Prohibit Nonce Account Self-Withdrawals

Summary

Reject a System Program WithdrawNonceAccount instruction when the nonce account and recipient resolve to the same address. After validating the instruction accounts and nonce state, the System Program returns InvalidArgument when the addresses match. Other nonce withdrawals are unchanged.

Motivation

Currently, a System Program WithdrawNonceAccount instruction may use the same account as the nonce account and recipient. Self-withdrawals from uninitialized nonce accounts and zero or partial self-withdrawals from initialized nonce accounts that pass existing checks subtract and then restore the requested lamports, making them no-ops.

A full self-withdrawal from an initialized nonce account that passes the existing checks behaves differently. It deinitializes the nonce account, subtracts its full balance, and restores the lamports to the same account. This leaves a funded, System Program-owned, uninitialized nonce account.

InitializeNonceAccount does not require the nonce account or new authority to sign. Another party can initialize the account with themselves as the new nonce authority and control the retained balance. Initiating the self-withdrawal requires the existing nonce authority, making this an operator footgun.

Self-withdrawals have no known use case. The Stake Program rejects them with InvalidArgument, and SIMD-0609 proposes the same rule for Vote Program withdrawals.

New Terminology

N/A

Detailed Design

System Program WithdrawNonceAccount processes an instruction in this order:

1. Validate the instruction account count and sysvars, require a writable source, and decode the nonce state.
2. Compare the addresses of the nonce account and recipient.
3. If the addresses match, return InvalidArgument.
4. If the addresses differ, continue existing withdrawal processing.

The new comparison runs before the initialized and uninitialized state branches and their checks for balance, rent, nonce expiration, and required signers. It therefore applies to zero, partial, full-balance, and over-balance withdrawals.

The instruction data and account interface do not change.

Alternatives Considered

• Reject only full self-withdrawals from initialized nonce accounts. This prevents deinitialization but preserves other no-op self-withdrawals.
• Skip deinitialization for full self-withdrawals. This makes them no-ops and leaves a special case that future changes must continue to handle safely.
• Reject self-withdrawals only in the SDK and CLI. Custom transaction builders can bypass client-side checks, so this does not enforce the invariant.
• Keep the existing behavior. This preserves the footgun.

Impact

After activation, nonce account self-withdrawals that reach the address check fail with InvalidArgument. Non-self withdrawals, instruction encoding, and account lists are unchanged.

Because the System Program is a native builtin, validator clients must implement the same feature-gated behavior before activation.

Security Considerations

N/A