@@ -18,6 +18,7 @@
 #define PY_SSIZE_T_CLEAN
 #include "Python.h"
 
+#include <limits.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -105,18 +106,25 @@ static int buffer_grow(buffer_t buffer, int min_length) {
  * Return non-zero and sets MemoryError on allocation failure.
  * Return non-zero and sets ValueError if `size` would exceed 2GiB. */
 static int buffer_assure_space(buffer_t buffer, int size) {
-    int new_size = buffer->position + size;
-    /* Check for overflow. */
-    if (new_size < buffer->position) {
+    long long new_size;
+    if (size < 0) {
         PyErr_SetString(PyExc_ValueError,
                         "Document would overflow BSON size limit");
         return 1;
     }
 
-    if (new_size <= buffer->size) {
+    /* Compute in a wider type so the addition cannot overflow `int`. */
+    new_size = (long long)buffer->position + (long long)size;
+    if (new_size > INT_MAX) {
+        PyErr_SetString(PyExc_ValueError,
+                        "Document would overflow BSON size limit");
+        return 1;
+    }
+
+    if ((int)new_size <= buffer->size) {
         return 0;
     }
-    return buffer_grow(buffer, new_size);
+    return buffer_grow(buffer, (int)new_size);
 }
 
 /* Save `size` bytes from the current position in `buffer` (and grow if needed).
 
@@ -1,6 +1,24 @@
 Changelog
 =========
 
+Changes in Version 4.18.2 (2026/09/24)
+--------------------------------------
+
+Version 4.18.2 is a bug fix release.
+
+- Hardened the bson buffer size guard against signed integer overflow.
+- Fixed connection string parsing to percent-decode each host individually.
+- Client-side field level encryption now rejects a KMS endpoint ending in
+  ``.sock``.
+
+Issues Resolved
+...............
+
+See the `PyMongo 4.18.2 release notes in JIRA`_ for the list of resolved issues
+in this release.
+
+.. _PyMongo 4.18.2 release notes in JIRA: https://jira.mongodb.org/secure/ReleaseNote.jspa?projectId=10004&version=52896
+
 Changes in Version 4.18.1 (2026/09/10)
 --------------------------------------
 
@@ -9,7 +27,7 @@ Version 4.18.1 is a bug fix release.
 - Use an exact match for the file ID in GridFS delete methods
   (`CVE-2026-88029`_).
 
-.. CVE-2026-88029: https://www.cve.org/CVERecord?id=CVE-2026-88029
+.. _CVE-2026-88029: https://www.cve.org/CVERecord?id=CVE-2026-88029
 
 Changes in Version 4.18.0 (2026/09/03)
 --------------------------------------
 
@@ -19,7 +19,7 @@
 import re
 from typing import Union
 
-__version__ = "4.18.2.dev0"
+__version__ = "4.18.2"
 
 
 def get_version_tuple(version: str) -> tuple[Union[int, str], ...]:
 
@@ -187,6 +187,8 @@ async def kms_request(self, kms_context: MongoCryptKmsContext) -> None:
             ssl_context=ctx,
         )
         address = parse_host(endpoint, _HTTPS_PORT)
+        if address[0].endswith(".sock"):
+            raise ConfigurationError(f"Invalid KMS endpoint {endpoint!r}")
         sleep_u = kms_context.usleep
         if sleep_u:
             sleep_sec = float(sleep_u) / 1e6
 
@@ -18,7 +18,6 @@
 from __future__ import annotations
 
 from typing import Any, Optional
-from urllib.parse import unquote_plus
 
 from pymongo.asynchronous.srv_resolver import _SrvResolver
 from pymongo.common import SRV_SERVICE_NAME, _CaseInsensitiveDictionary
@@ -159,7 +158,6 @@ async def _parse_srv(
     else:
         hosts = host_part
 
-    hosts = unquote_plus(hosts)
     srv_max_hosts = srv_max_hosts or options.get("srvMaxHosts")
     srv_allowed_hosts_suffix = srv_allowed_hosts_suffix or options.get("srvAllowedHostsSuffix")
     if is_srv:
 
@@ -186,6 +186,8 @@ def kms_request(self, kms_context: MongoCryptKmsContext) -> None:
             ssl_context=ctx,
         )
         address = parse_host(endpoint, _HTTPS_PORT)
+        if address[0].endswith(".sock"):
+            raise ConfigurationError(f"Invalid KMS endpoint {endpoint!r}")
         sleep_u = kms_context.usleep
         if sleep_u:
             sleep_sec = float(sleep_u) / 1e6
 
@@ -18,7 +18,6 @@
 from __future__ import annotations
 
 from typing import Any, Optional
-from urllib.parse import unquote_plus
 
 from pymongo.common import SRV_SERVICE_NAME, _CaseInsensitiveDictionary
 from pymongo.errors import ConfigurationError, InvalidURI
@@ -159,7 +158,6 @@ def _parse_srv(
     else:
         hosts = host_part
 
