Wormhole漏洞分析

Posted 2022-02-18 mutourend

1. 引言

前序博客有：

• Wormhole资产跨链项目代码解析

Wormhole为Solana上的跨链bridge。

Wormhole中引入了Validator角色——即guardians。
Wormhole不是区块链网络，其仅依赖共识和其bridge的链的finalization。
Wormhole中没有leader角色，所有的guardians都对其监听到的on-chain event执行相同的计算，同时对Validator Action Approval (VAA)签名。若有⅔+的大多数guardian节点使用各自私钥对同一event签名，则在所有链上的Wormhole合约都将自动认为其是有效的，并触发相应的mint/burn操作。（当前采用19个guardian，需达到13+个签名。）

Wormhole bridge采用wrapped token 方式，来lock tokens in one blockchain into a smart contract。

2022年2月3日，Wormhole中价值超过3亿美金的加密资产被盗。根本原因在于：Wormhole后台没有正确验证其guardian accounts。通过创建a fake signature account，黑客在Solana链上mint了12万个WETH（价值3亿美金），然后通过一系列操作，将其中的93,750个ETH转移至以太坊的私人钱包0x629e7da20197a5429d30da36e77d06cdf796b71a中：

• https://etherscan.io/tx/0xacd309b02e4b533484d148de9ab0adf367ed4e70ed751d1ff036152dc3bc0479：由黑客直接调用以太坊wrapped ether合约的withdraw方法。
• https://etherscan.io/tx/0x4d5201dd4a377f20e61fb8f42e6f929ec16bcec918f0584e39241d15b254a80f：由黑客直接调用以太坊Wormhole: Token Bridge合约的completeTransferAndUnwrapETH方法。对应的Solana上交易为：(https://solscan.io/tx/5UaqPus91wvAzKNve6L8YAHsESomZQ7GWi37gPFyzTHcXNMZA641bb8m8txo7bS7A5cAnzKDKYyiKcQC8GgDcAuf)。
• https://etherscan.io/tx/0xd31b155e259a403ebe69831fae0ec2b4bd33dfa090c43b605a57d5c72c4fbbc7：由黑客直接调用以太坊Wormhole: Token Bridge合约的completeTransferAndUnwrapETH方法。

在Wormhole 里面要mint ETH 的流程是要执行 complete_wrapped -> 然后需要transfer message -> transfer message 是透过post_vaa 这个function 产生-> 透过verify_signatures 去验证签名是不是合法的-> 然后用到了solana sdk 提供的一个function load_instruction_at，也是这次漏洞发生的主因，不需要透过系统的地址就可以执行。
骇客就先试打了0.1 ETH 拿到正常verify_signatures 的参数去做伪造，反正系统不会检查，这点相当的聪明。
然而Wormhole 也在被hack 之前就准备要更新成Solana 1.9.4 版本，骇客抓准了修复漏洞之前开始攻击，应该是已经潜伏已久。
所以这件事情其实影响到的范围是所有有用到load_instruction_at 的Dapp，如果还有其他协议没有更新新版的话应该还会有其他锅会爆炸。

Solana端Wormhole合约提供的接口函数主要有：

//Solana Wormhole brdige，适于token bridge和nft bridge
solitaire! 
    Initialize(InitializeData)                  => initialize,
    PostMessage(PostMessageData)                => post_message,
    PostVAA(PostVAAData)                        => post_vaa,
    SetFees(SetFeesData)                        => set_fees,
    TransferFees(TransferFeesData)              => transfer_fees,
    UpgradeContract(UpgradeContractData)        => upgrade_contract,
    UpgradeGuardianSet(UpgradeGuardianSetData)  => upgrade_guardian_set,
    VerifySignatures(VerifySignaturesData)      => verify_signatures,

//token_bridge
solitaire! 
    Initialize(InitializeData) => initialize,
    AttestToken(AttestTokenData) => attest_token,
    CompleteNative(CompleteNativeData) => complete_native,
    CompleteWrapped(CompleteWrappedData) => complete_wrapped,
    TransferWrapped(TransferWrappedData) => transfer_wrapped,
    TransferNative(TransferNativeData) => transfer_native,
    RegisterChain(RegisterChainData) => register_chain,
    CreateWrapped(CreateWrappedData) => create_wrapped,
    UpgradeContract(UpgradeContractData) => upgrade_contract,

