consensus: Add CompactSize range check to deserialization by erickcestari · Pull Request #5697 · rust-bitcoin/rust-bitcoin

erickcestari · 2026-02-18T17:09:09Z

Reject CompactSize values exceeding MAX_COMPACT_SIZE (0x02000000) during deserialization, matching Bitcoin Core's serialize.h limit.

Extract read_compact_size logic into a private free function to allow tests to bypass the range check when validating non-minimal encoding.

Add test for the new OversizedCompactSize error at the boundary.

Update bip152 error test to expect failure at deserialization rather than at index computation, since the range check now triggers earlier.

Found by differential fuzzing using bitcoinfuzz.

Reject CompactSize values exceeding MAX_COMPACT_SIZE (0x02000000) during deserialization, matching Bitcoin Core's serialize.h limit. Extract read_compact_size logic into a private free function to allow tests to bypass the range check when validating non-minimal encoding. Add test for the new SizeTooLargeCompactSize error at the boundary. Update bip152 error test to expect failure at deserialization rather than at index computation, since the range check now triggers earlier.

tcharding · 2026-02-18T22:18:20Z

Thanks man! Good work on the fuzzing.

A couple of comments/questions for the project more than just for you.

The patched code is more or less on its way out so if we need this fix we likely need it consensus_enocding as well.
We have done a fair bit of work with compact sizes and we don't have this check, that makes me think we discussed it and I can't remember? If not how did we miss this?

erickcestari · 2026-02-19T18:47:18Z

Thanks man! Good work on the fuzzing.

A couple of comments/questions for the project more than just for you.

The patched code is more or less on its way out so if we need this fix we likely need it consensus_enocding as well.

We have done a fair bit of work with compact sizes and we don't have this check, that makes me think we discussed it and I can't remember? If not how did we miss this?

More context:

I found this while doing differential fuzzing in the PSBT parser. Bitcoin Core always checks if the compact_size value is greater than 33,554,432 to throw an exception. While debugging, I found that the PSBT key has a type value that is the compact size encoded, which Bitcoin Core checks against the upper bound limit, but rust-bitcoin does not. Making rust-bitcoin accepting the PSBT and Bitcoin Core rejects it.

tcharding · 2026-02-19T20:46:30Z

Ha! Interesting timing then. I only recently raised this issue: #5622 which is around the key type and its usage of a CompactSize encoding. Note in the associated PR the code comment 'the protocol abuses ...' (which I suggested). Amusing that its not the only bug we have on this data field and I"m now not surprised we missed it.

tcharding · 2026-02-19T20:48:08Z

BTW we are trying to move PSBT dev over to rust-psbt. Would be mad if you fuzzed against that too if its not too much additional work. Would give us more confidence as we migrate dev that we don't make any mistakes. ref: #5700

jrakibi · 2026-02-19T21:40:59Z

We have done a fair bit of work with compact sizes and we don't have this check, that makes me think we discussed it and I can't remember? If not how did we miss this?

we previously had MAX_ENCODABLE_VALUE constant for this, but it was removed in 460976939 because it wasn’t enforced anywhere.

this was discussed here: #5057 (comment)

tcharding · 2026-02-20T02:26:37Z

Ouch so I removed it. Thanks @jrakibi!

erickcestari · 2026-02-20T15:30:53Z

BTW we are trying to move PSBT dev over to rust-psbt. Would be mad if you fuzzed against that too if its not too much additional work. Would give us more confidence as we migrate dev that we don't make any mistakes. ref: #5700

Yes, adding the rust-psbt is already on the plan.

bitcoinfuzz/bitcoinfuzz#419

I'll check to see if it gets done this week and report any issues I find.

tcharding · 2026-02-20T21:03:21Z

Boss!

apoelstra · 2026-02-27T15:58:04Z

I'm gonna go ahead and ACK/merge this. But

Does this need to be backported to 0.32.x to be picked up by your fuzzer?
Do we want to move any of this PR to rust-psbt?

apoelstra

ACK 1dd1d92; successfully ran local tests

tcharding · 2026-03-02T02:34:24Z

I created rust-bitcoin/rust-psbt#74 to track it over in rust-psbt and added the backport label (after checking we can backport the changes). And #5752 to track fixing it in consensus_encoding.

erickcestari · 2026-03-02T14:11:32Z

I'm gonna go ahead and ACK/merge this. But

Does this need to be backported to 0.32.x to be picked up by your fuzzer?

