Enforce minimum witness program length for fallback addresses in BOLT11 parsing #8219
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…dresses BIP-141 specifies that a witness program must be between 2 and 40 bytes in length. In our fallback address parsing, we were already checking the upper bound, but missing the lower bound check. This commit adds validation to ensure fallback address witness programs are at least 2 bytes long, bringing our implementation in line with the spec and other implementations like rust-lightning. Changelog-Fixed: Enforced minimum witness program length of 2 bytes for fallback addresses to comply with BIP-141 and prevent invalid decodings.
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vincenzopalazzo
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Apr 8, 2025
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This PR fixes an issue with BOLT11 fallback address validation discovered through differential fuzzing between Lightning implementations. Core Lightning was accepting invoices with non-standard witness address fallbacks that other implementations correctly reject.
While comparing parse results from LND, LDK, and Core Lightning using the bitcoinfuzz PR #132, I found that Core Lightning enforces the 40-byte upper limit for witness programs as per BIP-141, but doesn't check the 2-byte minimum length requirement.
The fix is minimal: adding a lower bound check to ensure witness programs are at least 2 bytes long, bringing Core Lightning in line with both the BIP-141 specification and other Lightning implementations.
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