Reject self ContextExtension keys >= 0x80 (JVM context-construction parity)#906
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Reject self ContextExtension keys >= 0x80 (JVM context-construction parity)#906mwaddip wants to merge 1 commit into
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ContextExtension keys are a signed Byte JVM-side. ErgoLikeContext .toSigmaContext builds the contextVars as new Array(maxKey+1) indexed by the signed key (ErgoLikeContext.scala:140-146), so a self-box extension carrying a key >= 0x80 (signed-negative) crashes context construction (NegativeArraySizeException / ArrayIndexOutOfBoundsException) BEFORE any bytecode runs — the spend never validates. The crash is on the key's PRESENCE, independent of whether the script reads it. sigma-rust stores extension keys as unsigned u8 and resolves them lazily at GetVar, so it would otherwise ACCEPT a >= 0x80 key where the JVM rejects — a consensus fork on the attacker-supplied spending extension (the node feeds the spending input's extension into ctx.extension before reduce_to_crypto). Reject any self-extension key outside 0..=127 at the reduction boundary, mirroring the construction-time crash as a clean rejection. This is the top-level self extension only; the per-input getVarFromInput path masks the id with & 0xff by design and is untouched, and a GetVar of a >= 0x80 id with the key absent still resolves to None. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
kushti
approved these changes
Jun 29, 2026
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ContextExtension keys are a signed
ByteJVM-side.ErgoLikeContext.toSigmaContextbuilds the context vars asnew Array(maxKey+1)indexed by the signed key (ErgoLikeContext.scala:140-146), so a self-box extension with a key>= 0x80crashes context construction (NegativeArraySize/ AIOOBE) before any bytecode — the spend never validates, on key PRESENCE.sigma-rust stores extension keys as unsigned
u8and resolves them lazily atGetVar, so it would otherwise ACCEPT a>= 0x80key where the JVM rejects — a consensus fork on the attacker-supplied spending extension. Reject any self-extension key outside0..=127at the reduction boundary. The per-inputgetVarFromInputpath (id masked& 0xff) is untouched, and aGetVarof an absent>= 0x80id still resolves toNone.