Note: this article is now at least two years old and may contain serious errors.

Expl0itch4ins

Update: 19/02/2017 - added discussion on hacker news. I also added an example.

Update: 21/02/2017 - added discussion of obfuscated exploits, early disclosure penalties, incentives, and scalability**


Bug bounties suck. Researchers routinely don't get paid for their work and vendors continue to get away with the same shitty behavior. It's a system that lacks any kind of accountability and only benefits the company.

Solution: Do it as a smart contract on a blockchain.

An example

  1. A smart contract to audit a C-based program is written. It includes a test case to see if a file with a specific name has been created under the process' permissions. It also includes information about the program.
  2. A researcher finds a bug and uses it to write a buffer overflow exploit. The exploit is designed to pass the test case and is written using a special domain specific language for exploitable code for security reasons.
  3. They go through the protocol to claim the reward by committing the exploit and a payment address.
  4. The network runs the exploit against the software within a virtual machine and runs the test case to check for the file. If a valid exploit was found the process should have been hijacked to write the file. The validity of an exploit thus forms part of the consensus rules for the exploit blockchain.

This process means that the complexity of a program to audit doesn't need to be accounted for and only the results of an exploit are worth checking. But one also needs to be careful with obfuscated exploits as these would make it much harder for the vendor to release a patch.

One possible solution to the obfuscated exploit issue is to create a special language specifically for describing exploitable code that can then be used to express highly compact exploits. Such a language might not necessary be turning complete to start with but it would be formulated in a way to make writing obfuscated code very difficult.

This exploit DSL would be closely tied to how the test-cases work so that many details of an exploit can be omitted to make code more readable. Also note how this process doesn't depend on trust since the results of all computations are verifiable. Hence there is no need to introduce a trusted third-party. It's also important to differentiate this from so-called "oracle" schemes which depend on a number of trusted participants who vouch for external state or some subset of trusted operations.

This scheme can be done entirely based on regular proof-of-work as every full node can easily check for themselves a transaction's validity based on the program's response to exploits. So this whole thing is really just another part of the consensus (which is what makes this a smart contract.)

Technical details

1. contract_tx = Hash(binary_file) as H1, URL(binary_file) as URL1

  • Setup the smart contract with the bug bounty information. Valid bugs are expressed in terms of input to the program via user-input, network, and/or files, in accordance to templates or custom rules. E.G. Bug validity is determined based on whether or not a bug can cause privileged actions to happen within the context of the process space. This is testable.

2. exploit_tx = Hash(exploit_code + payout_pub_key) as H2

  • Commit to a hash of an exploit and payout address that satisfies the bounty conditions against the binary_file.
  • Commit to a collateral payment that gets given to the miners if the exploit is invalid to avoid attacks.

3. disclosure_tx = RSA_Encrypt(exploit_code, vendor_pub_key) as E

  • Publicly release an encrypted version of the exploit using the vendor's public key. This is done with no random padding such that the same plaintext always produces the same ciphertext (YOLO.)

4. Vendor receives encrypted exploit

  • Vendor decrypts the exploit code with their RSA private key from the blockchain and validates it against their software.

5. The vendor (actually) pushes a patch to their customers on time. Because if they don't the smart contract punishes them. Can you hear the trolls singing?

6. confirm_tx (optimal) The vendor signals to the researcher that they may disclose the exploit now.

7. release_tx (the researcher actually gets paid for once) The researcher releases the exploit code and claims the reward. Input = Release exploit_code + payout address such that H(exploit_code + payout address) == H2. Output = anything. Sig = Must be signed with the ECDSA key pair used for payout address!

8. Validate the release TX

  • The network validates the exploit against the binary file in accordance with the rules listed for the contract. Exploits are executed in virtual machines and the validity forms the consensus rules for allocating rewards.
  • The network looks at whether or not the vulnerability was disclosed to the vendor by doing RSA_Encrypt(exploit_code, vendor_pub_key) == E. If it was it looks at how many blocks have elapsed since disclosure and release to calculate to what extent a penalty or reward is justified.
  • If an exploit was disclosed too early the smart contract can specify a penalty to the researcher (like reduced reward or even taking away researcher collateral if that is used to submit "solutions.") One issue with this idea is that if another person finds a vulnerability and discloses it early it is impossible to prove if the original researcher was the one who actually did this so this opens up an attack vector. After thinking about this some more from what I can tell it really only makes sense to have early disclosure penalties for highly paid flaws / serious bugs.
  • Optional: Reward is initially given as a fraction of the number of outstanding exploits against the software version, N / reward_size but as exploits are released you get to see who was first to claim a bounty so that the rest of the reserved fractions become available to the same researcher. This allows rewards to be split for duplicate bugs.

