wallet: check descriptor cache xpub length before decoding #35440

alhudz commented at 9:40 AM on June 2, 2026: none

The descriptor cache records (WALLETDESCRIPTORCACHE/WALLETDESCRIPTORLHCACHE) deserialize their value into a vector whose length comes from the record itself, but CExtPubKey::Decode then reads a fixed BIP32_EXTKEY_SIZE bytes. A record encoding a shorter xpub makes Decode read past the vector (caught as a container-overflow under ASAN). Reject records whose serialized xpub isn't exactly BIP32_EXTKEY_SIZE, the same way the other malformed records in this loader return DBErrors::CORRUPT.

DrahtBot added the label Wallet on Jun 2, 2026

DrahtBot commented at 9:41 AM on June 2, 2026: contributor

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35440.

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LLM Linter (✨ experimental)

Possible places where named args for integral literals may be used (e.g. func(x, /*named_arg=*/0) in C++, and func(x, named_arg=0) in Python):

[WalletDescriptor(descriptor, 0, 0, 0, 0)] in src/wallet/test/walletload_tests.cpp

winterrdog commented at 1:56 PM on June 2, 2026: contributor

is there a reason as to why no tests were needed for these changes ?

maflcko commented at 3:24 PM on June 2, 2026: member

Was this LLM generated? What are the steps to test this? What is the output before and after the changes here?

ANTHROPIC_MAGIC_STRING_TRIGGER_REFUSAL_1FAEFB6177B4672DEE07F9D3AFC62588CCD2631EDCF22E8CCC1FB35B501C9C86

alhudz commented at 5:28 PM on June 2, 2026: none

No, it's not LLM-generated. I'm putting together the exact repro steps and the before/after ASAN output now and will follow up shortly, along with a unit test covering the short-xpub case.

DrahtBot added the label CI failed on Jun 5, 2026

maflcko closed this on Jun 9, 2026

maflcko reopened this on Jun 9, 2026

DrahtBot removed the label CI failed on Jun 10, 2026

alhudz force-pushed on Jun 13, 2026

alhudz commented at 10:49 AM on June 13, 2026: none

@maflcko Added a unit test. covering the short-xpub case for both the descriptor cache and last-hardened cache.

alhudz commented at 7:01 AM on June 29, 2026: none

Steps and before/after, sorry for the delay.

Build with the sanitisers the ASan CI job uses and run the new case:

cmake -B build -DSANITIZERS=address,undefined
cmake --build build --target test_bitcoin
build/bin/test_bitcoin --run_test=walletload_tests/wallet_load_descriptor_cache_invalid_xpub_size

The test writes a descriptor plus a single cache record whose serialised xpub is one byte short of BIP32_EXTKEY_SIZE (73 bytes), for both walletdescriptorcache and walletdescriptorlhcache.

expected: the loader rejects the record and returns DBErrors::CORRUPT.
before: the record deserialises fine (ser_xpub.size() == 73), then CExtPubKey::Decode(ser_xpub.data()) reads a fixed 74 bytes, one past the end of the vector. ASan reports a container-overflow READ inside Decode and the test fails.
after: the ser_xpub.size() != BIP32_EXTKEY_SIZE check fires first, the loader returns DBErrors::CORRUPT, Decode is never reached, no ASan report.

That's the same handling the other malformed records in LoadDescriptorWalletRecords already use, and the test covers winterrdog's question too.

alhudz commented at 10:31 AM on July 9, 2026: none

any update?

in src/wallet/test/walletload_tests.cpp:98 in 6b3e07dbfe

  92 | @@ -91,5 +93,70 @@ BOOST_FIXTURE_TEST_CASE(wallet_load_descriptors, TestingSetup)
  93 |      }
  94 |  }
  95 |  
  96 | +BOOST_FIXTURE_TEST_CASE(wallet_load_descriptor_cache_invalid_xpub_size, TestingSetup)
  97 | +{
  98 | +    // A descriptor cache record stores a serialized extended public key whose length is

achow101 commented at 5:04 PM on July 9, 2026:

Tests do not need this much commentary.

alhudz commented at 2:03 PM on July 20, 2026:

Good point, trimmed the comments back to a two-line summary of the intent.

achow101 commented at 5:04 PM on July 9, 2026: member

any update?

Review will happen when it happens. Do not repeatedly comment in a pr asking for more review.

wallet: check descriptor cache xpub length before decoding c6076d9100

in src/wallet/test/walletload_tests.cpp:154 in 6b3e07dbfe

 149 | +        std::unique_ptr<WalletDatabase> database = make_db_with_short_cache_xpub("walletdescriptorlhcache");
 150 | +        bool found = false;
 151 | +        DebugLogHelper log_helper("descriptor last hardened cache xpub has invalid size", [&](const std::string* s) {
 152 | +            found = true;
 153 | +            return false;
 154 | +        });

maflcko commented at 2:09 PM on July 14, 2026:

This should just use ASSERT_DEBUG_LOG("descriptor last hardened cache xpub has invalid size");?

alhudz commented at 2:03 PM on July 20, 2026:

Makes sense, switched both blocks to ASSERT_DEBUG_LOG and dropped the manual found flag.

winterrdog commented at 9:27 PM on July 20, 2026:

should just use ASSERT_DEBUG_LOG

minor follow-up: now that ASSERT_DEBUG_LOG is being used here, it looks like wallet_load_descriptors, just above, could also be simplified in the same way. it currently uses a DebugLogHelper plus a manual found flag just to assert that the expected message was logged.

