diff --git a/docs/changelog/199.bugfix.rst b/docs/changelog/199.bugfix.rst
new file mode 100644
index 0000000..bfed634
--- /dev/null
+++ b/docs/changelog/199.bugfix.rst
@@ -0,0 +1 @@
+Reject format widths in interpreter sysconfig paths before they allocate oversized strings.
diff --git a/src/python_discovery/_py_info.py b/src/python_discovery/_py_info.py
index ecf6cc6..e3d4b23 100644
--- a/src/python_discovery/_py_info.py
+++ b/src/python_discovery/_py_info.py
@@ -11,7 +11,7 @@
 from collections import OrderedDict
 from itertools import product
 from os.path import normcase, realpath
-from string import digits
+from string import Formatter, digits
 from typing import TYPE_CHECKING, ClassVar, Final, NamedTuple, TypedDict
 
 from ._py_info_collect import PythonInfoCollector
@@ -136,6 +136,9 @@ def sysconfig_path(self, key: str, config_var: dict[str, str | int | None] | Non
         pattern = self.sysconfig_paths.get(key)
         if pattern is None:
             return ""
+        if any(format_spec for _, _, format_spec, _ in Formatter().parse(pattern)):
+            msg = f"sysconfig path {key!r} contains format specifiers"
+            raise ValueError(msg)
         if config_var is None:
             config_var = self.sysconfig_vars
         else:
diff --git a/tests/test_py_info_extra.py b/tests/test_py_info_extra.py
index b7642bd..f2548b7 100644
--- a/tests/test_py_info_extra.py
+++ b/tests/test_py_info_extra.py
@@ -303,6 +303,14 @@ def test_install_path() -> None:
     assert isinstance(CURRENT.install_path("purelib"), str)
 
 
+def test_install_path_rejects_format_width() -> None:
+    info = copy.deepcopy(CURRENT)
+    info.distutils_install = {}
+    info.sysconfig_paths["purelib"] = "{base:>1000000}/lib"
+    with pytest.raises(ValueError, match="format specifiers"):
+        info.install_path("purelib")
+
+
 def test_system_include() -> None:
     result = CURRENT.system_include
     assert isinstance(result, str)