Do we want to move any of this PR to rust-psbt?

Does this need to be backported to 0.32.x to be picked up by your fuzzer?

No, We can do fuzzing in any commit/branch.

Do we want to move any of this PR to rust-psbt?

Yes, I've tested with rust-psbt and it has the same issue as rust-bitcoin. Since rust-psbt v0 uses the Key from rust-bitcoin it also will accept a key type that core doesn't accept.

Manually backport rust-bitcoin#5697. On `master` the `VarInt` type is gone but the deserialization bug fixed by 5697 exists here none the less. Add the same range check to `Decodable for VarInt`.

tcharding · 2026-03-29T23:26:10Z

Backported in #5921

Manually backport rust-bitcoin#5697. On `master` the `VarInt` type is gone but the deserialization bug fixed by 5697 exists here none the less. Add the same range check to `Decodable for VarInt`.

11bb6a8 bitcoin: update CompactSize Kani proofs (Ismail Daif) Pull request description: There were 2 existing Kani proofs, checking a CompactSize serialization/deserialization roundtrip for all `u32` values and `u64` values greater than `0xffffffff`. These proofs no longer succeeded after #5697 implemented a range-check on CompactSize deserialization. This PR updates the proofs such that a roundtrip is tested for all values in the enforced range, and values outside that range are rejected with `ParseError::OversizedCompactSize`. Closes #5954 ACKs for top commit: tcharding: ACK 11bb6a8 apoelstra: ACK 11bb6a8; successfully ran local tests; nice! We will try to be more conscientious about the Kani tests in future.. Tree-SHA512: 94c0a702a79d7154a3e8452df2292ac1773515780d78e8c1f28f53c0620f6643c8a9c520b629c3dbefd59228380a7329deb04fd14b49319a538775443ece503e

Manually backport rust-bitcoin#5697. On `master` the `VarInt` type is gone but the deserialization bug fixed by 5697 exists here none the less. Add the same range check to `Decodable for VarInt`.

…rialization 4a71193 Remove doc_auto_cfg (Tobin C. Harding) 91a6e64 consensus: Add CompactSize range check to deserialization (Tobin C. Harding) 48ac68d Update nightly toolchain version (Tobin C. Harding) Pull request description: Manually backport #5697. On `master` the `VarInt` type is gone but the deserialization bug fixed by 5697 exists here none the less. Add the same range check to `Decodable for VarInt`. ACKs for top commit: apoelstra: ACK 4a71193; successfully ran local tests Tree-SHA512: c46ed1b90f0d44c994302b69f7051388308a40939631f4542459abd58908913adbcf0f406f0b04111ae7bb74ad2ad603fb769eab3e1470a1e4fece1667fef6c5

Manually revert 91a6e64 (`git revert` didn't work.) This was a backport of rust-bitcoin#5697 which added range checking to the `VarInt` type. On master we did this then realized there are legitimate use cases for the full range of a `u64`. We then added `u64` support to compact size (rust-bitcoin#5784). Note that we cannot break the API so the new stuff introduced in `0.32.9` has to stay in there. Comment the code as such. NB: 'compact size' is the new name for varint. This backport then broke downstream users.

