bitcoinj has a ScriptExecution P2PKH/P2WPKH Verification Bypass

Summary

ScriptExecution.correctlySpends() contains two fast-path verification bugs for standard P2PKH and native P2WPKH spends in core/src/main/java/org/bitcoinj/script/ScriptExecution.java.

In both branches, bitcoinj verifies an attacker-controlled signature/public-key pair but fails to verify that the public key is the one committed to by the output being spent. As a result, any attacker keypair can satisfy bitcoinj's local verification for arbitrary P2PKH and P2WPKH outputs.

This doesn't affect the SPV (simple payment verification) trust model, as this model follows PoW and doesn't verify input signatures at all.

Details

The issue is in the optimized branches of ScriptExecution.correctlySpends(...).

In the P2PKH fast path at core/src/main/java/org/bitcoinj/script/ScriptExecution.java:1042, the code:

• parses the attacker-supplied signature from scriptSig
• parses the attacker-supplied public key from scriptSig
• computes the sighash against the victim output's scriptPubKey
• checks only pubkey.verify(sigHash, signature)

It never enforces the missing P2PKH binding:

• HASH160(pubkey) == ScriptPattern.extractHashFromP2PKH(scriptPubKey)

That means the OP_DUP OP_HASH160 <hash> OP_EQUALVERIFY OP_CHECKSIG semantics are not actually enforced in this fast path.

Relevant code:

} else if (ScriptPattern.isP2PKH(scriptPubKey)) {
    if (chunks.size() != 2)
        throw new ScriptException(...);
    TransactionSignature signature;
    try {
        byte[] data = Objects.requireNonNull(chunks.get(0).data);
        signature = TransactionSignature.decodeFromBitcoin(data, true, true);
    } catch (SignatureDecodeException x) {
        throw new ScriptException(...);
    }
    ECKey pubkey = ECKey.fromPublicOnly(Objects.requireNonNull(chunks.get(1).data));
    Sha256Hash sigHash = txContainingThis.hashForSignature(scriptSigIndex, scriptPubKey,
            signature.sigHashMode(), false);
    boolean validSig = pubkey.verify(sigHash, signature);
    if (!validSig)
        throw new ScriptException(...);
}

In the native P2WPKH fast path at core/src/main/java/org/bitcoinj/script/ScriptExecution.java:1023, the bug is similar. The code:

• reads the attacker-supplied pubkey from witness
• builds scriptCode from that attacker pubkey with ScriptBuilder.createP2PKHOutputScript(pubkey)
• computes the BIP143 sighash using that attacker-derived scriptCode
• verifies the signature against the attacker pubkey

It never enforces:

• HASH160(pubkey) == ScriptPattern.extractHashFromP2WH(scriptPubKey)

So for P2WPKH, the attacker controls both the pubkey and the scriptCode used for signing.

Relevant code:

if (ScriptPattern.isP2WPKH(scriptPubKey)) {
    Objects.requireNonNull(witness);
    if (witness.getPushCount() < 2)
        throw new ScriptException(...);
    TransactionSignature signature;
    try {
        signature = TransactionSignature.decodeFromBitcoin(witness.getPush(0), true, true);
    } catch (SignatureDecodeException x) {
        throw new ScriptException(...);
    }
    ECKey pubkey = ECKey.fromPublicOnly(witness.getPush(1));
    Script scriptCode = ScriptBuilder.createP2PKHOutputScript(pubkey);
    Sha256Hash sigHash = txContainingThis.hashForWitnessSignature(scriptSigIndex, scriptCode, value,
            signature.sigHashMode(), false);
    boolean validSig = pubkey.verify(sigHash, signature);
    if (!validSig)
        throw new ScriptException(...);
}

Affected call sites include:

• core/src/main/java/org/bitcoinj/core/TransactionInput.java:546
• core/src/main/java/org/bitcoinj/wallet/Wallet.java:4520
• core/src/main/java/org/bitcoinj/signers/LocalTransactionSigner.java:84
• core/src/main/java/org/bitcoinj/signers/CustomTransactionSigner.java:77

These call sites use correctlySpends() for transaction/input validation and pre-signing checks. Any application that treats a successful result from this path as proof that a spend is valid is affected.

Fix

The issue is fixed on the release-0.17 branch via 2bc5653c41d260d840692bc554690d4d79208f9c, and on master via b575a682acf614b9ff95cacbdeb48f86c3ababe0. A 0.17.1 maintenance release has been made available on Maven Central.