Low 34 Bitcoin Committed Feb 23, 2026 · 12:46 UTC

fix(core/thp): disable THP ACK piggybacking as a workaround for #6506

586df8a0 View source ↗

Public commit record

What the developer wrote

Authored by Roman Zeyde

93/100 · Strong

fix(core/thp): disable THP ACK piggybacking as a workaround for #6506

The underlying bug can also happen without THP ACK piggybacking but
with much lower probability (it requires >200ms THP ACK delay over USB).

[no changelog]

✓ Specific, descriptive subject✓ Names a concrete action or component✓ Uses a recognizable type or scope✓ Provides detailed explanatory context✓ Links an issue, advisory, or supporting reference

The short version

What changed, and why it matters

This commit temporarily disables a performance feature called 'THP ACK piggybacking' in Trezor firmware to reduce the chance of triggering an unrelated bug referenced as issue #6506. It is described as a workaround, not a complete fix. The underlying bug can still occur, but only if a network acknowledgment is delayed by more than 200 milliseconds. The change lowers the protocol version minor number from 1 to 0 and stops the device from enabling ACK piggybacking during the handshake. There is no direct evidence in the commit that this is a security vulnerability, and no changelog entry is added.

Recommended action

Treat this as a defensive workaround pending a proper fix for issue #6506. Monitor the referenced issue for the root-cause patch and re-enable ACK piggybacking only after #6506 is resolved and the fix is reviewed. If this firmware is being evaluated for security certification, request details on #6506 from the vendor to assess whether the underlying bug has security implications.

Security signals we found

Workaround for an unspecified bug (#6506) with timing-related trigger condition (>200ms ACK delay)

Feature rollback (protocol version minor 1 -> 0) to avoid compatibility path

No changelog entry added despite user-facing protocol change

No CVE, advisory, or researcher attribution present in commit materials

Technical analysis

Evidence from the diff

The patch removes the changelog entry for ‘Support receive-side THP ACK piggybacking’ and disables the feature in two places: (1) the THP protocol version minor is rolled back from 1 to 0, and (2) the handshake handler no longer calls ABP.allow_ack_piggybacking() when the ACK bit is set in handshake_init_req. Comments indicate this is a workaround for issue #6506, which can still happen without piggybacking but with much lower probability (requires >200ms THP ACK delay over USB). The actual bug in #6506 is not shown, so the security relevance is inferred only from the workaround language.

Changed components

core/src/trezor/wire/thp/__init__.pycore/src/trezor/wire/thp/received_message_handler.pyTrezor Core THP (Trezor Host Protocol) implementation

Inspect captured patch +7 / −6

diff --git a/core/.changelog.d/6202.added b/core/.changelog.d/6202.added
deleted file mode 100644
index c50964dc..00000000
--- a/core/.changelog.d/6202.added
+++ /dev/null
@@ -1 +0,0 @@
-Support receive-side THP ACK piggybacking.
diff --git a/core/src/trezor/wire/thp/__init__.py b/core/src/trezor/wire/thp/__init__.py
index 0945178f..201b433f 100644
--- a/core/src/trezor/wire/thp/__init__.py
+++ b/core/src/trezor/wire/thp/__init__.py
@@ -210,7 +210,8 @@ def _get_device_properties(iface: WireInterface) -> ThpDeviceProperties:
         internal_model=utils.INTERNAL_MODEL,
         model_variant=model_variant,
         protocol_version_major=2,
-        protocol_version_minor=1,
+        # TODO: re-enable THP ACK piggybacking after #6506 is fixed
+        protocol_version_minor=0,
     )
 
 
diff --git a/core/src/trezor/wire/thp/received_message_handler.py b/core/src/trezor/wire/thp/received_message_handler.py
index b0b6e310..bd17f8f1 100644
--- a/core/src/trezor/wire/thp/received_message_handler.py
+++ b/core/src/trezor/wire/thp/received_message_handler.py
@@ -106,10 +106,11 @@ async def _handle_state_handshake(
     def _handshake_callback(ctrl_byte: int) -> bool:
         success = control_byte.is_handshake_init_req(ctrl_byte)
 
-        if success and control_byte.get_ack_bit(ctrl_byte) == 1:
-            # Newer Suite versions will send `handshake_init_req` with a non-zero ACK bit.
-            # The device should not use ACK piggybacking with older Suite versions.
-            ABP.allow_ack_piggybacking(ctx.channel_cache)
+        # TODO: re-enable THP ACK piggybacking after #6506 is fixed
+        # if success and control_byte.get_ack_bit(ctrl_byte) == 1:
+        #     # Newer Suite versions will send `handshake_init_req` with a non-zero ACK bit.
+        #     # The device should not use ACK piggybacking with older Suite versions.
+        #     ABP.allow_ack_piggybacking(ctx.channel_cache)
 
         if __debug__:
             ctx._log(

Risk score

Why this scored 34/100

Our methodology →

Potential impact 8/30

Exploitability 5/25

Stealth signal 6/15

Affected reach 7/15

Confidence 5/10

Evidence quality 3/5

Human-validated context

Community notes

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What the developer wrote

What changed, and why it matters

Security signals we found

Evidence from the diff

Changed components

Why this scored 34/100

Community notes

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