Add content from: Use mutation testing to find the bugs your tests don't catch

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src/SUMMARY.md

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 - [Post Exploitation](todo/post-exploitation.md)
 - [Investment Terms](todo/investment-terms.md)
 - [Cookies Policy](todo/cookies-policy.md)
+
+  - [Mutation Testing With Slither](blockchain/smart-contract-security/testing-and-analysis/mutation-testing-with-slither.md)

src/blockchain/blockchain-and-crypto-currencies/README.md

@@ -176,6 +176,14 @@ Transactions in Ethereum involve a sender and a recipient, which can be either u
 
 These practices and mechanisms are foundational for anyone looking to engage with cryptocurrencies while prioritizing privacy and security.
 
+## Smart Contract Security
+
+- Mutation testing to find blind spots in test suites:
+
+{{#ref}}
+../smart-contract-security/testing-and-analysis/mutation-testing-with-slither.md
+{{#endref}}
+
 ## References
 
 - [https://en.wikipedia.org/wiki/Proof_of_stake](https://en.wikipedia.org/wiki/Proof_of_stake)

src/blockchain/smart-contract-security/testing-and-analysis/mutation-testing-with-slither.md

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+# Mutation Testing for Solidity with Slither (slither-mutate)
+
+{{#include ../../../banners/hacktricks-training.md}}
+
+Mutation testing "tests your tests" by systematically introducing small changes (mutants) into your Solidity code and re-running your test suite. If a test fails, the mutant is killed. If the tests still pass, the mutant survives, revealing a blind spot in your test suite that line/branch coverage cannot detect.
+
+Key idea: Coverage shows code was executed; mutation testing shows whether behavior is actually asserted.
+
+## Why coverage can deceive
+
+Consider this simple threshold check:
+
+```solidity
+function verifyMinimumDeposit(uint256 deposit) public returns (bool) {
+    if (deposit >= 1 ether) {
+        return true;
+    } else {
+        return false;
+    }
+}
+```
+
+Unit tests that only check a value below and a value above the threshold can reach 100% line/branch coverage while failing to assert the equality boundary (==). A refactor to `deposit >= 2 ether` would still pass such tests, silently breaking protocol logic.
+
+Mutation testing exposes this gap by mutating the condition and verifying your tests fail.
+
+## Common Solidity mutation operators
+
+Slither’s mutation engine applies many small, semantics-changing edits, such as:
+- Operator replacement: `+` ↔ `-`, `*` ↔ `/`, etc.
+- Assignment replacement: `+=` → `=`, `-=` → `=`
+- Constant replacement: non-zero → `0`, `true` ↔ `false`
+- Condition negation/replacement inside `if`/loops
+- Comment out whole lines (CR: Comment Replacement)
+- Replace a line with `revert()`
+- Data type swaps: e.g., `int128` → `int64`
+
+Goal: Kill 100% of generated mutants, or justify survivors with clear reasoning.
+
+## Running mutation testing with slither-mutate
+
+Requirements: Slither v0.10.2+.
+
+- List options and mutators:
+
+```bash
+slither-mutate --help
+slither-mutate --list-mutators
+```
+
+- Foundry example (capture results and keep a full log):
+
+```bash
+slither-mutate ./src/contracts --test-cmd="forge test" &> >(tee mutation.results)
+```
+
+- If you don’t use Foundry, replace `--test-cmd` with how you run tests (e.g., `npx hardhat test`, `npm test`).
+
+Artifacts and reports are stored in `./mutation_campaign` by default. Uncaught (surviving) mutants are copied there for inspection.
+
+### Understanding the output
+
+Report lines look like:
+
+```text
+INFO:Slither-Mutate:Mutating contract ContractName
+INFO:Slither-Mutate:[CR] Line 123: 'original line' ==> '//original line' --> UNCAUGHT
+```
+
+- The tag in brackets is the mutator alias (e.g., `CR` = Comment Replacement).
+- `UNCAUGHT` means tests passed under the mutated behavior → missing assertion.
+
+## Reducing runtime: prioritize impactful mutants
+
+Mutation campaigns can take hours or days. Tips to reduce cost:
+- Scope: Start with critical contracts/directories only, then expand.
+- Prioritize mutators: If a high-priority mutant on a line survives (e.g., entire line commented), you can skip lower-priority variants for that line.
+- Parallelize tests if your runner allows it; cache dependencies/builds.
+- Fail-fast: stop early when a change clearly demonstrates an assertion gap.
+
+## Triage workflow for surviving mutants
+
+1) Inspect the mutated line and behavior.
+   - Reproduce locally by applying the mutated line and running a focused test.
+
+2) Strengthen tests to assert state, not only return values.
+   - Add equality-boundary checks (e.g., test threshold `==`).
+   - Assert post-conditions: balances, total supply, authorization effects, and emitted events.
+
+3) Replace overly permissive mocks with realistic behavior.
+   - Ensure mocks enforce transfers, failure paths, and event emissions that occur on-chain.
+
+4) Add invariants for fuzz tests.
+   - E.g., conservation of value, non-negative balances, authorization invariants, monotonic supply where applicable.
+
+5) Re-run slither-mutate until survivors are killed or explicitly justified.
+
+## Case study: revealing missing state assertions (Arkis protocol)
+
+A mutation campaign during an audit of the Arkis DeFi protocol surfaced survivors like:
+
+```text
+INFO:Slither-Mutate:[CR] Line 33: 'cmdsToExecute.last().value = _cmd.value' ==> '//cmdsToExecute.last().value = _cmd.value' --> UNCAUGHT
+```
+
+Commenting out the assignment didn’t break the tests, proving missing post-state assertions. Root cause: code trusted a user-controlled `_cmd.value` instead of validating actual token transfers. An attacker could desynchronize expected vs. actual transfers to drain funds. Result: high severity risk to protocol solvency.
+
+Guidance: Treat survivors that affect value transfers, accounting, or access control as high-risk until killed.
+
+## Practical checklist
+
+- Run a targeted campaign:
+  - `slither-mutate ./src/contracts --test-cmd="forge test"`
+- Triage survivors and write tests/invariants that would fail under the mutated behavior.
+- Assert balances, supply, authorizations, and events.
+- Add boundary tests (`==`, overflows/underflows, zero-address, zero-amount, empty arrays).
+- Replace unrealistic mocks; simulate failure modes.
+- Iterate until all mutants are killed or justified with comments and rationale.
+
+## References
+
+- [Use mutation testing to find the bugs your tests don't catch (Trail of Bits)](https://blog.trailofbits.com/2025/09/18/use-mutation-testing-to-find-the-bugs-your-tests-dont-catch/)
+- [Arkis DeFi Prime Brokerage Security Review (Appendix C)](https://github.com/trailofbits/publications/blob/master/reviews/2024-12-arkis-defi-prime-brokerage-securityreview.pdf)
+- [Slither (GitHub)](https://github.com/crytic/slither)
+
+{{#include ../../../banners/hacktricks-training.md}}

src/crypto-and-stego/blockchain-and-crypto-currencies.md

@@ -176,6 +176,14 @@ Transactions in Ethereum involve a sender and a recipient, which can be either u
 
 These practices and mechanisms are foundational for anyone looking to engage with cryptocurrencies while prioritizing privacy and security.
 
+## Smart Contract Security
+
+- Mutation testing to find blind spots in test suites:
+
+{{#ref}}
+../blockchain/smart-contract-security/testing-and-analysis/mutation-testing-with-slither.md
+{{#endref}}
+
 ## References
 
 - [https://en.wikipedia.org/wiki/Proof_of_stake](https://en.wikipedia.org/wiki/Proof_of_stake)