Add fuzz target for the v1 payjoin roundtrip - #1872
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Walk the full v1 exchange: the sender builds a request from an original PSBT, the receiver runs it through the typestate chain, and the sender parses the resulting proposal. A custom mutator keeps a known-good original PSBT intact and spends the mutation budget on the BIP 78 query parameters, where the attacker-controlled values live. Raw inputs cannot reach the receiver: the sender builder needs funded inputs, an output paying the URI address and consistent values, so seeding a valid PSBT is what puts the state machine rather than the PSBT parser under test. The receiver's checks all answer unconditionally, since a real receiver's answers depend on its own UTXO set.
caarloshenriq
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DanGould,
benalleng and
spacebear21
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September 9, 2026 20:33
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Coverage Report for CI Build 34401726297Coverage remained the same at 86.646%Details
Uncovered ChangesNo uncovered changes found. Coverage RegressionsNo coverage regressions found. Coverage Stats
💛 - Coveralls |
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Summary
Adds a libFuzzer target covering the full v1 payjoin roundtrip: sender builds a request, receiver walks the typestate chain, sender parses the proposal that comes back.
What is covered
The target exercises both sides of the exchange and the handoff between them:
Sender:
SenderBuilder::new,build_non_incentivizing,create_v1_post_request,V1Context::process_responseReceiver:
UncheckedOriginalPayload::from_request,check_inputs_not_owned,check_no_inputs_seen_before,identify_receiver_outputs,commit_outputs,commit_inputs,apply_fee_range,finalize_proposalThe receiver's ownership checks answer unconditionally: a real receiver's answers depend on its own UTXO set, which is not what this target is exercising. The goal is the state machine's robustness against attacker-controlled input, not wallet logic.
Mutator design
The input is
[len:2][psbt][query]. The custom mutator keeps the PSBT section intact and mutates the query, where the BIP 78 parameters live. 25% of mutations pass straight tofuzzer_mutate, which is also the only path that reaches the PSBT bytes and so doubles as coverage of the sender builder's rejection paths.Seeding a valid PSBT is what makes the target work. Measured over 200k runs with raw input, roughly 4% deserialize as a PSBT at all and none of those survive
build_non_incentivizing, which needs funded inputs, an output paying the URI address and consistent values. Without the seed the target only exercises the rust-bitcoin PSBT parser. The seed is theBIP 78 test vector already used by payjoin-test-utils, and the URI address is derived from its output 1 rather than hardcoded, so the two cannot drift apart.
Fuzzing results
From an empty corpus on x86_64 Linux, 6 forks, 61 seconds:
Addresses part of #1267.
Disclosure: co-authored by Claude Opus 5
Pull Request Checklist
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