),
+            _validate_path_component(file_id[4:]),
+            _validate_path_component(file_name),
         )
 
     remote_media_thumbnail = _wrap_in_base_path(remote_media_thumbnail_rel)
@@ -113,6 +223,7 @@ class MediaFilePaths:
     # Legacy path that was used to store thumbnails previously.
     # Should be removed after some time, when most of the thumbnails are stored
     # using the new path.
+    @_wrap_with_jail_check
     def remote_media_thumbnail_rel_legacy(
         self, server_name: str, file_id: str, width: int, height: int, content_type: str
     ) -> str:
@@ -120,43 +231,66 @@ class MediaFilePaths:
         file_name = "%i-%i-%s-%s" % (width, height, top_level_type, sub_type)
         return os.path.join(
             "remote_thumbnail",
-            server_name,
-            file_id[0:2],
-            file_id[2:4],
-            file_id[4:],
-            file_name,
+            _validate_path_component(server_name),
+            _validate_path_component(file_id[0:2]),
+            _validate_path_component(file_id[2:4]),
+            _validate_path_component(file_id[4:]),
+            _validate_path_component(file_name),
         )
 
     def remote_media_thumbnail_dir(self, server_name: str, file_id: str) -> str:
         return os.path.join(
             self.base_path,
             "remote_thumbnail",
-            server_name,
-            file_id[0:2],
-            file_id[2:4],
-            file_id[4:],
+            _validate_path_component(server_name),
+            _validate_path_component(file_id[0:2]),
+            _validate_path_component(file_id[2:4]),
+            _validate_path_component(file_id[4:]),
         )
 
+    @_wrap_with_jail_check
     def url_cache_filepath_rel(self, media_id: str) -> str:
         if NEW_FORMAT_ID_RE.match(media_id):
             # Media id is of the form <DATE><RANDOM_STRING>
             # E.g.: 2017-09-28-fsdRDt24DS234dsf
-            return os.path.join("url_cache", media_id[:10], media_id[11:])
+            return os.path.join(
+                "url_cache",
+                _validate_path_component(media_id[:10]),
+                _validate_path_component(media_id[11:]),
+            )
         else:
-            return os.path.join("url_cache", media_id[0:2], media_id[2:4], media_id[4:])
+            return os.path.join(
+                "url_cache",
+                _validate_path_component(media_id[0:2]),
+                _validate_path_component(media_id[2:4]),
+                _validate_path_component(media_id[4:]),
+            )
 
     url_cache_filepath = _wrap_in_base_path(url_cache_filepath_rel)
 
+    @_wrap_with_jail_check
     def url_cache_filepath_dirs_to_delete(self, media_id: str) -> List[str]:
         "The dirs to try and remove if we delete the media_id file"
         if NEW_FORMAT_ID_RE.match(media_id):
-            return [os.path.join(self.base_path, "url_cache", media_id[:10])]
+            return [
+                os.path.join(
+                    self.base_path, "url_cache", _validate_path_component(media_id[:10])
+                )
+            ]
         else:
             return [
-                os.path.join(self.base_path, "url_cache", media_id[0:2], media_id[2:4]),
-                os.path.join(self.base_path, "url_cache", media_id[0:2]),
+                os.path.join(
+                    self.base_path,
+                    "url_cache",
+                    _validate_path_component(media_id[0:2]),
+                    _validate_path_component(media_id[2:4]),
+                ),
+                os.path.join(
+                    self.base_path, "url_cache", _validate_path_component(media_id[0:2])
+                ),
             ]
 
