Cross Contract State

Cross-contract state consistency detection

When this applies

Trigger on any of:

• A value (price, total supply, share rate, debt) lives in contract A but is read by B
• Cached/mirrored state (cachedRate, lastTotalAssets) that must be refreshed
• Multi-contract operations that mutate several contracts non-atomically
• Modules/plugins/hooks that read core state mid-update (read-during-callback)
• Proxy + implementation, or vault + strategy, or controller + market splits
• Accounting assuming "A and B are always in sync"

Detection patterns

Stale read of another contract's mid-update state (HIGH)

// Vault.deposit() transfers to Strategy, then:
uint256 tvl = strategy.totalAssets();   // ← strategy hasn't accounted the deposit yet
shares = amount * totalSupply / tvl;    // wrong denominator

Signal: B reads A while A's update is incomplete (or vice versa). Share/price math uses a denominator that's about to change, letting an attacker mint mispriced shares. This is the read-only-reentrancy family generalized to ordinary call ordering.

Cached value drift (HIGH)

uint256 public cachedPrice;            // updated by poke()
function valueOf(uint256 amt) external view returns (uint256) {
    return amt * cachedPrice / 1e18;   // ← may be hours stale
}

Signal: a mirrored value with no freshness guarantee or no atomic refresh-before-use. Trades/loans price off a cache that diverged from the source contract.

Non-atomic multi-contract update (HIGH)

registry.setActive(id, true);
// external call here can observe registry=active but vault=uninitialized
vault.initialize(id);

Signal: state spread across contracts is updated in sequence; a reentrant or interleaved call observes a half-applied transaction (one contract updated, the other not). Invariants that span both contracts are temporarily violated.

Shared dependency divergence (MEDIUM / HIGH)

Two contracts both read the same oracle/token but at different blocks or via different cached copies, then reconcile assuming equality. Signal: assert(A.totalSupply() == B.mirroredSupply())-style assumptions with independent update paths.

Read-during-callback (HIGH)

A hook/plugin invoked mid-operation calls back and reads the core contract's not-yet-finalized accounting — the Curve/Balancer read-only reentrancy shape, but also plain composability (e.g. an ERC-4626 vault read by a money-market during the vault's own deposit).

Severity rubric

| Pattern | Severity | Notes | |---|---|---| | Stale cross-contract read in share/price math | High | Mispriced mint/redeem | | Read-during-callback of unfinalized state | High | Read-only reentrancy class | | Non-atomic multi-contract invariant break | High | Half-applied state observable | | Cached value drift used for valuation | High | Stale pricing | | Synchronized-state assumption across modules | Medium | Reconciliation error |

Remediation patterns

1. Single source of truth — derive values from one authoritative contract at read time; avoid mirrored copies, or make the mirror push-updated atomically in the same tx.
2. Update-then-read ordering — ensure A finalizes accounting before B reads; apply CEI across the call boundary, not just within a function.
3. Atomic multi-contract updates — batch via a single entrypoint / multicall that completes all writes before any external observation; guard with a system-wide reentrancy lock.
4. Freshness gates on caches — require(block.timestamp - lastUpdate <= maxAge) or refresh-on-read.
5. Reentrancy-guard the read path too (Balancer ensureNotInVaultContext / Curve lock) when callbacks can observe state.

False-positive notes

• A mirror that is push-updated atomically in the same transaction as its source is consistent — don't flag.
• Independent contracts that never reconcile or compare state aren't subject to this; the risk needs a shared invariant.
• Eventually-consistent designs that explicitly tolerate drift (and bound it) are by-design — note the assumption.

Related

• [[reentrancy]] — read-only reentrancy is the callback variant of this
• [[oracle-redundancy]] — shared feed staleness/divergence
• [[storage-layout]] — proxy/implementation storage must also stay consistent

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: omermaksutii
• Source: omermaksutii/RugProof
• License: MIT
• Homepage: https://omermaksutii.github.io/RugProof

Install and usage instructions live in the source repository linked above.