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tpap: harden pre-auth DoS + constant-time confirm-MAC compare #9

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tpap: harden pre-auth DoS + constant-time confirm-MAC compare #9

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10 changes: 8 additions & 2 deletions kasa/transports/tpaptransport.py
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Original file line number Diff line number Diff line change
Expand Up @@ -54,6 +54,10 @@ class TpapEncryptionSession:
PAKE_CONTEXT_TAG = b"PAKE V1"
TAG_LEN = 16
NONCE_LEN = 12
# PBKDF2-HMAC-SHA256 iteration count from an unauthenticated pake_register
# response is otherwise unbounded and runs synchronously on the event loop;
# cap it well above any realistic device value to prevent a pre-auth DoS.
MAX_PAKE_ITERATIONS = 100_000
CIPHER_PARAMETERS = {
"aes_128_ccm": (
b"tp-kdf-salt-aes128-key",
Expand Down Expand Up @@ -795,7 +799,7 @@ def _build_share_params_from_register(
"TPAP register response has invalid iterations"
) from exc

if iterations <= 0:
if iterations <= 0 or iterations > self.MAX_PAKE_ITERATIONS:
raise KasaException("TPAP register response has invalid iterations")

encryption = str(register_result.get("encryption") or "")
Expand Down Expand Up @@ -931,7 +935,9 @@ def _establish_session_from_share_result(
dev_confirm = str(share_result.get("dev_confirm") or "").lower()
if not dev_confirm:
raise KasaException("TPAP share response missing dev_confirm")
if dev_confirm != (self._expected_dev_confirm or "").lower():
if not hmac.compare_digest(
dev_confirm, (self._expected_dev_confirm or "").lower()
):
raise KasaException("TPAP confirmation mismatch")

if self._use_dac_certification():
Expand Down