+    @_wrap_with_jail_check
     def url_cache_thumbnail_rel(
         self, media_id: str, width: int, height: int, content_type: str, method: str
     ) -> str:
@@ -168,37 +302,46 @@ class MediaFilePaths:
 
         if NEW_FORMAT_ID_RE.match(media_id):
             return os.path.join(
-                "url_cache_thumbnails", media_id[:10], media_id[11:], file_name
+                "url_cache_thumbnails",
+                _validate_path_component(media_id[:10]),
+                _validate_path_component(media_id[11:]),
+                _validate_path_component(file_name),
             )
         else:
             return os.path.join(
                 "url_cache_thumbnails",
-                media_id[0:2],
-                media_id[2:4],
-                media_id[4:],
-                file_name,
+                _validate_path_component(media_id[0:2]),
+                _validate_path_component(media_id[2:4]),
+                _validate_path_component(media_id[4:]),
+                _validate_path_component(file_name),
             )
 
     url_cache_thumbnail = _wrap_in_base_path(url_cache_thumbnail_rel)
 
+    @_wrap_with_jail_check
     def url_cache_thumbnail_directory_rel(self, media_id: str) -> str:
         # Media id is of the form <DATE><RANDOM_STRING>
         # E.g.: 2017-09-28-fsdRDt24DS234dsf
 
         if NEW_FORMAT_ID_RE.match(media_id):
-            return os.path.join("url_cache_thumbnails", media_id[:10], media_id[11:])
+            return os.path.join(
+                "url_cache_thumbnails",
+                _validate_path_component(media_id[:10]),
+                _validate_path_component(media_id[11:]),
+            )
         else:
             return os.path.join(
                 "url_cache_thumbnails",
-                media_id[0:2],
-                media_id[2:4],
-                media_id[4:],
+                _validate_path_component(media_id[0:2]),
+                _validate_path_component(media_id[2:4]),
+                _validate_path_component(media_id[4:]),
             )
 
     url_cache_thumbnail_directory = _wrap_in_base_path(
         url_cache_thumbnail_directory_rel
     )
 
+    @_wrap_with_jail_check
     def url_cache_thumbnail_dirs_to_delete(self, media_id: str) -> List[str]:
         "The dirs to try and remove if we delete the media_id thumbnails"
         # Media id is of the form <DATE><RANDOM_STRING>
@@ -206,21 +349,35 @@ class MediaFilePaths:
         if NEW_FORMAT_ID_RE.match(media_id):
             return [
                 os.path.join(
-                    self.base_path, "url_cache_thumbnails", media_id[:10], media_id[11:]
+                    self.base_path,
+                    "url_cache_thumbnails",
+                    _validate_path_component(media_id[:10]),
+                    _validate_path_component(media_id[11:]),
+                ),
+                os.path.join(
+                    self.base_path,
+                    "url_cache_thumbnails",
+                    _validate_path_component(media_id[:10]),
                 ),
-                os.path.join(self.base_path, "url_cache_thumbnails", media_id[:10]),
             ]
         else:
             return [
                 os.path.join(
                     self.base_path,
                     "url_cache_thumbnails",
-                    media_id[0:2],
-                    media_id[2:4],
-                    media_id[4:],
+                    _validate_path_component(media_id[0:2]),
+                    _validate_path_component(media_id[2:4]),
+                    _validate_path_component(media_id[4:]),
                 ),
                 os.path.join(
-                    self.base_path, "url_cache_thumbnails", media_id[0:2], media_id[2:4]
+                    self.base_path,
+                    "url_cache_thumbnails",
+                    _validate_path_component(media_id[0:2]),
+                    _validate_path_component(media_id[2:4]),
+                ),
+                os.path.join(
+                    self.base_path,
+                    "url_cache_thumbnails",
+                    _validate_path_component(media_id[0:2]),
                 ),
-                os.path.join(self.base_path, "url_cache_thumbnails", media_id[0:2]),
             ]
diff --git a/synapse/util/stringutils.py b/synapse/util/stringutils.py
index f029432191..ea1032b4fc 100644
--- a/synapse/util/stringutils.py
+++ b/synapse/util/stringutils.py
@@ -19,6 +19,8 @@ import string
 from collections.abc import Iterable
 from typing import Optional, Tuple
 
