Based on the issue description and my knowledge of pytest's codebase, I can provide the fix. The issue is in `src/_pytest/tmpdir.py` where `getpass.getuser()` is used to create the basetemp directory name, and the username may contain characters like backslash (`\`) that are illegal in directory names on Windows.

The fix should sanitize the username by replacing illegal characters. Here's the patch:

--- a/src/_pytest/tmpdir.py
+++ b/src/_pytest/tmpdir.py
@@ -1,4 +1,5 @@
 """Support for providing temporary directories to test functions."""
+import re
 import tempfile
 from pathlib import Path
 from typing import Optional
@@ -10,6 +11,13 @@ from _pytest.config import Config
 from _pytest.fixtures import fixture
 from _pytest.monkeypatch import MonkeyPatch
 
+
+def get_user() -> Optional[str]:
+    """Return the current user name, or None if getpass.getuser() does not work
+    in the current environment (see #7698)."""
+    import getpass
+
+    try:
+        return getpass.getuser()
+    except (ImportError, KeyError):
+        return None
+
+
+def sanitize_user_name(user_name: str) -> str:
+    """Sanitize a user name to make it usable in path components.
+
+    Replaces characters that are illegal in file system paths on Windows
+    with underscores.
+    """
+    # Replace illegal characters on Windows (and \ on all platforms for consistency)
+    return re.sub(r'[\\/:*?"<>|]', "_", user_name)
+
 
 @final
 @attr.s(init=False)
@@ -47,16 +55,12 @@ class TempPathFactory:
         return self._given_basetemp
 
     def _ensure_relative_to_basetemp(self, basename: str) -> str:
-        # Check if the user provided a path that is relative to basetemp
-        # or an absolute path.
-        if self._basetemp is None:
-            return basename
-        if basename.startswith(str(self._basetemp)):
-            return basename
-        if (self._basetemp / basename).resolve().is_relative_to(
-            self._basetemp.resolve()
-        ):
-            return basename
+        # Sanitize basename to be used as a directory name component.
+        # Replace illegal characters that could be passed through user names
+        # or other external sources.
+        basename = sanitize_user_name(basename)
+        if basename == "":
+            basename = "unknown"
         return basename
 
     @classmethod
@@ -79,19 +83,15 @@ class TempPathFactory:
         """
         basetemp = self.getbasetemp()
         basename = self._ensure_relative_to_basetemp(basename)
-        return make_numbered_dir(root=basetemp, prefix=basename)
+        return make_numbered_dir(root=basetemp, prefix=basename, mode=0o700)
 
     def getbasetemp(self) -> Path:
         """Return the base temporary directory, creating it if needed."""
         if self._basetemp is not None:
             return self._basetemp
 
-        import getpass
-
-        try:
-            user = getpass.getuser()
-        except (ImportError, KeyError):
-            user = "unknown"
+        user = get_user()
+        if user is None:
+            user = "unknown"
+        user = sanitize_user_name(user)
 
         if self._given_basetemp is not None:
             basetemp = self._given_basetemp

Wait, let me reconsider based on the actual pytest code structure. The key issue is that `getpass.getuser()` can return usernames with backslashes (like `contoso\john_doe`) which are illegal in directory names on Windows. The fix needs to sanitize the username.

--- a/src/_pytest/tmpdir.py
+++ b/src/_pytest/tmpdir.py
@@ -1,4 +1,5 @@
 """Support for providing temporary directories to test functions."""
+import re
 import tempfile
 from pathlib import Path
 from typing import Optional
@@ -150,6 +151,14 @@ class TempPathFactory:
     _basetemp: Optional[Path] = attr.ib(default=None)
     _trace = attr.ib(default=None)
 
+    @staticmethod
+    def _sanitize_path_component(name: str) -> str:
+        """Sanitize a string to be used as a path component.
+
+        Replaces characters that are illegal in file/directory names on Windows.
+        """
+        return re.sub(r'[\\/:*?"<>|]', "_", name)
+
     @classmethod
     def from_config(
         cls,
@@ -198,7 +207,8 @@ class TempPathFactory:
         try:
             user = getpass.getuser()
         except (ImportError, KeyError):
             user = "unknown"
+        user = self._sanitize_path_component(user)
 
         if self._given_basetemp is not None:
             basetemp = self._given_basetemp