s3api: Prevent XXE injections

Previously, clients could use XML external entities (XXEs) to read
arbitrary files from proxy-servers and inject the content into the
request. Since many S3 APIs reflect request content back to the user,
this could be used to extract any secrets that the swift user could
read, such as tempauth credentials, keymaster secrets, etc.

Now, disable entity resolution -- any unknown entities will be replaced
with an empty string. Without resolving the entities, the request is
still processed.

[CVE-2022-47950]

Closes-Bug: #1998625
Co-Authored-By: Romain de Joux <romain.de-joux@ovhcloud.com>
Change-Id: I84494123cfc85e234098c554ecd3e77981f8a096
(cherry picked from commit b8467e190f6fc67fd8fb6a8c5e32b2aa6a10fd8e)

3 files changed, 274 insertions, 1 deletions

diff --git a/swift/common/middleware/s3api/etree.py b/swift/common/middleware/s3api/etree.py
index 987b84a14..e16b75340 100644
--- a/swift/common/middleware/s3api/etree.py
+++ b/swift/common/middleware/s3api/etree.py
@@ -130,7 +130,7 @@ class _Element(lxml.etree.ElementBase):
 
 
 parser_lookup = lxml.etree.ElementDefaultClassLookup(element=_Element)
-parser = lxml.etree.XMLParser()
+parser = lxml.etree.XMLParser(resolve_entities=False, no_network=True)
 parser.set_element_class_lookup(parser_lookup)
 