核心关键在于：黑客如何在Solana上mint出了12万个WETH？
mint 12万个WETH的交易为：https://solscan.io/tx/2zCz2GgSoSS68eNJENWrYB48dMM1zmH8SZkgYneVDv2G4gRsVfwu5rNXtK5BKFxn7fSqX9BvrBc1rdPAeBEcD6Es
在该笔交易中，会调用complete_wrapped函数，该函数需要a valid VAA。

fn claimable_vaa(
    bridge_id: Pubkey,
    message_key: Pubkey,
    vaa: PostVAAData,
) -> (AccountMeta, AccountMeta) 
    let claim_key = Claim::<'_,  AccountState::Initialized >::key(
        &ClaimDerivationData 
            emitter_address: vaa.emitter_address,
            emitter_chain: vaa.emitter_chain,
            sequence: vaa.sequence,
        ,
        &bridge_id,
    );

    (
        AccountMeta::new_readonly(message_key, false),
        AccountMeta::new(claim_key, false),
    )

pub fn complete_wrapped(
    program_id: Pubkey,
    bridge_id: Pubkey,
    payer: Pubkey,
    message_key: Pubkey,//GvAarWUV8khMLrTRouzBh3xSr8AeLDXxoKNJ6FgxGyg5 利用了该地址之前的有效VAA。
    vaa: PostVAAData,
    payload: PayloadTransfer,
    to: Pubkey,
    fee_recipient: Option<Pubkey>,
    data: CompleteWrappedData,
) -> solitaire::Result<Instruction> 
    let config_key = ConfigAccount::<'_,  AccountState::Uninitialized >::key(None, &program_id);
    let (message_acc, claim_acc) = claimable_vaa(program_id, message_key, vaa.clone());
    let endpoint = Endpoint::<'_,  AccountState::Initialized >::key(
        &EndpointDerivationData 
            emitter_chain: vaa.emitter_chain,
            emitter_address: vaa.emitter_address,
        ,
        &program_id,
    );
    let mint_key = WrappedMint::<'_,  AccountState::Uninitialized >::key(
        &WrappedDerivationData 
            token_chain: payload.token_chain,
            token_address: payload.token_address,
        ,
        &program_id,
    );
    let meta_key = WrappedTokenMeta::<'_,  AccountState::Uninitialized >::key(
        &WrappedMetaDerivationData  mint_key ,
        &program_id,
    );
    let mint_authority_key = MintSigner::key(None, &program_id);

    Ok(Instruction 
        program_id,
        accounts: vec![
            AccountMeta::new(payer, true),
            AccountMeta::new_readonly(config_key, false),
            message_acc,
            claim_acc,
            AccountMeta::new_readonly(endpoint, false),
            AccountMeta::new(to, false),
            if let Some(fee_r) = fee_recipient 
                AccountMeta::new(fee_r, false)
             else 
                AccountMeta::new(to, false)
            ,
            AccountMeta::new(mint_key, false),
            AccountMeta::new_readonly(meta_key, false),
            AccountMeta::new_readonly(mint_authority_key, false),
            // Dependencies
            AccountMeta::new_readonly(solana_program::sysvar::rent::id(), false),
            AccountMeta::new_readonly(solana_program::system_program::id(), false),
            // Program
            AccountMeta::new_readonly(bridge_id, false),
            AccountMeta::new_readonly(spl_token::id(), false),
        ],
        data: (crate::instruction::Instruction::CompleteWrapped, data).try_to_vec()?,
    )

但是攻击者如何获得有效的VAA呢？message_key（一个有效的VAA account）为之前已调用Solana端bridge主合约的post_vaa函数创建，具体见交易：
https://solscan.io/tx/2SohoVoPDSdzgsGCgKQPByKQkLAXHrYmvtE7EEqwKi3qUBTGDDJ7DcfYS7YJC2f8xwKVVa6SFUpH5MZ5xcyn1BCK

pub fn post_vaa(ctx: &ExecutionContext, accs: &mut PostVAA, vaa: PostVAAData) -> Result<()> 
    let msg_derivation = PostedVAADerivationData 
        payload_hash: accs.signature_set.hash.to_vec(),
    ;

    accs.message
        .verify_derivation(ctx.program_id, &msg_derivation)?;
    accs.guardian_set
        .verify_derivation(ctx.program_id, &(&vaa).into())?;

    if accs.message.is_initialized() 
        return Ok(());
    

    // Verify any required invariants before we process the instruction.
    check_active(&accs.guardian_set, &accs.clock)?;
    check_valid_sigs(&accs.guardian_set, &accs.signature_set)?;
    check_integrity(&vaa, &accs.signature_set)?;

    // Count the number of signatures currently present.
    let signature_count: usize = accs.signature_set.signatures.iter().filter(|v| **v).count();

