diff --git a/changelog/8414.bugfix.rst b/changelog/8414.bugfix.rst
new file mode 100644
index 000000000..a8b791ae2
--- /dev/null
+++ b/changelog/8414.bugfix.rst
@@ -0,0 +1,5 @@
+pytest used to create directories under ``/tmp`` with world-readable
+permissions. This means that any user in the system was able to read
+information written by tests in temporary directories (such as those created by
+the ``tmp_path``/``tmpdir`` fixture). Now the directories are created with
+private permissions.
diff --git a/src/_pytest/pathlib.py b/src/_pytest/pathlib.py
index eaae09083..e86811f12 100644
--- a/src/_pytest/pathlib.py
+++ b/src/_pytest/pathlib.py
@@ -205,7 +205,7 @@ def _force_symlink(
         pass
 
 
-def make_numbered_dir(root: Path, prefix: str) -> Path:
+def make_numbered_dir(root: Path, prefix: str, mode: int = 0o700) -> Path:
     """Create a directory with an increased number as suffix for the given prefix."""
     for i in range(10):
         # try up to 10 times to create the folder
@@ -213,7 +213,7 @@ def make_numbered_dir(root: Path, prefix: str) -> Path:
         new_number = max_existing + 1
         new_path = root.joinpath(f"{prefix}{new_number}")
         try:
-            new_path.mkdir()
+            new_path.mkdir(mode=mode)
         except Exception:
             pass
         else:
@@ -345,13 +345,17 @@ def cleanup_numbered_dir(
 
 
 def make_numbered_dir_with_cleanup(
-    root: Path, prefix: str, keep: int, lock_timeout: float
+    root: Path,
+    prefix: str,
+    keep: int,
+    lock_timeout: float,
+    mode: int,
 ) -> Path:
     """Create a numbered dir with a cleanup lock and remove old ones."""
     e = None
     for i in range(10):
         try:
-            p = make_numbered_dir(root, prefix)
+            p = make_numbered_dir(root, prefix, mode)
             lock_path = create_cleanup_lock(p)
             register_cleanup_lock_removal(lock_path)
         except Exception as exc:
diff --git a/src/_pytest/pytester.py b/src/_pytest/pytester.py
index 968a53651..5a29c8eae 100644
--- a/src/_pytest/pytester.py
+++ b/src/_pytest/pytester.py
@@ -1456,7 +1456,7 @@ class Pytester:
             :py:class:`Pytester.TimeoutExpired`.
         """
         __tracebackhide__ = True
-        p = make_numbered_dir(root=self.path, prefix="runpytest-")
+        p = make_numbered_dir(root=self.path, prefix="runpytest-", mode=0o700)
         args = ("--basetemp=%s" % p,) + args
         plugins = [x for x in self.plugins if isinstance(x, str)]
         if plugins:
@@ -1475,7 +1475,7 @@ class Pytester:
         The pexpect child is returned.
         """
         basetemp = self.path / "temp-pexpect"
-        basetemp.mkdir()
+        basetemp.mkdir(mode=0o700)
         invoke = " ".join(map(str, self._getpytestargs()))
         cmd = f"{invoke} --basetemp={basetemp} {string}"
         return self.spawn(cmd, expect_timeout=expect_timeout)
diff --git a/src/_pytest/tmpdir.py b/src/_pytest/tmpdir.py
index 309754fdd..275ec7334 100644
--- a/src/_pytest/tmpdir.py
+++ b/src/_pytest/tmpdir.py
@@ -94,14 +94,14 @@ class TempPathFactory:
         basename = self._ensure_relative_to_basetemp(basename)
         if not numbered:
             p = self.getbasetemp().joinpath(basename)
-            p.mkdir()
+            p.mkdir(mode=0o700)
         else:
-            p = make_numbered_dir(root=self.getbasetemp(), prefix=basename)
+            p = make_numbered_dir(root=self.getbasetemp(), prefix=basename, mode=0o700)
             self._trace("mktemp", p)
         return p
 
     def getbasetemp(self) -> Path:
-        """Return base temporary directory."""
+        """Return the base temporary directory, creating it if needed."""
         if self._basetemp is not None:
             return self._basetemp
 
@@ -109,7 +109,7 @@ class TempPathFactory:
             basetemp = self._given_basetemp
             if basetemp.exists():
                 rm_rf(basetemp)
-            basetemp.mkdir()
+            basetemp.mkdir(mode=0o700)
             basetemp = basetemp.resolve()
         else:
             from_env = os.environ.get("PYTEST_DEBUG_TEMPROOT")
@@ -119,13 +119,17 @@ class TempPathFactory:
             # make_numbered_dir() call
             rootdir = temproot.joinpath(f"pytest-of-{user}")
             try:
-                rootdir.mkdir(exist_ok=True)
+                rootdir.mkdir(mode=0o700, exist_ok=True)
             except OSError:
                 # getuser() likely returned illegal characters for the platform, use unknown back off mechanism
                 rootdir = temproot.joinpath("pytest-of-unknown")
-                rootdir.mkdir(exist_ok=True)
+                rootdir.mkdir(mode=0o700, exist_ok=True)
             basetemp = make_numbered_dir_with_cleanup(
-                prefix="pytest-", root=rootdir, keep=3, lock_timeout=LOCK_TIMEOUT
+                prefix="pytest-",
+                root=rootdir,
+                keep=3,
+                lock_timeout=LOCK_TIMEOUT,
+                mode=0o700,
             )
         assert basetemp is not None, basetemp
         self._basetemp = basetemp
diff --git a/testing/test_tmpdir.py b/testing/test_tmpdir.py
index 63450eab2..2ae23ffca 100644
--- a/testing/test_tmpdir.py
+++ b/testing/test_tmpdir.py
@@ -454,3 +454,19 @@ def test_tmp_path_factory_handles_invalid_dir_characters(
     monkeypatch.setattr(tmp_path_factory, "_given_basetemp", None)
     p = tmp_path_factory.getbasetemp()
     assert "pytest-of-unknown" in str(p)
+
+
+@pytest.mark.skipif(not hasattr(os, "getuid"), reason="checks unix permissions")
+def test_tmp_path_factory_create_directory_with_safe_permissions(
+    tmp_path: Path, monkeypatch: MonkeyPatch
+) -> None:
+    """Verify that pytest creates directories under /tmp with private permissions."""
+    # Use the test's tmp_path as the system temproot (/tmp).
+    monkeypatch.setenv("PYTEST_DEBUG_TEMPROOT", str(tmp_path))
+    tmp_factory = TempPathFactory(None, lambda *args: None, _ispytest=True)
+    basetemp = tmp_factory.getbasetemp()
+
+    # No world-readable permissions.
+    assert (basetemp.stat().st_mode & 0o077) == 0
+    # Parent too (pytest-of-foo).
+    assert (basetemp.parent.stat().st_mode & 0o077) == 0