fix(axon): reject requests with missing signature in default_verify

[Thykof]

Bug

Issue: #3392

Summary: Axon.default_verify (bittensor/core/axon.py) skips signature verification entirely when the incoming request carries no signature, allowing any caller to impersonate any dendrite hotkey.

Description of the Change

default_verify reconstructs the signed message from the request headers and then runs three checks in order: it rejects a missing nonce ("Missing Nonce"), rejects a stale nonce, and finally verifies the signature.

The signature check was written as a single combined condition:

  if synapse.dendrite.signature and not keypair.verify(message, synapse.dendrite.signature):
      raise Exception("Signature mismatch ...")

Because signature presence is and-ed with the verification, an empty or absent synapse.dendrite.signature short-circuits the expression to False, so keypair.verify(...) is never called. The request then falls through the rest of
default_verify as authenticated — the nonce is even recorded. There is a "Missing Nonce" guard but no matching "Missing Signature" guard.

This change adds the missing presence guard (mirroring the nonce guard) and then verifies unconditionally:

  if not synapse.dendrite.signature:
      raise Exception("Missing signature")

  if not keypair.verify(message, synapse.dendrite.signature):
      raise Exception(f"Signature mismatch with {message} and {synapse.dendrite.signature}")

A present-but-invalid signature was already rejected correctly (keypair.verify → False → raise). The only hole was the empty/absent case, which this closes.

Impact: an attacker can claim any hotkey in bt_header_dendrite_hotkey — including a high-stake validator — and send no signature. The forged request passes default_verify and reaches the subnet's blacklist/forward. On the standard miner
blacklist (gated on validator_permit + stake), this lets an attacker harvest miner outputs while masquerading as a legitimate validator, with no secret material required.

Alternate Designs

• Keep one combined condition (if not signature or not keypair.verify(...): raise). Functionally equivalent, but it collapses two distinct failure modes into one message and loses parity with the existing "Missing Nonce" guard. The explicit
  two-step form is clearer and gives an actionable error.
• Let keypair.verify handle the empty input (drop the presence check, always call verify). This relies on keypair.verify("") returning False for every keypair backend — implementation-dependent and easy to regress. An explicit presence
  check is safer and self-documenting.

The proposed version was chosen for explicitness, a clear distinct error message, and symmetry with the surrounding nonce checks.

Possible Drawbacks

• Requests that previously "succeeded" with no signature will now be rejected. That path was the vulnerability itself, so this is the intended behavior — no legitimately signed client is affected (the dendrite always attaches a signature).
• No change to the public API, headers, or the valid-signature path; the only behavioral change is that the previously-bypassing unsigned case now fails closed.

Verification Process

• Unit: a synapse with dendrite.signature = "" (and None) now raises "Missing signature" from default_verify instead of returning successfully.
• Unit (regression): a correctly signed synapse still verifies; a present-but-wrong signature still raises "Signature mismatch".
• Integration: an unsigned request claiming a registered validator hotkey is rejected at the axon (HTTP 401) and never reaches forward.
• Run: pytest tests/unit_tests/test_axon.py.

Release Notes

Reject requests with missing signature

Branch Acknowledgement

• I am acknowledging that I am opening this branch against staging.

[basfroman]

Nice fix, the change is correct and well-documented.

One suggestion: test_default_verify_empty_signature_raises and test_default_verify_missing_signature_raises test the same code path (if not synapse.dendrite.signature) with different falsy values. Consider merging them into a single parametrized test to avoid near-duplication:

@pytest.mark.parametrize("empty_sig", ["", None])
async def test_default_verify_missing_signature_raises(empty_sig):
...

Otherwise LGTM. Please address this and re-request review.

[Thykof]

Hi @basfroman, thank you for the review. I have pushed the requested changes.