Introduction
The Stacks Blockchain[1] is a layer-1 platform designed to work alongside the Bitcoin blockchain, enabling smart contracts and decentralized apps on Bitcoin. To facilitate this, Stacks employs its native smart contract language called Clarity[2]. Unlike some other contract languages, Clarity is decidable, allowing developers to predict contract behavior without execution, thereby enhancing security and reducing unforeseen behaviors.
Clarity smart contracts have a potential blind spot: misuse of the `tx-sender
` variable during authentication. This oversight mirrors vulnerabilities like those in OWASP’s threat list and SWC-115[3] from Solidity.
In Clarity, `tx-sender
` identifies the initiating principal (an address in Stacks):
While useful for verifying a function caller, it can, if misused, be an entry point for malicious contracts.
The Vulnerability Unpacked
Imagine a token contract using `tx-sender
` to grant transaction permissions. If a malicious contract can trick a user into initiating a function, it can call the original contract’s `transfer
` function. Why? Because `tx-sender
` doesn’t change, making it seem like the original user is authorizing transfers.
Take the following token contract as example:
(define-fungible-token vulnerable-token u100000) (define-constant err-not-token-owner (err u4001)) (define-public (transfer (amount uint) (sender principal) (recipient principal)) (begin (asserts! (is-eq tx-sender sender) err-not-token-owner) (try! (ft-transfer? vulnerable-token amount sender recipient)) (ok true)))
Here’s the exploit:
- Set up a rogue contract targeting the `
transfer
` function. - Lure a user into activating a function within this contract — using phishing or social engineering.
- The rogue contract hits the `
transfer
` function in the token contract, moving tokens due to the static `tx-sender`.
This rogue contract would look like this:
(define-constant TARGET '<SOME_PRINCIPAL>.vulnerable-token) (define-constant OWNER tx-sender) (define-public (airdrop) (contract-call? TARGET transfer u10000 tx-sender OWNER))
Defense Tactics
Several measures can plug this security hole:
Use `contract-caller
` for Stronger Authentication:
`contract-caller
` in Clarity points to the account or contract making the call. It’s a tighter security measure than `tx-sender
`.
With this countermeasure implemented in the vulnerable token, the attack previously described would not work.
Whitelist Trusted Contracts
When an intermediary contract is required, create a list of trusted contracts. Cross-check `contract-caller
` against this list to weed out the untrusted ones.
Add Post-Conditions
With Stacks 2.0, users can set conditions for contract calls. If a post-call check fails, the transaction aborts. However, this works only for token transfers, not for other phishing opportunities (e.g., if the vulnerable contract would include a function that changes the contract owner authenticating using tx-sender, then the post-condition countermeasure will not work since no token is being transferred).
The updated token contract code:
(define-fungible-token vulnerable-token u100000) (define-constant err-not-token-owner (err u4001)) (define-map trusted-contracts principal bool) (define-public (transfer (amount uint) (sender principal) (recipient principal)) (let ((is-trusted (default-to false (map-get? trusted-contracts contract-caller)))) (asserts! (or (is-eq contract-caller sender) (and is-trusted (is-eq tx-sender sender))) err-not-token-owner) (try! (ft-transfer? vulnerable-token amount sender recipient)) (ok true))) (define-public (add-trusted-contract (contract principal)) (begin (map-set trusted-contracts contract true) (ok true)))
Conclusion
Misusing `tx-sender
` can expose Clarity smart contracts to risks. But with a grasp of the problem and the right tools at hand, developers can create secure, efficient blockchain solutions.
#notes .lqd-highlight-inner{height:0.275em!important;bottom:0px!important;}
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- Source: https://blog.coinfabrik.com/tx-sender-in-clarity-smart-contracts/
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