{ "number": 36386, "repo": "bitcoin/bitcoin", "url": "https://github.com/bitcoin/bitcoin/pull/36386", "title": "compressor: oversized script decoding can leave a spendable script", "author": "l0rinc", "author_association": "MEMBER", "created_at": "2026-09-30T01:25:49Z", "updated_at": "2026-10-31T18:21:52Z", "description_hash": "cfa3f44704470552", "age_days": 32, "draft": false, "labels": [], "milestone": null, "base": "master", "head_sha": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "head_ref": "l0rinc/script-compression-replace-oversized", "head_repo": "l0rinc/bitcoin", "head_history": [ { "t": "2026-09-30T01:28:20Z", "sha": "6a30cea3cd313ca6a4b8178d129f62728e514c5f" }, { "t": "2026-09-30T18:10:26Z", "sha": "f313faff443e5cdbd858a54c6bb939799d7ce714" }, { "t": "2026-10-15T13:57:26Z", "sha": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd" } ], "additions": 17, "deletions": 1, "changed_files": 2, "commit_count": 2, "size_bucket": "S", "mergeable_state": "clean", "bot": { "drahtbot": { "present": true, "reviews": { "ack": [ { "login": "fametrano", "url": "https://github.com/bitcoin/bitcoin/pull/36386#issuecomment-6396973053" }, { "login": "hodlinator", "url": "https://github.com/bitcoin/bitcoin/pull/36386#pullrequestreview-5650692444" }, { "login": "maflcko", "url": "https://github.com/bitcoin/bitcoin/pull/36386#issuecomment-6440879122" }, { "login": "optout21", "url": "https://github.com/bitcoin/bitcoin/pull/36386#issuecomment-6444909520" } ], "stale_ack": [ { "login": "jcastros", "url": "https://github.com/bitcoin/bitcoin/pull/36386#issuecomment-5997586155" } ] }, "conflicts": [] } }, "acks_parsed": { "jcastros": { "kind": "ack", "hash": "f313faff443e5cdbd858a54c6bb939799d7ce714", "t": "2026-10-05T15:27:09Z", "stale": true }, "hodlinator": { "kind": "ack", "hash": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "t": "2026-10-26T10:45:52Z", "stale": false }, "optout21": { "kind": "ack", "hash": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "t": "2026-10-27T14:40:12Z", "stale": false } }, "acks_tally": { "ack": 2, "stale_ack": 1, "concept_ack": 0, "approach_ack": 0, "nack": 0, "concept_nack": 0, "approach_nack": 0 }, "reviews": { "approved": 1, "changes_requested": 0, "distinct_reviewers": [ "fametrano", "hodlinator", "jcastros", "maflcko", "optout21" ] }, "signals": { "needs_rebase": false, "ci_failed": false, "mergeable_state": "clean", "last_author_activity": "2026-10-15T13:57:26Z", "last_reviewer_activity": "2026-10-27T14:40:12Z", "last_reviewer": "optout21", "author_silent_days": 17, "waiting_on_author_days": 5, "days_since_update": 1 }, "refs": { "mentioned": [ 36280 ], "depends_on": [], "fixes": [], "linked_issues": [], "references": [ { "number": 36280, "type": "pull", "state": "open", "merged": false, "merged_at": null, "title": "validation: throw when a compressed script can't be decompressed" } ], "conflicts": [] }, "stack": { "shares_commits_with": [], "based_on": [], "base_for": [] }, "review_paths": [ "src/compressor.h", "src/test/compress_tests.cpp" ], "body": "**Problem:** The oversized-script fallback appends `OP_RETURN` to the destination instead of replacing it. If the destination already contains an unfinished push, the appended byte becomes pushed data and the result can remain spendable.\n\n**Fix:** Replace the destination with `OP_RETURN` rather than appending it.