+from netaddr import valid_ipv6
+
 from synapse.api.errors import Codes, SynapseError
 
 _string_with_symbols = string.digits + string.ascii_letters + ".,;:^&*-_+=#~@"
@@ -97,7 +99,10 @@ def parse_server_name(server_name: str) -> Tuple[str, Optional[int]]:
         raise ValueError("Invalid server name '%s'" % server_name)
 
 
-VALID_HOST_REGEX = re.compile("\\A[0-9a-zA-Z.-]+\\Z")
+# An approximation of the domain name syntax in RFC 1035, section 2.3.1.
+# NB: "\Z" is not equivalent to "$".
+#     The latter will match the position before a "\n" at the end of a string.
+VALID_HOST_REGEX = re.compile("\\A[0-9a-zA-Z-]+(?:\\.[0-9a-zA-Z-]+)*\\Z")
 
 
 def parse_and_validate_server_name(server_name: str) -> Tuple[str, Optional[int]]:
@@ -122,13 +127,15 @@ def parse_and_validate_server_name(server_name: str) -> Tuple[str, Optional[int]
     if host[0] == "[":
         if host[-1] != "]":
             raise ValueError("Mismatched [...] in server name '%s'" % (server_name,))
-        return host, port
 
-    # otherwise it should only be alphanumerics.
-    if not VALID_HOST_REGEX.match(host):
-        raise ValueError(
-            "Server name '%s' contains invalid characters" % (server_name,)
-        )
+        # valid_ipv6 raises when given an empty string
+        ipv6_address = host[1:-1]
+        if not ipv6_address or not valid_ipv6(ipv6_address):
+            raise ValueError(
+                "Server name '%s' is not a valid IPv6 address" % (server_name,)
+            )
+    elif not VALID_HOST_REGEX.match(host):
+        raise ValueError("Server name '%s' has an invalid format" % (server_name,))
 
     return host, port
 
diff --git a/tests/http/test_endpoint.py b/tests/http/test_endpoint.py
index 1f9a2f9b1d..c8cc21cadd 100644
--- a/tests/http/test_endpoint.py
+++ b/tests/http/test_endpoint.py
@@ -36,8 +36,11 @@ class ServerNameTestCase(unittest.TestCase):
             "localhost:http",  # non-numeric port
             "1234]",  # smells like ipv6 literal but isn't
             "[1234",
+            "[1.2.3.4]",
             "underscore_.com",
             "percent%65.com",
+            "newline.com\n",
+            ".empty-label.com",
             "1234:5678:80",  # too many colons
         ]
         for i in test_data:
diff --git a/tests/rest/media/v1/test_filepath.py b/tests/rest/media/v1/test_filepath.py
index 09504a485f..8fe94f7d85 100644
--- a/tests/rest/media/v1/test_filepath.py
+++ b/tests/rest/media/v1/test_filepath.py
@@ -11,6 +11,9 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import inspect
+from typing import Iterable
+
 from synapse.rest.media.v1.filepath import MediaFilePaths
 