-    hosts = unquote_plus(hosts)
     srv_max_hosts = srv_max_hosts or options.get("srvMaxHosts")
     srv_allowed_hosts_suffix = srv_allowed_hosts_suffix or options.get("srvAllowedHostsSuffix")
     if is_srv:
 
@@ -450,10 +450,23 @@ def split_hosts(hosts: str, default_port: Optional[int] = DEFAULT_PORT) -> list[
         if not entity:
             raise ConfigurationError("Empty host (or extra comma in host list)")
         port = default_port
-        # Unix socket entities don't have ports
+        node = entity
+        # Decoding happens per entity, after splitting on ",".
         if entity.endswith(".sock"):
+            # Unix socket entities don't have ports. Socket paths are the
+            # only host identifiers permitted to contain reserved
+            # characters (e.g. "/") that require escaping.
+            node = unquote_plus(entity)
             port = None
-        nodes.append(parse_host(entity, port))
+        elif entity.startswith("["):
+            # An IPv6 zone index is escaped as "%25" (RFC 6874).
+            node = entity.replace("%25", "%")
+        elif "%" in entity:
+            raise InvalidURI(
+                "Percent-encoding is only allowed in Unix domain socket paths "
+                f"and IPv6 zone indexes, not in hostnames: {entity}"
+            )
+        nodes.append(parse_host(node, port))
     return nodes
 
 
@@ -576,7 +589,6 @@ def _validate_uri(
     if "/" in hosts:
         raise InvalidURI(f"Any '/' in a unix domain socket must be percent-encoded: {host_part}")
 
-    hosts = unquote_plus(hosts)
     fqdn = None
     srv_max_hosts = srv_max_hosts or options.get("srvMaxHosts")
     if is_srv:
 
@@ -1299,6 +1299,14 @@ async def test_04_kmip_endpoint_invalid_port(self):
         with self.assertRaisesRegex(EncryptionError, "localhost:12345"):
             await self.client_encryption.create_data_key("kmip", master_key=master_key)
 
+    async def test_kmip_endpoint_unix_socket_rejected(self):
+        # PYTHON-5990: a masterKey.endpoint ending in ".sock" must not be
+        # treated as a Unix domain socket path. KMS endpoints must be a TCP
+        # host[:port].
+        master_key = {"keyId": "1", "endpoint": "example.sock"}
+        with self.assertRaisesRegex(EncryptionError, "Invalid KMS endpoint"):
+            await self.client_encryption.create_data_key("kmip", master_key=master_key)
+
     @unittest.skipUnless(any(AWS_CREDS.values()), "AWS environment credentials are not set")
     async def test_05_aws_endpoint_wrong_region(self):
         master_key = {
 
@@ -691,6 +691,14 @@ def test_overflow(self):
         self.assertTrue(encode({"x": -9223372036854775808}))
         self.assertRaises(OverflowError, encode, {"x": -9223372036854775809})
 
+    @unittest.skipUnless(bson.has_c(), "This test requires the C extension")
+    def test_encode_size_limit(self):
+        # PYTHON-5996: encoding must raise when a document's encoded size
+        # exceeds the BSON size limit.
+        big_value = "a" * (1 << 30)
+        with self.assertRaises(ValueError):
+            encode({"a": big_value, "b": big_value, "c": big_value})
+
     def test_small_long_encode_decode(self):
         encoded1 = encode({"x": 256})
         decoded1 = decode(encoded1)["x"]
Original file line number	Diff line number	Diff line change
`@@ -187,6 +187,8 @@ async def kms_request(self, kms_context: MongoCryptKmsContext) -> None:`
`187`	`187`	`ssl_context=ctx,`
`188`	`188`	`)`
`189`	`189`	`address = parse_host(endpoint, _HTTPS_PORT)`
	`190`	`+ if address[0].endswith(".sock"):`
	`191`	`+ raise ConfigurationError(f"Invalid KMS endpoint {endpoint!r}")`
`190`	`192`	`sleep_u = kms_context.usleep`
`191`	`193`	`if sleep_u:`
`192`	`194`	`sleep_sec = float(sleep_u) / 1e6`
Original file line number	Diff line number	Diff line change
`@@ -186,6 +186,8 @@ def kms_request(self, kms_context: MongoCryptKmsContext) -> None:`
`186`	`186`	`ssl_context=ctx,`
`187`	`187`	`)`
`188`	`188`	`address = parse_host(endpoint, _HTTPS_PORT)`
	`189`	`+ if address[0].endswith(".sock"):`
	`190`	`+ raise ConfigurationError(f"Invalid KMS endpoint {endpoint!r}")`
`189`	`191`	`sleep_u = kms_context.usleep`
`190`	`192`	`if sleep_u:`
`191`	`193`	`sleep_sec = float(sleep_u) / 1e6`