\n\nFound while reviewing #36280. This predates that PR and appears latent because oversized outputs are excluded from the UTXO set, but it could affect malformed stored records decoded into a reused destination.", "commits": [ { "sha": "1c8ba894ed9c5d7aa39d94eec2de60f11488a76e", "date": "2026-10-15T13:53:46Z", "message": "test: characterize oversized script decoding into a reused target\n\nDecode an oversized script into a target containing an incomplete push.\nRecord that the resulting script fails IsUnspendable().\nThe following fix requires the decoded script to be unspendable." }, { "sha": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "date": "2026-10-15T13:55:58Z", "message": "compressor: replace oversized scripts instead of appending\n\nThe oversized-script fallback appends OP_RETURN to the destination rather than replacing it as documented.\nA reused target retains its previous contents, and an incomplete push can consume OP_RETURN as data.\nAssign a new OP_RETURN script before skipping the oversized payload so the result is independent of the destination's prior contents." } ], "timeline": [ { "t": "2026-09-30T01:28:20Z", "kind": "force_push", "who": "l0rinc", "commit": "6a30cea3cd313ca6a4b8178d129f62728e514c5f", "id": "push:6a30cea3cd313ca6a4b8178d129f62728e514c5f" }, { "t": "2026-09-30T07:58:40Z", "kind": "comment", "who": "maflcko", "assoc": "MEMBER", "text": "[quoted text omitted]\n\nNot sure, reusing a destination would be an unrelated bug? So I am half suggesting to drop the test in a third commit again, or at least clarify in the comment that this isn't supposed to test any real production behavior.\n\nlgtm either way.", "id": "c:5906809717", "url": "https://github.com/bitcoin/bitcoin/pull/36386#issuecomment-5906809717", "hash": "ca7c4f1d9eebf6ab", "edited": null }, { "t": "2026-09-30T18:10:26Z", "kind": "force_push", "who": "l0rinc", "commit": "f313faff443e5cdbd858a54c6bb939799d7ce714", "id": "push:f313faff443e5cdbd858a54c6bb939799d7ce714" }, { "t": "2026-09-30T18:12:54Z", "kind": "comment", "who": "l0rinc", "assoc": "MEMBER", "text": "[quoted text omitted]\n\nWe do have some existing reuses, e.g. in https://github.com/bitcoin/bitcoin/blob/e7aef7e86da79000aa42f5ba6d13b0d123d9da7c/src/rpc/blockchain.cpp#L3435-L3462 the snapshot export reuses the same `Coin` in the loop. Regardless, added a comment clarifying that this tests the fallback directly, since normal UTXO insertion excludes oversized scripts.", "id": "c:5917022886", "url": "https://github.com/bitcoin/bitcoin/pull/36386#issuecomment-5917022886", "hash": "a99e5868bc1a0602", "edited": null }, { "t": "2026-10-05T15:27:09Z", "kind": "comment", "who": "jcastros", "assoc": "CONTRIBUTOR", "text": "Tested ACK f313faff443e5cdbd858a54c6bb939799d7ce714\n\nI built test_bitcoin (GCC 13.3, RelWithDebInfo) and ran compress_tests on this and on 11494efb01d0. Both are clean. Putting `script << OP_RETURN` back makes compress_oversized_script fail, so the new check covers the fix.", "id": "c:5997586155", "url": "https://github.com/bitcoin/bitcoin/pull/36386#issuecomment-5997586155", "hash": "0dfa2a88806339d6", "edited": null }, { "t": "2026-10-14T09:36:39Z", "kind": "review_comment", "who": "hodlinator", "assoc": "MEMBER", "path": "src/test/compress_tests.cpp", "commit": "f313faff443e5cdbd858a54c6bb939799d7ce714", "in_reply_to": null, "text": "This has a bit too much happening at once IMO (+ redundant `CScript`-ctor-call). Would rather have something closer to\n```suggestion\n DataStream oversized_compressed{DataStream{} << Using(CScript{} << std::vector(MAX_SCRIPT_SIZE + 1, uint8_t{0}))};\n oversized_compressed >> Using(restored);\n```", "id": "rc:4269121804", "url": "https://github.com/bitcoin/bitcoin/pull/36386#discussion_r4269121804", "hash": "eb0d763540aa5cde", "edited": "2026-10-14T09:43:43Z" }, { "t": "2026-10-14T09:39:00Z", "kind": "review_comment", "who": "hodlinator", "assoc": "MEMBER", "path": "src/test/compress_tests.cpp", "commit": "f313faff443e5cdbd858a54c6bb939799d7ce714", "in_reply_to": null, "text": "nit: Could verify that `restored` starts out spendable before the compression round-trip makes it unspendable?\n```suggestion\n CScript restored{ParseScript(\"0x01\")}; // Incomplete push: appending OP_RETURN would make it pushed data\n BOOST_CHECK(!restored.IsUnspendable());\n (DataStream{} << Using(CScript{CScript() << std::vector(MAX_SCRIPT_SIZE + 1, uint8_t{0})})) >> Using(restored);\n```", "id": "rc:4269144356", "url": "https://github.com/bitcoin/bitcoin/pull/36386#discussion_r4269144356", "hash": "b4d5921442a83e78", "edited": "2026-10-14T09:43:43Z" }, { "t": "2026-10-14T09:43:02Z", "kind": "review_comment", "who": "hodlinator", "assoc": "MEMBER", "path": "src/test/compress_tests.cpp", "commit": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "in_reply_to": null, "text": "Ralph suggested also pinning down the resulting script (https://git.fish.foo/bitcoin/bitcoin/pulls/36386#issuecomment-608275), which seems fair enough.\n\nGuess it could be done through:\n```suggestion\n BOOST_CHECK(restored.IsUnspendable());\n const CScript expected{OP_RETURN};\n BOOST_CHECK_EQUAL_COLLECTIONS(restored.begin(), restored.end(), expected.begin(), expected.end());\n```", "id": "rc:4269186201", "url": "https://github.com/bitcoin/bitcoin/pull/36386#discussion_r4269186201", "hash": "5a4e22ae552fb290", "edited": "2026-10-14T09:43:43Z" }, { "t": "2026-10-14T09:43:43Z", "kind": "review", "who": "hodlinator", "assoc": "MEMBER", "state": "COMMENTED", "commit": "f313faff443e5cdbd858a54c6bb939799d7ce714", "text": "Concept ACK f313faff443e5cdbd858a54c6bb939799d7ce714", "id": "r:5515914011", "url": "https://github.com/bitcoin/bitcoin/pull/36386#pullrequestreview-5515914011", "hash": "5eff56a778bd1c7e", "edited": null }, { "t": "2026-10-15T13:57:26Z", "kind": "force_push", "who": "l0rinc", "commit": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "id": "push:426f29c3f1639e4c97be63cdf4c2e76bb86b39dd" }, { "t": "2026-10-25T11:19:22Z", "kind": "comment", "who": "fametrano", "assoc": "NONE", "text": "tACK 426f29c3f1639e4c97be63cdf4c2e76bb86b39dd\n\nI decoded 1307 compressed scripts (raw sizes 0 to 39 and from just under `MAX_SCRIPT_SIZE` to 200000, truncated streams, every special type) into a fresh target and five reused ones, a P2PKH among them, and compared each result with what a library I maintain decodes from the same bytes. On the base, every complete oversized script decoded into a reused target keeps the old bytes and appends `OP_RETURN` (35 decodes). On this head every raw and oversized case matches for every target.