Merge tag 'v1.47.1'

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).

1 files changed, 14 insertions, 7 deletions

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