9. patch_tx = Hash(new_binary_file) as H3, URL(new_binary_file) as URL2, code_changes as diff

  • Vendor uses an ECDSA private key to sign a new transaction to point their software to an updated version. This updates the contract meta-data and proves to the network that the exploit has been fixed.

10. Validate the patch TX.

  • Patches can be expressed as small programs that alter the code for the original software. The network can then use these additions to rebuild the original binary files from a deterministic build system, run test cases, and run the original exploit against it to see if it succeeds or fails.
  • The network also looks at how long it took the vendor to initiate a patch TX. Rewards and penalties are therefore assigned based on the rules originally set forth within the contract_tx.

Goto: step 1 again.

1. Continue running exploits and allocating rewards.

  • Because exploits are written against a particular version its possible to track at what point in the chain a patch defeated a particular set of exploits. This makes it possible to see what researchers submitted duplicate bugs and to give them a split of the reward (if specified.)
  • To avoid attacks all exploits must be disclosed within a reasonable number of blocks regardless of a patch TX. This ensures that rewards are given out on time and that rewards that have been allocated as a potential split between researchers are progressively given out to the first person in the chain to have demonstrated the bug. Perhaps penalties for false exploits can be given to the first researcher whose time was wasted.
  • A DAO between researchers and vendors can be established to verify patches if this patch system doesn't work.

Most of the complexity for this scheme is to stop people from stealing exploits and saying that they wrote them. Because if you didn't use commitments someone can just copy the exploit hash directly from your transaction and try confirm it before you. I suppose this isn't that much different from how blockchain notaries work if that helps.

Scalability and incentives

The problem of scaling such an expensive blockchain and designing it in a way that has proper full node incentives is a huge problem by itself. I still need to think more about this issue but so far I am liking the idea of having incentivized full nodes.

This will be a reward system for maintaining the blockchain. In Bitcoin rewards are given out for ordering events and validating transactions. In the exploit chain rewards will be given for checking exploits on behalf of researchers and auditing rewards.

There is likely going to need to be cryptographic proof done on the blockchain and a file storage protocol will work for proving that a node actually has a full copy. Periodic audits will need to be done to check that full nodes are actually providing the service that they need to.

I will keep thinking on the scalability issue.

Optional crowd sourcing

The contracts can specify that patch updates can be given by anyone rather than always by the vendor with as little or as much controls in place as required (manual approval or must pass test suites.)

Edit: Most likely this won't work. There's no way to autonomously prove that additional bugs haven't been added to the software. At best you can use a smart contract to automatically decide to flag a patch as being ready to a group of human oracles based on tests passing but that is really no different to how software is already written. I've left the original scheme in bellow.

1. good_samaritan_tx = Commit to Hash(patch_code + pay_out_address) as H4.

2. samaritan_claim_tx Input = patch_code + payout_address such that Hash(patch_code + payout address) == H4 output = anything sig = Signed with payout address.

3. Patch code is validated against the exploit and if it defeats it the transaction is valid.

Private exploit sales

In theory zero-knowledge proofs can be used to prove that a person has produced a valid exploit for a buyer. This satisfies the requirement of doing trustless private purchases for an exploit (as the existing contract requires the exploit to be revealed for the reward to be given.)

The ZK proof approach means that after you encrypt the exploit your proof is given in zero knowledge. If you try to get smart and produce a valid proof but encrypt an invalid exploit the vendor can always release what you provided as the encrypted exploit to prove to the network that you are cheating. This requires a clearing phase for security.

Thus autonomous private exploit markets are possible in theory.

Btw: Something else I just realized is that this protocol has its place within the bug bounties too. The use-case would be to reward researchers for their work much earlier on without having to wait for the the patch time-frame set forth within the contract to elapse before doing a disclosure.

Why does this matter

Under the current bug bounty system:

  • Researchers aren't guaranteed payment for their work
  • Vendors aren't accountable for security
  • Researchers are forced to operate under their identities
  • Bug bounties are vague or poorly defined
  • The vendor is free to delay payment for bugs
  • There is no codified penalties for early disclosure

By representing the interests of the customer, researcher, and vendor as a smart contract we can build an autonomous bug bounty system / exploit marketplace that will produce much better results.

Feedback so far

There's a good discussion for this going on hacker news at the moment.

Some thoughts that people have raised:

  • Security - code runs in VMs on full nodes so how do you protect the host from compromise? I'm open to ideas.
  • Rewards - will vendors agree to this process?
  • Design - this will only work for specific exploit types to start with. Is that going to be useful enough for researchers?
  • Architecture - what platforms will the VMs target?
  • Scalability - how is something that needs to burn so much bandwidth and computations going to scale? This is a big problem.

There are still a lot of open questions on how best to achieve this so I'd love to know what your thoughts are on this dear reader.