fix(auth): compare a short secret whole when scanning for a reflected credential

The reflection scan only ever compared eight-character windows of the response
against the secret, so a secret with fewer credential characters than that could
never match once an endpoint echoed it percent-encoded or split up, and the
verbatim check needs the raw form. Cap the run at the secret's length so a short
client secret is compared whole in every wire shape.

litellm/llms/base_llm/auth/token_exchange.py

@@ -169,17 +169,23 @@ def _drop_reflected_assertion(rendered: str, assertion: SecretStr | None) -> str
 def _shares_a_credential_run(rendered: str, compacted_secret: str) -> bool:
     """``unquote`` covers a credential sent form-encoded, without every caller enumerating that
     shape for itself: percent-escaping is reversible and applies to any field, query string
-    included."""
+    included.
+
+    A secret shorter than the probe run is compared whole: a window longer than the secret can
+    never be found inside it, which would leave a short client secret unprotected in every shape
+    but the verbatim one.
+    """
     # unquote covers %XX; unquote_plus additionally covers the "+" a form-encoded body uses for a
     # space. Both are kept rather than only the wider one, because "+" is a base64 character and
     # decoding it away would lose a run that the undecoded candidate still matches on.
+    run: Final = min(_REFLECTION_MIN_RUN, len(compacted_secret))
     compacted_candidates: Final = tuple(
         _CREDENTIAL_CHARS.sub("", candidate) for candidate in (rendered, unquote(rendered), unquote_plus(rendered))
     )
     return any(
-        compacted[start : start + _REFLECTION_MIN_RUN] in compacted_secret
+        compacted[start : start + run] in compacted_secret
         for compacted in compacted_candidates
-        for start in range(len(compacted) - _REFLECTION_MIN_RUN + 1)
+        for start in range(len(compacted) - run + 1)
     )
 
 

tests/test_litellm/llms/base_llm/auth/test_token_exchange.py

@@ -703,6 +703,19 @@ class TestRedactionAndCaps:
 
         assert "short-secret" not in result.redacted_body
 
+    def test_a_short_secret_echoed_in_its_wire_shape_is_dropped(self):
+        """Regression: the run scan only ever compared eight-character windows, so a secret
+        with fewer credential characters than that could never match once it came back
+        percent-encoded rather than verbatim, and the whole-value check needs the raw form."""
+        secret = SecretStr("p@ss w0rd!")
+        echoed = quote(secret.get_secret_value(), safe="")
+        body = {"error": "invalid_client", "error_description": f"rejected {echoed}"}
+
+        assert secret.get_secret_value() not in echoed
+        result = redact_oauth_error_body(400, json.dumps(body), secret)
+
+        assert echoed not in result.redacted_body
+
     def test_an_unrelated_body_is_not_falsely_redacted(self):
         """The scan must not fire on a body that merely shares short runs with the assertion."""
         assertion = SecretStr("eyJhbGciOiJSUzI1NiJ9." + "Z" * 60 + ".signature")