\n\nOne path still depends on the target: an uncompressed P2PK (type 4 or 5) whose x is not on the curve leaves a reused target untouched, so a P2PKH stays a P2PKH. #36280 makes that path throw. Its `compressor.h` change merges cleanly with this one; only `compress_tests.cpp` conflicts. With both applied, no decode in the set depends on what the target held before. I can share the vectors and the test.", "id": "c:6396973053", "url": "https://github.com/bitcoin/bitcoin/pull/36386#issuecomment-6396973053", "hash": "109cb0908d157879", "edited": null }, { "t": "2026-10-26T10:35:02Z", "kind": "review_comment", "who": "hodlinator", "assoc": "MEMBER", "path": "src/test/compress_tests.cpp", "commit": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "in_reply_to": null, "text": "nit: Could compute at compile time:\n```diff\n--- a/src/test/compress_tests.cpp\n+++ b/src/test/compress_tests.cpp\n@@ -28,6 +28,8 @@\n // amounts 50 .. 21000000\n #define NUM_MULTIPLES_50BTC 420000\n\n+using namespace util::hex_literals;\n+\n BOOST_FIXTURE_TEST_SUITE(compress_tests, BasicTestingSetup)\n\n bool static TestEncode(uint64_t in) {\n@@ -172,7 +174,7 @@ BOOST_AUTO_TEST_CASE(compress_p2pk_scripts_not_on_curve)\n BOOST_AUTO_TEST_CASE(compress_oversized_script)\n {\n // Test the fallback directly, since normal UTXO insertion excludes oversized scripts\n- CScript restored{ParseScript(\"0x01\")}; // Incomplete push: appending OP_RETURN would make it pushed data\n+ auto restored{CScript{} << \"01\"_hex}; // Incomplete push: appending OP_RETURN would make it pushed data\n BOOST_CHECK(!restored.IsUnspendable());\n DataStream oversized_compressed{DataStream{} << Using(CScript{} << std::vector(MAX_SCRIPT_SIZE + 1, uint8_t{0}))};\n oversized_compressed >> Using(restored);\n```", "id": "rc:4376274486", "url": "https://github.com/bitcoin/bitcoin/pull/36386#discussion_r4376274486", "hash": "f0120c39d727e472", "edited": "2026-10-26T10:45:52Z" }, { "t": "2026-10-26T10:45:52Z", "kind": "review", "who": "hodlinator", "assoc": "MEMBER", "state": "APPROVED", "commit": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "text": "ACK 426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "id": "r:5650692444", "url": "https://github.com/bitcoin/bitcoin/pull/36386#pullrequestreview-5650692444", "hash": "3db45a42bccba877", "edited": null }, { "t": "2026-10-27T11:34:28Z", "kind": "comment", "who": "maflcko", "assoc": "MEMBER", "text": "review ACK 426f29c3f1639e4c97be63cdf4c2e76bb86b39dd \ud83c\udfc9\n\nShow signature\n\nSignature:\n\n```\nuntrusted comment: signature from minisign secret key on empty file; verify via: minisign -Vm \"${path_to_any_empty_file}\" -P RWTRmVTMeKV5noAMqVlsMugDDCyyTSbA3Re5AkUrhvLVln0tSaFWglOw -x \"${path_to_this_whole_four_line_signature_blob}\"\nRUTRmVTMeKV5npGrKx1nqXCw5zeVHdtdYURB/KlyA/LMFgpNCs+SkW9a8N95d+U4AP1RJMi+krxU1A3Yux4bpwZNLvVBKy0wLgM=\ntrusted comment: review ACK 426f29c3f1639e4c97be63cdf4c2e76bb86b39dd \ud83c\udfc9\nZMFrq8y3VWJkvdmEHZqxUEXnr07PVirQi5JeVZZ0cI9PWixMNhtsYF/RYb7eALrDfdaoDa4OPwLQP3GmubegDg==\n```", "id": "c:6440879122", "url": "https://github.com/bitcoin/bitcoin/pull/36386#issuecomment-6440879122", "hash": "c1f6f33e00d73123", "edited": null }, { "t": "2026-10-27T13:57:47Z", "kind": "review_comment", "who": "optout21", "assoc": "CONTRIBUTOR", "path": "src/compressor.h", "commit": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "in_reply_to": null, "text": "[quoted text omitted]\n\nThe commit message states: \"so the result is independent of the destination's prior contents\". However, this is not always the case. In this branch, `DecompressScript` could fail in such a way that it does not modify `script` (in the `0x04`/`0x05` prefix + error case).