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Test Solidity security by writing down the properties the contract must preserve, then checking them with both individual calls and sequences of calls. Use hostile callback contracts to probe reentrancy, authorized and unauthorized actors to probe permissions, and boundary inputs to probe arithmetic. Add fuzzing, invariant testing, static analysis, and—where suitable—formal analysis as complementary checks, not substitutes for a correct specification and code review.
Start with properties, not tool output
A test is meaningful only if it states what must remain true. Translate the protocol’s intended behavior into explicit properties before choosing tools. Adapt each property to the contract; these examples are not universal guarantees:
- No account can withdraw more than its credited share.
- A privileged function rejects callers without the required role.
- A paused system cannot perform transitions that the protocol prohibits while paused.
- Aggregate liabilities remain covered under the protocol’s accounting model.
Test properties at the right scope. A single-call test can show that one withdrawal behaves as expected in a known setup. A sequence test can expose a failure that requires a deposit, role change, callback, or earlier state transition first.
Pin the build context
Record the exact Solidity compiler version, optimizer and build settings, dependencies, and EVM target used for the tests. Arithmetic behavior depends on compiler version and whether an operation is inside an unchecked block, so a result without its build context can be misleading. Solidity’s versioned Security Considerations documentation explains checked and unchecked arithmetic; its 0.8.38-develop documentation describes a development version and should not be treated as a stable-release specification.
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How do I test a Solidity contract for reentrancy?
Reentrancy is a control-flow risk: when contract A calls contract B, B can receive control and call back into A before A’s first operation has finished. The Solidity Security Considerations documentation states: “Any interaction from a contract (A) with another contract (B) and any transfer of Ether hands over control to that contract (B).” This means the review should cover external contract calls broadly, not only Ether withdrawals.
Map call edges and affected state
Trace each external call, including token callbacks, hooks, and calls to contracts treated as trusted that could themselves invoke other code. For each edge, identify the state that has been read or changed and the invariants that must hold before and after the callee runs. Reentrancy may cross functions or contracts, so include related balances, shares, allowances, debt, and protocol-wide accounting where they apply.
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Build an adversarial callback test
- Set up the contract with state that makes the sensitive action valid, such as an account with a withdrawable balance.
- Use a receiver contract that attempts to call back into the target during the external interaction.
- Exercise the callback into the original function and into any other entry point that changes related state.
- Assert the relevant invariants after the call sequence, including both local balances and cross-contract accounting.
A reentrancy guard or Checks-Effects-Interactions can reduce risk, but neither is a substitute for testing the property. Checks-Effects-Interactions means validate inputs and authorization first, write the intended state changes next, and make external interactions last. Treat it as a design guideline: tests should still try callbacks at reachable interaction points and check the resulting state.
How do I test access control in Solidity?
For every privileged function, make an authorization matrix that records the required role, callers that should succeed, callers that should fail, and any relevant protocol state. Test both sides of the permission rather than checking only that a valid administrator can act.
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| Action or transition | Test with an authorized actor | Test with an unauthorized actor |
|---|---|---|
| Initialization | Expected initializer succeeds in the intended initial state. | Repeated or otherwise unauthorized initialization is rejected, where the design requires it. |
| Privileged operation | Actor with the required role can perform the operation. | Actor without that role cannot perform it. |
| Role or ownership change | Permitted actor can grant, transfer, or revoke authority as designed. | Actor without the relevant permission cannot alter authority. |
| Paused operation | Allowed pause or recovery action works for its permitted caller. | Prohibited action or unauthorized pause control is rejected. |
| Proxy or upgrade initialization | Expected initialization or upgrade path works in the intended state. | Unauthorized or repeated path cannot seize or alter control. |
Include public helpers that may change authority indirectly, revoked roles, and state transitions that change which actions are permitted. Use multiple sender identities in stateful tests. An attacker must not become owner or gain a privileged capability unless the specified process permits it; the Ethereum.org Echidna tutorial uses attacker ownership as an example access-control invariant.
How do I test for integer overflow and underflow?
First establish the compiler version and whether each operation is checked or occurs inside unchecked. Solidity’s default checked arithmetic reverts on overflow or underflow; unchecked arithmetic permits wrapping. Encode the intended outcome explicitly rather than assuming that a revert or wrap is always correct.
