diff --git a/CHANGELOG.md b/CHANGELOG.md
index 11c78475..5c652d12 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,11 @@
 
 <!-- ✨ You do not need to add a pull request reference or an author, this will be added automatically by CI. ✨ -->
 
+ - If a variable's value meets certain entropy criteria and matches an environment variable value,
+   pdoc will now emit a warning and display the variable's name as a placeholder instead.
+   This heuristic is meant to prevent accidental leakage of environment secrets and can be disabled by setting
+   `PDOC_DISPLAY_ENV_VARS=1`.
+   ([#622](https://github.com/mitmproxy/pdoc/pull/622), @mhils)
 
 ## 2023-09-10: pdoc 14.1.0
 
diff --git a/pdoc/doc.py b/pdoc/doc.py
index 1c74481f..2f00b1c8 100644
--- a/pdoc/doc.py
+++ b/pdoc/doc.py
@@ -1106,6 +1106,23 @@ def default_value_str(self) -> str:
         if self.default_value is empty:
             return ""
 
+        # This is not perfect, but a solid attempt at preventing accidental leakage of secrets.
+        # If you have input on how to improve the heuristic, please send a pull request!
+        value_taken_from_env_var = (
+            isinstance(self.default_value, str)
+            and len(self.default_value) >= 8
+            and self.default_value in _environ_lookup()
+        )
+        if value_taken_from_env_var and not os.environ.get("PDOC_DISPLAY_ENV_VARS", ""):
+            env_var = "$" + _environ_lookup()[self.default_value]
+            warnings.warn(
+                f"The default value of {self.fullname} matches the {env_var} environment variable. "
+                f"To prevent accidental leakage of secrets, the default value is not displayed. "
+                f"Disable this behavior by setting PDOC_DISPLAY_ENV_VARS=1 as an environment variable.",
+                RuntimeWarning,
+            )
+            return env_var
+
         try:
             pretty = repr(self.default_value)
         except Exception as e:
@@ -1124,6 +1141,14 @@ def annotation_str(self) -> str:
             return ""
 
 
+@cache
+def _environ_lookup():
+    """
+    A reverse lookup of os.environ. This is a cached function so that it is evaluated lazily.
+    """
+    return {value: key for key, value in os.environ.items()}
+
+
 class _PrettySignature(inspect.Signature):
     """
     A subclass of `inspect.Signature` that pads __str__ over several lines
diff --git a/test/test_doc.py b/test/test_doc.py
index 84d7186d..98208460 100644
--- a/test/test_doc.py
+++ b/test/test_doc.py
@@ -11,6 +11,7 @@
 from pdoc.doc import Class
 from pdoc.doc import Module
 from pdoc.doc import Variable
+from pdoc.doc import _environ_lookup
 from pdoc.doc_types import empty
 
 here = Path(__file__).parent
@@ -143,3 +144,34 @@ def test_raising_submodules():
             assert m.submodules
     finally:
         f.write_bytes(b"# syntax error will be inserted by test here\n")
+
+
+def test_default_value_masks_env_vars(monkeypatch):
+    monkeypatch.setenv("SUPER_SECRET_TOKEN", "correct horse battery staple")
+    monkeypatch.setenv("VERSION_NUMBER", "42.0.1")
+    _environ_lookup.cache_clear()
+    try:
+        v1 = Variable(
+            "module",
+            "var",
+            taken_from=("module", "var"),
+            docstring="",
+            annotation=empty,
+            default_value="correct horse battery staple",
+        )
+        with pytest.warns(
+            match=r"The default value of module.var matches the \$SUPER_SECRET_TOKEN environment variable."
+        ):
+            assert v1.default_value_str == "$SUPER_SECRET_TOKEN"
+
+        v2 = Variable(
+            "module",
+            "version",
+            taken_from=("module", "version"),
+            docstring="",
+            annotation=empty,
+            default_value="42.0.1",
+        )
+        assert v2.default_value_str == "'42.0.1'"
+    finally:
+        _environ_lookup.cache_clear()