    // Calculate how many signatures are required to reach consensus. This calculation is in
    // expanded form to ease auditing.
    let required_consensus_count = 
        let len = accs.guardian_set.keys.len();
        // Fixed point number transformation with one decimal to deal with rounding.
        let len = (len * 10) / 3;
        // Multiplication by two to get a 2/3 quorum.
        let len = len * 2;
        // Division to bring number back into range.
        len / 10 + 1
    ;

    if signature_count < required_consensus_count 
        return Err(PostVAAConsensusFailed.into());
    

    // Persist VAA data
    accs.message.nonce = vaa.nonce;
    accs.message.emitter_chain = vaa.emitter_chain;
    accs.message.emitter_address = vaa.emitter_address;
    accs.message.sequence = vaa.sequence;
    accs.message.payload = vaa.payload;
    accs.message.consistency_level = vaa.consistency_level;
    accs.message.vaa_version = vaa.version;
    accs.message.vaa_time = vaa.timestamp;
    accs.message.vaa_signature_account = *accs.signature_set.info().key;
    accs.message
        .create(&msg_derivation, ctx, accs.payer.key, Exempt)?;

    Ok(())

但是，攻击者如何能通过post_vaa中的check_active,check_valid_sigs,check_integrity等签名检查呢？攻击者需要提供相应的SignatureSet。
攻击者在另一笔交易中调用了Solana端Wormhole bridge主合约中的verify_signatures函数，具体交易见：
https://solscan.io/tx/25Zu1L2Q9uk998d5GMnX43t9u9eVBKvbVtgHndkc2GmUFed8Pu73LGW6hiDsmGXHykKUTLkvUdh4yXPdL3Jo4wVS

pub fn verify_signatures(
    ctx: &ExecutionContext,
    accs: &mut VerifySignatures,
    data: VerifySignaturesData,
) -> Result<()> 
    .......

    let current_instruction = solana_program::sysvar::instructions::load_current_index(
        &accs.instruction_acc.try_borrow_mut_data()?,
    );
    if current_instruction == 0 
        return Err(InstructionAtWrongIndex.into());
    

    // The previous ix must be a secp verification instruction
    let secp_ix_index = (current_instruction - 1) as u8;
    let secp_ix = solana_program::sysvar::instructions::load_instruction_at(//此处。自Solana 1.8起已deprecate，Unsafe because the sysvar accounts address is not checked, please use load_instruction_at_checked instead
        secp_ix_index as usize,
        &accs.instruction_acc.try_borrow_mut_data()?,
    )
    .map_err(|_| ProgramError::InvalidAccountData)?;

    // Check that the instruction is actually for the secp program
    if secp_ix.program_id != solana_program::secp256k1_program::id() 
        return Err(InvalidSecpInstruction.into());
    

    ........

在verify_signatures函数中，会将guardians提供的a set of signature pack为 a SignatureSet，但是，在该函数中并不会直接进行验签，而是将验签操作委托给secp256k1合约。问题就出在这，solana_program::sysvar::instructions mod意味着与Instruction sysvar（a sort of precompile on Solana）一起使用。但是，Wormhole中使用的solana_program版本中，并未验证所使用的合约地址：
solana_program::sysvar::instructions::load_instruction_at(//此处。自Solana 1.8起已deprecate，Unsafe because the sysvar accounts address is not checked, please use load_instruction_at_checked instead

使用过期的load_instruction_at方法，意味着，黑客可创建自己的account，存储与Instructions sysvar中相同的数据，然后在调用’verify_signatures’时将该帐户替换为Instruction sysvar，就可完全绕过签名验证。

事实上，黑客就是按如上方法操作的，在数小时前，黑客创建了相应的account，包含了a single serialized instruction corresponding to a call to the Secp256k1 contract，然后将该account传入作为Instruction sysvar，创建该account的交易见：
https://solscan.io/account/2tHS1cXX2h1KBEaadprqELJ6sV9wLoaSdX68FqsrrZRd
将该account作为Instructions sysvar参数传入调用verify_signatures，实际可验签通过：

至此，黑客就拥有了造假的SignatureSet，可用该假的SignatureSet来生成有效的VAA，并触发unauthorized mint to their own account。

漏洞修复后的合约代码为：

参考资料

[1] How $323M in crypto was stolen from a blockchain bridge called Wormhole
[2] Wormhole漏洞解析
[3] Wormhole漏洞分析
[4] 被盗的3.2亿美元以太坊：这个锅谁该来扛？