GHSA-x442-m7cc-hr92 kora-lib: Unrecognized Instruction Types Create Empty Stubs That Bypass Fee Payer Policy

• crates.io kora-lib: < 2.0.5 → fixed in 2.0.5

Summary When inner CPI instructions use instruction types not recognized by Kora's parser (including Token-2022 extension instructions like `ConfidentialTransfer`, `TransferFeeExtension::WithdrawWithheldTokens`, etc.), they are reconstructed as stub instructions with empty accounts and empty data. These stubs fail deserialization during fee payer policy validation and are silently skipped, meaning any fee payer usage within those instructions goes completely unchecked. Severity Medium Affected Component • File: `crates/lib/src/transaction/instruction_util.rs` • Functions: `reconstruct_system_instruction()`, `reconstruct_spl_token_instruction()` • Lines: 750–753, 1187–1189 Root Cause The instruction reconstruction functions have a catch-all `_ =>` arm for unrecognized instruction types that creates a stub `CompiledInstruction` with the correct `program_id_index` but empty `accounts` and empty `data`. When this stub reaches the fee payer policy parsing (`parse_system_instructions` / `parse_token_instructions`), deserialization of empty data fails. The parsing functions also have a catch-all `_ => {}` that silently skips the failed instruction. The result: the instruction exists in `all_instructions` (so program allowlist checks pass), but fee payer policy is never enforced on it. Vulnerable Code Stub Creation ```rust // crates/lib/src/transaction/instruction_util.rs:750-753 // System program — unrecognized instruction type: _ => { log::error!("Unsupported system instruction type: {}", instruction_type); Ok(Self::build_default_compiled_instruction(program_id_index)) } // crates/lib/src/transaction/instruction_util.rs:1187-1189 // SPL Token program — unrecognized instruction type: _ => { log::error!("Unsupported token instruction type: {}", instruction_type); Ok(Self::build_default_compiled_instruction(program_id_index)) } ``` The stub builder: ```rust pub fn build_default_compiled_instruction(program_id_index: u8) -> CompiledInstruction { CompiledInstruction { program_id_index, accounts: vec![], // (&instruction.data) { match system_instruction { // ... known types handled ... _ => {} // {} // arm • Returns: CompiledInstruction { program_id_index, accounts: [], data: [] } 4. Stub is added to all_instructions → validate_programs() sees Token-2022 program ID → PASS (allowed) → validate_disallowed_accounts() sees no accounts in the stub → PASS 5. parse_token_instructions() processes the stub: • TokenInstruction::unpack(&[]) → Err (empty data) • if let Ok(...) block skipped entirely • Instruction not added to any ParsedSPLInstructionType map 6. validate_fee_payer_usage() iterates parsed SPL instructions: • No entry for "withdrawWithheldTokensFromMint" • Fee payer's usage as withdraw_withheld_authority is NEVER checked 7. Transaction is signed by Kora 8. On-chain: fee_payer (as withdraw authority) withdraws withheld transfer fees from the mint to attacker's account ``` Verifiable Test ```rust [test] fn test_unrecognized_instruction_produces_empty_stub() { // Simulate what happens for an unrecognized Token-2022 instruction let program_id_index: u8 = 3; // Token-2022 at index 3 // This is what the catch-all arm produces: let stub = IxUtils::build_default_compiled_instruction(program_id_index); assert_eq!(stub.accounts.len(), 0); // No accounts assert_eq!(stub.data.len(), 0); // No data // Attempt to parse it: let result = TokenInstruction::unpack(&stub.data); assert!(result.is_err()); // Cannot parse empty data // Therefore: fee payer policy is never applied to this instruction // The fee payer could be the withdraw_withheld_authority in the // REAL instruction, but the stub has zero accounts — invisible. } ``` Impact • Fee Payer Policy Bypass: Token-2022 extension instructions that use the fee payer as an authority are invisible to policy enforcement. • Forward-Looking Risk: As Solana and SPL Token-2022 add new instruction types, they will automatically bypass all fee payer policy checks in Kora. • Precondition: Requires the fee payer to hold some authority role (e.g., `withdraw_withheld_authority`, `permanent_delegate`) for Token-2022 accounts. This is unlikely in typical deployments but possible in misconfigured setups. Recommendation Reject transactions containing inner instructions with unrecognized types (fail-secure): ```rust // In reconstruct_system_instruction: _ => { return Err(KoraError::InvalidTransaction(format!( "Unrecognized system instruction type '{}' in CPI — \ cannot validate fee payer policy. Transaction rejected.", instruction_type ))); } // In reconstruct_spl_token_instruction: _ => { return Err(KoraError::InvalidTransaction(format!( "Unrecognized SPL Token instruction type '{}' in CPI — \ cannot validate fee payer policy. Transaction rejected.", instruction_type ))); } ``` Alternatively, maintain a list of known-safe instruction types that don't involve authority checks, and only reject truly unknown types. References • `crates/lib/src/transaction/instruction_util.rs:750-753` — system instruction catch-all • `crates/lib/src/transaction/instruction_util.rs:1187-1189` — SPL token instruction catch-all • `crates/lib/src/transaction/instruction_util.rs:316-319` — `build_default_compiled_instruction` • SPL Token-2022 instruction types — full list of extension instructions