tx.origin / timestamp / block-number misuse

When this applies

• Auth checks using tx.origin
• Time-based logic (lockups, deadlines, vesting, auctions)
• Block-number-based rate limits or cooldowns
• L2 deployments where block semantics differ from L1
• Randomness derived from block data

Detection patterns

tx.origin for authorization (HIGH)

require(tx.origin == owner);   // ← phishable via intermediate contract

A contract the owner has interacted with can drain by calling back into this function — owner-EOA is tx.origin even if direct caller is the malicious contract.

Exception: tx.origin == msg.sender check to enforce EOA-only is sometimes valid for limiting bots — but EIP-7702 will invalidate even this. Flag as outdated pattern.

block.timestamp for high-precision deadlines (MEDIUM)

Miners (post-merge: validators) can skew timestamp ±~15s. Don't use for sub-15s precision.

block.timestamp as randomness (HIGH)

uint r = uint(keccak256(abi.encode(block.timestamp, msg.sender)));   // ← validator can simulate

block.number on L2 (HIGH)

Arbitrum: block.number is L1 block number, NOT L2. block.timestamp is L2. Optimism: block.number is L2. Get this wrong and rate limits fire 12x too fast (or too slow).

blockhash(n) with n outside [block.number-256, block.number-1] returns 0 (MEDIUM)

Random distribution becomes biased / predictable.

block.coinbase (validator address) used in logic (HIGH)

Validator can rotate addresses; using coinbase for auth or whitelisting is unsafe.

block.prevrandao for high-value randomness (HIGH)

Post-merge prevrandao is the previous block's randao; validators can sometimes choose to skip producing a block to influence it (limited but real). Don't use for valuable mints.

block.basefee as anti-MEV gate (LOW-MEDIUM)

Basefee can be manipulated within bounds by miners. Use sparingly.

Timestamp-based vesting cliffs (LOW)

Generally fine, but document tolerance for ±15s.

Severity rubric

Remediation patterns

• Auth: always msg.sender. If you need to know "is this caller a contract", check caller.code.length > 0 but understand its limits.
• Randomness: Chainlink VRF or commit-reveal.
• L2 block-number: use chain-specific helper (ArbSys.arbBlockNumber() on Arbitrum, block.number on Optimism).
• Timestamps: round to ~minute granularity for fairness-sensitive flows.
• Validators-can-skew tolerance: bake in slack in deadlines (≥1 minute).

False-positive notes

• block.timestamp for week/month-scale vesting → fine.
• tx.origin in events for analytics → fine, not used for auth.
• L2 chain-id detection via block.chainid is correct (always returns the canonical chain id).

Related

• [[mev-frontrunning]] — randomness/RNG
• [[access-control]]
• [[signature-replay]]