Manually revert 91a6e64 Note that we cannot break the API so the new stuff introduced in `0.32.9` has to stay in there. Comment the code as such. This was a backport of rust-bitcoin#5697 which added range checking to the `VarInt` type. On master we did this then realized there are legitimate use cases for the full range of a `u64`. We then added `u64` support to compact size (rust-bitcoin#5784). NB: 'compact size' is the new name for varint. This backport then broke downstream users.

Manually revert 91a6e64 Note that we cannot break the API so the new stuff introduced in `0.32.9` has to stay in there. Comment the code as such. This was a backport of #5697 which added range checking to the `VarInt` type. On master we did this then realized there are legitimate use cases for the full range of a `u64`. We then added `u64` support to compact size (#5784). NB: 'compact size' is the new name for varint. This backport then broke downstream users.

3bcd7c9 Manually revert VarInt range check (Tobin C. Harding) Pull request description: Manually revert 91a6e64 Note that we cannot break the API so the new stuff introduced in `0.32.9` has to stay in there. Comment the code as such. This was a backport of #5697 which added range checking to the `VarInt` type. On master we did this then realized there are legitimate use cases for the full range of a `u64`. We then added `u64` support to compact size (#5784). NB: 'compact size' is the new name for varint. This backport then broke downstream users. Close: #6138 ACKs for top commit: apoelstra: ACK 3bcd7c9; successfully ran local tests Tree-SHA512: d01078f4f8fae59c439fd404a643abfb353bcec1757ebfbcabde487ff8620d927387f91d0e749a7bbd36163f2386290d301f17c0cff9e628e37585329daf116b

github-actions Bot added C-bitcoin PRs modifying the bitcoin crate C-p2p labels Feb 18, 2026

erickcestari force-pushed the add-max-size-compactsize branch from 3f3f5ec to 1dd1d92 Compare February 18, 2026 17:16

erickcestari mentioned this pull request Feb 21, 2026

feat: add rust-psbt module bitcoinfuzz/bitcoinfuzz#444

Merged

apoelstra approved these changes Feb 28, 2026

View reviewed changes

tcharding added the bug label Mar 2, 2026

tcharding mentioned this pull request Mar 2, 2026

Investigate if compact size decoding bug needs fixing here rust-bitcoin/rust-psbt#74

Closed

tcharding added the Needs Backport label Mar 2, 2026

tcharding mentioned this pull request Mar 2, 2026

consensus_encoding: Add range check to when decoding a compact size #5752

Closed

apoelstra merged commit 9129379 into rust-bitcoin:master Mar 14, 2026
28 checks passed

tcharding mentioned this pull request Mar 29, 2026

backport: consensus: Add CompactSize range check to deserialization #5921

Merged

This was referenced Apr 4, 2026

Kani CI failing since adding range-check to CompactSize deserialization #5954

Closed

bitcoin: update CompactSize Kani proofs #5955

Merged

apoelstra mentioned this pull request May 6, 2026

VarInt breaking change in 0.32.9 #6138

Open

tcharding mentioned this pull request May 14, 2026

Manually revert VarInt range check #6182

Merged

Uh oh!

Conversation

erickcestari commented Feb 18, 2026

Uh oh!

tcharding commented Feb 18, 2026

Uh oh!

erickcestari commented Feb 19, 2026

Uh oh!

tcharding commented Feb 19, 2026

Uh oh!

tcharding commented Feb 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

jrakibi commented Feb 19, 2026

Uh oh!

tcharding commented Feb 20, 2026

Uh oh!

erickcestari commented Feb 20, 2026

Uh oh!

tcharding commented Feb 20, 2026

Uh oh!

apoelstra commented Feb 27, 2026

Uh oh!

apoelstra left a comment

Choose a reason for hiding this comment

Uh oh!

tcharding commented Mar 2, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

erickcestari commented Mar 2, 2026

Uh oh!

Uh oh!

tcharding commented Mar 29, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

tcharding commented Feb 19, 2026 •

edited

Loading

tcharding commented Mar 2, 2026 •

edited

Loading