\nEither adjust this codepath as well (reset it in case of error), or soften the claim.", "id": "rc:4389555092", "url": "https://github.com/bitcoin/bitcoin/pull/36386#discussion_r4389555092", "hash": "b79932b22f6c4c7d", "edited": "2026-10-27T14:16:42Z" }, { "t": "2026-10-27T14:15:15Z", "kind": "review_comment", "who": "optout21", "assoc": "CONTRIBUTOR", "path": "src/compressor.h", "commit": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "in_reply_to": null, "text": "[quoted text omitted]\n\nWould it be possible to clear the `script` output variable at the beginning of this method? That would ensure the claim (independence of previous value).", "id": "rc:4389749344", "url": "https://github.com/bitcoin/bitcoin/pull/36386#discussion_r4389749344", "hash": "219159f1e636dbc1", "edited": "2026-10-27T14:16:42Z" }, { "t": "2026-10-27T14:16:42Z", "kind": "review", "who": "optout21", "assoc": "CONTRIBUTOR", "state": "COMMENTED", "commit": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "text": "Reviewed, left some comments. Happy to re-check/ack once answered.\n(426f29c3f1639e4c97be63cdf4c2e76bb86b39dd)", "id": "r:5668222358", "url": "https://github.com/bitcoin/bitcoin/pull/36386#pullrequestreview-5668222358", "hash": "4028d1d337da7e54", "edited": null }, { "t": "2026-10-27T14:20:26Z", "kind": "review_comment", "who": "optout21", "assoc": "CONTRIBUTOR", "path": "src/compressor.h", "commit": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "in_reply_to": 4389555092, "text": "I've just realized that #36280 addresses exactly this, so I think it's fine as it is in this PR.", "id": "rc:4389807813", "url": "https://github.com/bitcoin/bitcoin/pull/36386#discussion_r4389807813", "hash": "c135e409e4b7c042", "edited": "2026-10-27T14:20:57Z" }, { "t": "2026-10-27T14:40:12Z", "kind": "comment", "who": "optout21", "assoc": "CONTRIBUTOR", "text": "ACK 426f29c3f1639e4c97be63cdf4c2e76bb86b39dd\nUpgraded my previous review result, upon reconsideration. This change is a net improvement, independent of #36280 . I'd appreciate answers to my comments, nonetheless.", "id": "c:6444909520", "url": "https://github.com/bitcoin/bitcoin/pull/36386#issuecomment-6444909520", "hash": "03fa8284ac3dc4cc", "edited": null } ], "labels_log": [ { "t": "2026-09-30T01:29:31Z", "action": "labeled", "label": "CI failed", "who": "DrahtBot" }, { "t": "2026-09-30T02:20:52Z", "action": "unlabeled", "label": "CI failed", "who": "DrahtBot" } ], "state_log": [], "text_chars": 6951, "text_tokens_estimate": 1737, "changed_paths": [ "src/compressor.h", "src/test/compress_tests.cpp" ], "files": [ { "path": "src/compressor.h", "add": 1, "del": 1 }, { "path": "src/test/compress_tests.cpp", "add": 16, "del": 0 } ], "test_lines": 16, "git": { "head": "426f29c3f1639e4c97be63cdf4c2e76bb86b39dd", "head_matches_backup": true, "base": "cce32b16874e85e8a00cfa1a2190f0b6f4dcaf66", "patch_id": "93b578d8b06c8d01", "commits": [ { "sha": "1c8ba894ed", "subject": "test: characterize oversized script decoding into a reused target", "files": 1, "add": 15, "del": 0 }, { "sha": "426f29c3f1", "subject": "compressor: replace oversized scripts instead of appending", "files": 2, "add": 3, "del": 2 } ], "patch_truncated": false }, "patch_id": "93b578d8b06c8d01", "input_hash": "562d038df93c6252", "extracted_at": "2026-11-02T01:09:49+00:00" }