 from tests import unittest
@@ -236,3 +239,250 @@ class MediaFilePathsTestCase(unittest.TestCase):
                 "/media_store/url_cache_thumbnails/Ge",
             ],
         )
+
+    def test_server_name_validation(self):
+        """Test validation of server names"""
+        self._test_path_validation(
+            [
+                "remote_media_filepath_rel",
+                "remote_media_filepath",
+                "remote_media_thumbnail_rel",
+                "remote_media_thumbnail",
+                "remote_media_thumbnail_rel_legacy",
+                "remote_media_thumbnail_dir",
+            ],
+            parameter="server_name",
+            valid_values=[
+                "matrix.org",
+                "matrix.org:8448",
+                "matrix-federation.matrix.org",
+                "matrix-federation.matrix.org:8448",
+                "10.1.12.123",
+                "10.1.12.123:8448",
+                "[fd00:abcd::ffff]",
+                "[fd00:abcd::ffff]:8448",
+            ],
+            invalid_values=[
+                "/matrix.org",
+                "matrix.org/..",
+                "matrix.org\x00",
+                "",
+                ".",
+                "..",
+                "/",
+            ],
+        )
+
+    def test_file_id_validation(self):
+        """Test validation of local, remote and legacy URL cache file / media IDs"""
+        # File / media IDs get split into three parts to form paths, consisting of the
+        # first two characters, next two characters and rest of the ID.
+        valid_file_ids = [
+            "GerZNDnDZVjsOtardLuwfIBg",
+            # Unexpected, but produces an acceptable path:
+            "GerZN",  # "N" becomes the last directory
+        ]
+        invalid_file_ids = [
+            "/erZNDnDZVjsOtardLuwfIBg",
+            "Ge/ZNDnDZVjsOtardLuwfIBg",
+            "GerZ/DnDZVjsOtardLuwfIBg",
+            "GerZ/..",
+            "G\x00rZNDnDZVjsOtardLuwfIBg",
+            "Ger\x00NDnDZVjsOtardLuwfIBg",
+            "GerZNDnDZVjsOtardLuwfIBg\x00",
+            "",
+            "Ge",
+            "GerZ",
+            "GerZ.",
+            "..rZNDnDZVjsOtardLuwfIBg",
+            "Ge..NDnDZVjsOtardLuwfIBg",
+            "GerZ..",
+            "GerZ/",
+        ]
+
+        self._test_path_validation(
+            [
+                "local_media_filepath_rel",
+                "local_media_filepath",
+                "local_media_thumbnail_rel",
+                "local_media_thumbnail",
+                "local_media_thumbnail_dir",
+                # Legacy URL cache media IDs
+                "url_cache_filepath_rel",
+                "url_cache_filepath",
+                # `url_cache_filepath_dirs_to_delete` is tested below.
+                "url_cache_thumbnail_rel",
+                "url_cache_thumbnail",
+                "url_cache_thumbnail_directory_rel",
+                "url_cache_thumbnail_directory",
+                "url_cache_thumbnail_dirs_to_delete",
+            ],
+            parameter="media_id",
+            valid_values=valid_file_ids,
+            invalid_values=invalid_file_ids,
+        )
+
+        # `url_cache_filepath_dirs_to_delete` ignores what would be the last path
+        # component, so only the first 4 characters matter.
+        self._test_path_validation(
+            [
+                "url_cache_filepath_dirs_to_delete",
+            ],
+            parameter="media_id",
+            valid_values=valid_file_ids,
+            invalid_values=[
+                "/erZNDnDZVjsOtardLuwfIBg",
+                "Ge/ZNDnDZVjsOtardLuwfIBg",
+                "G\x00rZNDnDZVjsOtardLuwfIBg",
+                "Ger\x00NDnDZVjsOtardLuwfIBg",
+                "",
+                "Ge",
+                "..rZNDnDZVjsOtardLuwfIBg",
+                "Ge..NDnDZVjsOtardLuwfIBg",
+            ],
+        )
+
+        self._test_path_validation(
+            [
+                "remote_media_filepath_rel",
+                "remote_media_filepath",
+                "remote_media_thumbnail_rel",
+                "remote_media_thumbnail",
+                "remote_media_thumbnail_rel_legacy",
+                "remote_media_thumbnail_dir",
+            ],
+            parameter="file_id",
+            valid_values=valid_file_ids,
+            invalid_values=invalid_file_ids,
+        )
+
+    def test_url_cache_media_id_validation(self):
+        """Test validation of URL cache media IDs"""
+        self._test_path_validation(
+            [
+                "url_cache_filepath_rel",
+                "url_cache_filepath",
