After Bitcoin Core 0.14.0 and before Bitcoin Core 29.0, validating a specially-crafted block may cause the node to access previously freed memory.
During validation, necessary data required for checking inputs for each transaction is pre-calculated and cached. For specially crafted invalid blocks, it was possible for this data to be destroyed while it was still being accessed by a background validation thread. An attacker capable of mining a block with sufficient proof-of-work could have exploited this to crash victim nodes. Because of the nature of use-after-free bugs, it is possible that the crash could have been used for remote code execution, though constraints on the input (block) data make this unlikely.
This issue is considered High severity.
Details
By default, script validation for new blocks is dispatched to background threads via a vector of
CScriptCheck functors. Each CScriptCheck holds a pointer to a PrecomputedTransactionData object
which stores some data needed by each input in the transaction. Because it stores a pointer and not
the data itself, care must be taken to ensure that the PrecomputedTransactionData outlives the
CScriptCheck.
The script checks lifetime is enforced by an RAII class, CCheckQueueControl. However, the control
is intantiated before the precomputed transaction data. Because local objects in C++ are
destructed in reverse order of construction,
this means the vector of PrecomputedTransactionData is destroyed before the
CCheckQueueControl.
This is not an issue when the block is valid, as CCheckQueueControl::Wait() will be called before
the function returns and the PrecomputedTransactionData gets destroyed. However, in case of an
early return (when a separate check fails) a background script thread may read the precomputed
transaction data after it was destroyed. An attacker could exploit this to crash victim nodes at the
expense of a valid PoW at tip.
Attribution
Cory Fields (MIT DCI) discovered this vulnerability and responsibly disclosed it in a detailed report containing a proof of concept for reproduction and a proposed mitigation.
Timeline
- 2024-11-02 Cory Fields privately reports the bug
- 2024-11-06 Pieter Wuille pushes a covert fix to already open PR #31112 which works around the issue by removing the early returns
- 2024-12-03 PR #31112 is merged
- 2025-04-12 Bitcoin Core version 29.0 is released with a fix
- 2026-04-19 The last vulnerable Bitcoin Core version (28.x) goes end of life
- 2026-05-05 Public disclosure.