Solana Swap Emulator: SwapAToB Account Puzzle

For H7CTF 2026 the AI-category target here is a toy Solana SPL swap program. A raw-socket harness drives a self-contained re-implementation of the program loader, and solving it is: submit one correct account list for the swap instruction, encoded exactly how a real Solana runtime would need it.

Files

• lib.rs - the on-chain program. Borsh-defined Pool account plus an InitPool/SwapAToB/SwapBToA instruction enum.
• solve.py - the driver. Opens a socket, parses the banner into address/value names, submits accounts then a Borsh-encoded instruction.

Analysis

The target program's lib.rs is a small AMM skeleton with no real pricing. The interesting piece is the direction check inside swap:

let (want_src, want_dst) = if a_to_b { (p.vault_a, p.vault_b) } else { (p.vault_b, p.vault_a) };

Because the pool explicitly names which vault is source and which is destination, the account submission has to map cleanly onto that check, otherwise it fails on direction. For SwapAToB, funds for the incoming leg flow from a user-held user_a account into vault_a; the outgoing leg pays out of vault_b into user_b; the checking must keep exactly this mapping so the pooled deposited tokens become the intended direction, i.e. not merely the converse, for both the input and the payout legs. The user wallet is the only signer-checked authority and holds both user_a and user_b.

The Borsh encoding is trivial: the instruction enum tags are InitPool=0, SwapAToB=1, SwapBToA=2, and SwapAToB { amount } is bytes([1]) + struct.pack("<Q", amount).

Exploit

solve.py parses per-key values from the banner text with a regex (^name: value in multiline mode) for pool, authority, user, mint_a, mint_b, vault_a, vault_b, user_a, user_b, token_program, then encodes the accounts:

accounts = [f"r {pool}", f"r {authority}", f"s {user}", f"w {user_a}",
            f"w {user_b}", f"w {vault_a}", f"w {vault_b}",
            f"r {mint_a}", f"r {mint_b}", f"r {token_program}"]

The flags are r read-only, s signer, w writable - the standard Solana AccountMeta semantics. The swap instruction is then:

instruction = bytes([1]) + struct.pack("<Q", 101010101)

The amount is arbitrary; 101010101 is a non-zero, un-suspicious filler amount, with high bits set so it does not look like a degenerate sell-all test.

Flag

Flag: not recovered in the writeup. No flag string appears anywhere in solve.py or lib.rs.