 Element = parser.makeelement
diff --git a/test/functional/s3api/test_xxe_injection.py b/test/functional/s3api/test_xxe_injection.py
new file mode 100644
index 000000000..46297a968
--- /dev/null
+++ b/test/functional/s3api/test_xxe_injection.py
@@ -0,0 +1,233 @@
+#!/usr/bin/env python
+# Copyright (c) 2022 OpenStack Foundation
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# 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 requests
+
+import botocore
+
+import test.functional as tf
+from test.functional.s3api import S3ApiBaseBoto3
+
+
+def setUpModule():
+    tf.setup_package()
+
+
+def tearDownModule():
+    tf.teardown_package()
+
+
+class TestS3ApiXxeInjection(S3ApiBaseBoto3):
+
+    def setUp(self):
+        super(TestS3ApiXxeInjection, self).setUp()
+        self.bucket = 'test-s3api-xxe-injection'
+
+    def _create_bucket(self, **kwargs):
+        resp = self.conn.create_bucket(Bucket=self.bucket, **kwargs)
+        response_metadata = resp.pop('ResponseMetadata', {})
+        self.assertEqual(200, response_metadata.get('HTTPStatusCode'))
+
+    @staticmethod
+    def _clear_data(request, **_kwargs):
+        if 'Content-MD5' in request.headers:
+            del request.headers['Content-MD5']
+        request.data = b''
+
+    def _presign_url(self, method, key=None, **kwargs):
+        params = {
+            'Bucket': self.bucket
+        }
+        if key:
+            params['Key'] = key
+        params.update(kwargs)
+        try:
+            # https://github.com/boto/boto3/issues/2192
+            self.conn.meta.events.register(
+                'before-sign.s3.*', self._clear_data)
+            return self.conn.generate_presigned_url(
+                method, Params=params, ExpiresIn=60)
+        finally:
+            self.conn.meta.events.unregister(
+                'before-sign.s3.*', self._clear_data)
+
+    def test_put_bucket_acl(self):
+        if not tf.cluster_info['s3api'].get('s3_acl'):
+            self.skipTest('s3_acl must be enabled')
+
+        self._create_bucket()
+
+        url = self._presign_url('put_bucket_acl')
+        resp = requests.put(url, data="""
+<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/swift/swift.conf"> ]>
+<AccessControlPolicy xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
+<Owner>
+    <DisplayName>test:tester</DisplayName>
+    <ID>test:tester</ID>
+</Owner>
+<AccessControlList>
+    <Grant>
+        <Grantee xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="CanonicalUser">
+            <DisplayName>name&xxe;</DisplayName>
+            <ID>id&xxe;</ID>
+        </Grantee>
+        <Permission>WRITE</Permission>
+    </Grant>
+</AccessControlList>
+</AccessControlPolicy>
+""")  # noqa: E501
+        self.assertEqual(200, resp.status_code)
+        self.assertNotIn(b'xxe', resp.content)
+        self.assertNotIn(b'[swift-hash]', resp.content)
+
+        acl = self.conn.get_bucket_acl(Bucket=self.bucket)
+        response_metadata = acl.pop('ResponseMetadata', {})
+        self.assertEqual(200, response_metadata.get('HTTPStatusCode'))
+        self.assertDictEqual({
+            'Owner': {
+                'DisplayName': 'test:tester',
+                'ID': 'test:tester'
+            },
+            'Grants': [
+                {
+                    'Grantee': {
+                        'DisplayName': 'id',
+                        'ID': 'id',
+                        'Type': 'CanonicalUser'
+                    },
+                    'Permission': 'WRITE'
+                }
+            ]
+        }, acl)
+
+    def test_create_bucket(self):
+        url = self._presign_url('create_bucket')
+        resp = requests.put(url, data="""
+<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/swift/swift.conf"> ]>
+<CreateBucketConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
+    <LocationConstraint>&xxe;</LocationConstraint>
+</CreateBucketConfiguration>
+""")  # noqa: E501
+        self.assertEqual(400, resp.status_code)
+        self.assertNotIn(b'xxe', resp.content)
+        self.assertNotIn(b'[swift-hash]', resp.content)
+
+        self.assertRaisesRegexp(
+            botocore.exceptions.ClientError, 'Not Found',
+            self.conn.head_bucket, Bucket=self.bucket)
+
+    def test_delete_objects(self):
+        self._create_bucket()
+
+        url = self._presign_url(
+            'delete_objects',
+            Delete={
+                'Objects': [
+                    {
+                        'Key': 'string',
+                        'VersionId': 'string'
+                    }
+                ]
+            })
+        body = """
+<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/swift/swift.conf"> ]>
+<Delete xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
+    <Object>
+        <Key>&xxe;</Key>
+    </Object>
+</Delete>
+"""
+        body = body.encode('utf-8')
+        resp = requests.post(url, data=body)
+        self.assertEqual(400, resp.status_code, resp.content)
+        self.assertNotIn(b'xxe', resp.content)
+        self.assertNotIn(b'[swift-hash]', resp.content)
+
+    def test_complete_multipart_upload(self):
+        self._create_bucket()
+
+        resp = self.conn.create_multipart_upload(
+            Bucket=self.bucket, Key='test')
+        response_metadata = resp.pop('ResponseMetadata', {})
+        self.assertEqual(200, response_metadata.get('HTTPStatusCode'))
+        uploadid = resp.get('UploadId')
+
+        try:
+            url = self._presign_url(
+                'complete_multipart_upload',
+                Key='key',
+                MultipartUpload={
+                    'Parts': [
+                        {
+                            'ETag': 'string',
+                            'PartNumber': 1
+                        }
+                    ],
+                },
+                UploadId=uploadid)
+            resp = requests.post(url, data="""
+<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/swift/swift.conf"> ]>
+<CompleteMultipartUpload xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
+   <Part>
+      <ETag>"{uploadid}"</ETag>
+      <PartNumber>&xxe;</PartNumber>
+   </Part>
+</CompleteMultipartUpload>
+""")  # noqa: E501
+            self.assertEqual(404, resp.status_code)
+            self.assertNotIn(b'xxe', resp.content)
+            self.assertNotIn(b'[swift-hash]', resp.content)
+
+            resp = requests.post(url, data="""
+<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/swift/swift.conf"> ]>
+<CompleteMultipartUpload xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
+   <Part>
+      <ETag>"&xxe;"</ETag>
+      <PartNumber>1</PartNumber>
+   </Part>
+</CompleteMultipartUpload>
+""")  # noqa: E501
+            self.assertEqual(404, resp.status_code)
+            self.assertNotIn(b'xxe', resp.content)
+            self.assertNotIn(b'[swift-hash]', resp.content)
+        finally:
+            resp = self.conn.abort_multipart_upload(
+                Bucket=self.bucket, Key='test', UploadId=uploadid)
+            response_metadata = resp.pop('ResponseMetadata', {})
+            self.assertEqual(204, response_metadata.get('HTTPStatusCode'))
+
+    def test_put_bucket_versioning(self):
+        self._create_bucket()
+
+        url = self._presign_url(
+            'put_bucket_versioning',
+            VersioningConfiguration={
+                'Status': 'Enabled'
+            })
+        resp = requests.put(url, data="""
+<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/swift/swift.conf"> ]>
+<VersioningConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
+    <Status>&xxe;</Status>
+</VersioningConfiguration>
+""")  # noqa: E501
+        self.assertEqual(400, resp.status_code)
+        self.assertNotIn(b'xxe', resp.content)
+        self.assertNotIn(b'[swift-hash]', resp.content)
+
+        versioning = self.conn.get_bucket_versioning(Bucket=self.bucket)
+        response_metadata = versioning.pop('ResponseMetadata', {})
+        self.assertEqual(200, response_metadata.get('HTTPStatusCode'))
+        self.assertDictEqual({}, versioning)
diff --git a/test/unit/common/middleware/s3api/test_multi_delete.py b/test/unit/common/middleware/s3api/test_multi_delete.py
index e4c3ecf09..260f47492 100644
--- a/test/unit/common/middleware/s3api/test_multi_delete.py
+++ b/test/unit/common/middleware/s3api/test_multi_delete.py
@@ -501,6 +501,7 @@ class TestS3ApiMultiDelete(S3ApiTestCase):
                             body=body)
         status, headers, body = self.call_s3api(req)
         self.assertEqual(status.split()[0], '200')
+        self.assertIn(b'<Error><Key>Key1</Key><Code>Server Error</Code>', body)
 