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Choose inputs around the boundary
- Test zero and one, the maximum representable value, and values immediately around relevant limits.
- For signed values, test the minimum and maximum as well as nearby values.
- Probe intermediate operations, not only the final storage type: multiplication before division, accumulated totals, fees, casts, narrow types, and loop bounds.
- Exercise division by zero where the path is reachable.
- Inside intentional
uncheckedarithmetic, assert the exact wraparound result and confirm that wrapping cannot bypass balance, authorization, or supply constraints.
The Solidity Security Considerations documentation illustrates overflow with uint8(255) + 1. A checked revert is not automatically safe: if the operation cannot be avoided, the contract or a user flow could become stuck. Test the intended failure path and whether the protocol can recover or continue operating.
Combine scenario tests with sequence testing
Use ordinary tests for named, reproducible cases: expected behavior, boundary inputs, expected reverts, initialization, role transfer, and known callback entry points. These tests are easy to understand and make useful regression cases, but cover only the scenarios their authors write.
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Then fuzz inputs and sequences. Foundry invariant testing executes randomized sequences of calls from configured contracts and checks user assertions during the run. Its runs and depth settings affect campaign breadth. Echidna generates transaction sequences to try to falsify user-defined properties. Configure the harness to include relevant actors, targets, and state-changing actions—especially role changes and callbacks—rather than assuming a tool will explore paths the harness cannot express.
When a campaign finds a failing sequence, preserve the callers, inputs, preconditions, and state transitions. Reduce the failure to a clear regression test, then rerun both the test suite and the campaign. A counterexample is most useful when another developer can understand exactly which property failed and reproduce it.
What each testing approach can and cannot establish
| Approach | Useful for | Limit |
|---|---|---|
| Scenario and unit tests | Known attacks, business rules, specific callbacks, unauthorized callers, and boundary regressions. | They cover only the cases written by the test authors. |
| Foundry fuzz and invariant testing | Randomized inputs and configured call sequences checked against invariants. | Coverage depends on actor and target configuration, campaign settings, and harness design. |
| Echidna | Searching transaction sequences for ones that falsify Solidity properties. | Results depend on property and harness design, reachable sequences, and runtime; a passing run is not proof of all behavior. |
| Slither | Static, pattern-based review, including detectors for reentrancy classes. | Findings require code-path and invariant review; a pattern finding is not automatically an exploitable bug, and absence of a finding is not proof of safety. |
| Solidity SMTChecker and formal analysis | Checking specified properties under supported modeling assumptions and settings. | Solver support, assumptions, abstractions, and property coverage matter; the specification itself may not capture intended behavior. |
Use the axes that matter to the threat: single-call versus multi-call coverage, hostile callers and callbacks, property checking versus pattern detection, reproducibility of counterexamples, supported models, setup effort, and runtime. Slither’s detector documentation describes detector scopes and recommendations; treat output as a prompt for contextual review. Solidity’s formal verification guidance cautions that verification can show code satisfies a formal specification, but does not establish that the specification is the right one.
A repeatable testing workflow
- Pin the environment. Record compiler version, optimizer and build settings, dependencies, and EVM target.
- Write properties. State accounting, authorization, pause, and other protocol rules in terms that tests can assert.
- Cover named scenarios. Add normal-path, boundary, expected-revert, initialization, role-transition, and callback tests.
- Model hostile behavior. Add receiver contracts for reentry and configure relevant sender identities and state-changing calls.
- Fuzz sequences. Use Foundry invariants or Echidna properties to exercise combinations of calls and state transitions.
- Run static analysis. Review Slither findings against actual reachable code paths and the properties they could violate.
- Preserve failures. Minimize counterexamples into understandable regression tests and rerun the suite and campaign.
- Review intent and implementation independently. Check that properties match protocol behavior and that the harness and analysis model cover the relevant system.
These approaches answer different questions. A passing campaign says that the configured exploration did not find a counterexample to the properties it checked; it does not establish that every path was explored or that the properties express the intended security guarantees. Combine behavioral tests with specification review and code review.
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