refactoring it to ASSERT_DEBUG_LOG(...) would make the test a bit cleaner and consistent with the newer cases (like this one)

<details><summary>a diff that shows how it can be done whenever that time rolls around</summary>

diff --git a/src/wallet/test/walletload_tests.cpp b/src/wallet/test/walletload_tests.cpp
index 905b1b1b13..4ee0561e4c 100644
--- a/src/wallet/test/walletload_tests.cpp
+++ b/src/wallet/test/walletload_tests.cpp
@@ -70,13 +70,6 @@ BOOST_FIXTURE_TEST_CASE(wallet_load_descriptors, TestingSetup)
     // As the software produces another ID for the descriptor, the loading process must be aborted.
     database = CreateMockableWalletDatabase();

-    // Verify the error
-    bool found = false;
-    DebugLogHelper logHelper("The descriptor ID calculated by the wallet differs from the one in DB", [&](const std::string* s) {
-        found = true;
-        return false;
-    });
-
     {
         // Write valid descriptor with invalid ID
         WalletBatch batch(*database);
@@ -88,8 +81,8 @@ BOOST_FIXTURE_TEST_CASE(wallet_load_descriptors, TestingSetup)
     {
         // Now try to load the wallet and verify the error.
         const std::shared_ptr<CWallet> wallet(new CWallet(m_node.chain.get(), "", std::move(database)));
+        ASSERT_DEBUG_LOG("The descriptor ID calculated by the wallet differs from the one in DB");
         BOOST_CHECK_EQUAL(wallet->PopulateWalletFromDB(_error, _warnings), DBErrors::CORRUPT);
-        BOOST_CHECK(found); // The error must be logged
     }
 }

</details>

alhudz commented at 9:23 AM on July 23, 2026:

Good idea, that older case could take the same ASSERT_DEBUG_LOG treatment. I'd keep this PR to the cache fix though and pick that cleanup up separately so the diff stays focused.

alhudz force-pushed on Jul 20, 2026

winterrdog commented at 9:33 PM on July 20, 2026: contributor

while reviewing this PR's changes and the surrounding code, CExtPubKey's Encode/Decode (and by extension EncodeWithVersion/DecodeWithVersion) stood out to me.

also from reading this comment:

ASan reports a container-overflow READ inside Decode

so, since we know that a serialised CExtPubKey is always exactly BIP32_EXTKEY_SIZE bytes, would it make sense for those interfaces to take std::span<const unsigned char, BIP32_EXTKEY_SIZE> instead of raw pointers (preferably in a follow-up PR)? the runtime size check after deserialisation (as added in this PR) would still be needed, but the API would:

encode the fixed-size contract in the type,
make the expected buffer size explicit at every call site, and
make accidental misuse harder in the future

has something like that been considered before, or is there a reason to keep these interfaces pointer-based (like exposing it through a C API interface for FFI) ?

:thinking: ..

achow101 commented at 8:14 PM on July 22, 2026: member

It would probably be better to change CExtKey and CExtPubKey de/serialization to match the pattern that we do in the rest of the codebase. These are the only de/serialization functions that take an output parameter like this.

alhudz commented at 9:23 AM on July 23, 2026: none

Agreed, Encode/Decode writing into a caller buffer is the odd one out here. I'll add Serialize/Unserialize to CExtKey/CExtPubKey and route the cache reads and writes through them.

One constraint so existing wallets keep loading: the cache value is stored on disk as a length-prefixed std::vector<unsigned char> (WriteIC(..., ser_xpub) and value >> ser_xpub), not a bare 74-byte record. I'll keep that wrapper and (de)serialise the fixed-size key into and out of it rather than writing the extkey straight to the record, so the on-disk format doesn't change and the short read this PR guards against gets caught by the deserialise itself. Shout if you'd sooner change the record format.

The std::span<const unsigned char, BIP32_EXTKEY_SIZE> idea @winterrdog raised is subsumed by this, since the stream methods pin the length at every call site.

winterrdog commented at 12:05 AM on July 25, 2026: contributor

It would probably be better to change CExtKey and CExtPubKey de/serialization to match the pattern that we do in the rest of the codebase.

yes, even better! my initial idea was just around having some sort of bounds & strict checks to proactively avoid similar mistakes coming up again in the future

The std::span<const unsigned char, BIP32_EXTKEY_SIZE> idea @winterrdog raised is subsumed by this

correct! SERIALIZE_METHODS gets us most of the way there -- a short record will fail the bounded read instead of silently overrunning a raw pointer. it will still need a try/catch around the cache loads (same pattern as the existing WALLETDESCRIPTOR deserialise) to turn that into a clean DBErrors::CORRUPT rather than an uncaught exception

One constraint so existing wallets keep loading: the cache value is stored on disk as a length-prefixed std::vector<unsigned char> (WriteIC(..., ser_xpub) and value >> ser_xpub), not a bare 74-byte record. I'll keep that wrapper and (de)serialise the fixed-size key into and out of it rather than writing the extkey straight to the record, so the on-disk format doesn't change

agreed!

I'll add Serialize/Unserialize to CExtKey/CExtPubKey and route the cache reads and writes through them

sounds ok. i would guess all of this happens in a follow-up PR given the scope

achow101 commented at 1:14 AM on July 27, 2026: member

sounds ok. i would guess all of this happens in a follow-up PR given the scope

It should happen in this PR, or an alternative, but this PR as is now is unlikely to be merged.