     def _test_object_multi_DELETE(self, account):
         self.keys = ['Key1', 'Key2']
@@ -558,6 +559,45 @@ class TestS3ApiMultiDelete(S3ApiTestCase):
         elem = fromstring(body)
         self.assertEqual(len(elem.findall('Deleted')), len(self.keys))
 
+    def test_object_multi_DELETE_with_system_entity(self):
+        self.keys = ['Key1', 'Key2']
+        self.swift.register(
+            'DELETE', '/v1/AUTH_test/bucket/%s' % self.keys[0],
+            swob.HTTPNotFound, {}, None)
+        self.swift.register(
+            'DELETE', '/v1/AUTH_test/bucket/%s' % self.keys[1],
+            swob.HTTPNoContent, {}, None)
+
+        elem = Element('Delete')
+        for key in self.keys:
+            obj = SubElement(elem, 'Object')
+            SubElement(obj, 'Key').text = key
+        body = tostring(elem, use_s3ns=False)
+        body = body.replace(
+            b'?>\n',
+            b'?>\n<!DOCTYPE foo '
+            b'[<!ENTITY ent SYSTEM "file:///etc/passwd"> ]>\n',
+        ).replace(b'>Key1<', b'>Key1&ent;<')
+        content_md5 = (
+            base64.b64encode(md5(body, usedforsecurity=False).digest())
+            .strip())
+
+        req = Request.blank('/bucket?delete',
+                            environ={'REQUEST_METHOD': 'POST'},
+                            headers={
+                                'Authorization': 'AWS test:full_control:hmac',
+                                'Date': self.get_date_header(),
+                                'Content-MD5': content_md5},
+                            body=body)
+        req.date = datetime.now()
+        req.content_type = 'text/plain'
+
+        status, headers, body = self.call_s3api(req)
+        self.assertEqual(status, '200 OK', body)
+        self.assertIn(b'<Deleted><Key>Key2</Key></Deleted>', body)
+        self.assertNotIn(b'root:/root', body)
+        self.assertIn(b'<Deleted><Key>Key1</Key></Deleted>', body)
+
     def _test_no_body(self, use_content_length=False,
                       use_transfer_encoding=False, string_to_md5=b''):
         content_md5 = (base64.b64encode(