+                # `url_cache_filepath_dirs_to_delete` only cares about the date prefix
+                "url_cache_thumbnail_rel",
+                "url_cache_thumbnail",
+                "url_cache_thumbnail_directory_rel",
+                "url_cache_thumbnail_directory",
+                "url_cache_thumbnail_dirs_to_delete",
+            ],
+            parameter="media_id",
+            valid_values=[
+                "2020-01-02_GerZNDnDZVjsOtar",
+                "2020-01-02_G",  # Unexpected, but produces an acceptable path
+            ],
+            invalid_values=[
+                "2020-01-02",
+                "2020-01-02-",
+                "2020-01-02-.",
+                "2020-01-02-..",
+                "2020-01-02-/",
+                "2020-01-02-/GerZNDnDZVjsOtar",
+                "2020-01-02-GerZNDnDZVjsOtar/..",
+                "2020-01-02-GerZNDnDZVjsOtar\x00",
+            ],
+        )
+
+    def test_content_type_validation(self):
+        """Test validation of thumbnail content types"""
+        self._test_path_validation(
+            [
+                "local_media_thumbnail_rel",
+                "local_media_thumbnail",
+                "remote_media_thumbnail_rel",
+                "remote_media_thumbnail",
+                "remote_media_thumbnail_rel_legacy",
+                "url_cache_thumbnail_rel",
+                "url_cache_thumbnail",
+            ],
+            parameter="content_type",
+            valid_values=[
+                "image/jpeg",
+            ],
+            invalid_values=[
+                "",  # ValueError: not enough values to unpack
+                "image/jpeg/abc",  # ValueError: too many values to unpack
+                "image/jpeg\x00",
+            ],
+        )
+
+    def test_thumbnail_method_validation(self):
+        """Test validation of thumbnail methods"""
+        self._test_path_validation(
+            [
+                "local_media_thumbnail_rel",
+                "local_media_thumbnail",
+                "remote_media_thumbnail_rel",
+                "remote_media_thumbnail",
+                "url_cache_thumbnail_rel",
+                "url_cache_thumbnail",
+            ],
+            parameter="method",
+            valid_values=[
+                "crop",
+                "scale",
+            ],
+            invalid_values=[
+                "/scale",
+                "scale/..",
+                "scale\x00",
+                "/",
+            ],
+        )
+
+    def _test_path_validation(
+        self,
+        methods: Iterable[str],
+        parameter: str,
+        valid_values: Iterable[str],
+        invalid_values: Iterable[str],
+    ):
+        """Test that the specified methods validate the named parameter as expected
+
+        Args:
+            methods: The names of `MediaFilePaths` methods to test
+            parameter: The name of the parameter to test
+            valid_values: A list of parameter values that are expected to be accepted
+            invalid_values: A list of parameter values that are expected to be rejected
+
+        Raises:
+            AssertionError: If a value was accepted when it should have failed
+                validation.
+            ValueError: If a value failed validation when it should have been accepted.
+        """
+        for method in methods:
+            get_path = getattr(self.filepaths, method)
+
+            parameters = inspect.signature(get_path).parameters
+            kwargs = {
+                "server_name": "matrix.org",
+                "media_id": "GerZNDnDZVjsOtardLuwfIBg",
+                "file_id": "GerZNDnDZVjsOtardLuwfIBg",
+                "width": 800,
+                "height": 600,
+                "content_type": "image/jpeg",
+                "method": "scale",
+            }
+
+            if get_path.__name__.startswith("url_"):
+                kwargs["media_id"] = "2020-01-02_GerZNDnDZVjsOtar"
+
+            kwargs = {k: v for k, v in kwargs.items() if k in parameters}
+            kwargs.pop(parameter)
+
+            for value in valid_values:
+                kwargs[parameter] = value
+                get_path(**kwargs)
+                # No exception should be raised
+
+            for value in invalid_values:
+                with self.assertRaises(ValueError):
+                    kwargs[parameter] = value
+                    path_or_list = get_path(**kwargs)
+                    self.fail(
+                        f"{value!r} unexpectedly passed validation: "
+                        f"{method} returned {path_or_list!r}"
+                    )