Onchain Heroes — a Mythical weapon never mints weapon shards

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 61819-h-02-hero-owner-with-mythical-weapon-can-never-receive-weapo. Standalone Foundry PoC and full write-up: 61819-h-02-hero-owner-with-mythical-weapon-can-never-receive-weapo_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/logic/reward-calculation

Reproduction: a self-contained Foundry PoC that compiles and runs offline with only forge-std — no fork and no RPC. Trace: output.txt. PoC: test/61819-h-02-hero-owner-with-mythical-weapon-can-never-receive-weapo_exp.sol.

Key info#

TL;DR#

WEAPON_SHARD_TABLE is 0xD282321E140A, 48 bits, six rarity bytes. Mythical weapons are rarity 6. The reward function shifts the table by 6 * 8 bits, the masked byte is 0, and the hero is minted nothing.

The published Mythic coefficient is 210 (0xD2). Written into a 56-bit table it is 0xD2D282321E140A. At zone 3 that chance is 2,100, so 2 shards are guaranteed. With request id 0 and random 0 the leftover roll is 117 and does not add a third shard. N = 2. The Mythical claim returns 0. A Legendary hero, rarity 5, reads byte 210 from the same short table and is paid 2.

The vulnerable code#

uint256 weaponShardChance = (((WEAPON_SHARD_TABLE >> (rarity * 8)) & 0xff) * (25 + (zone * 25))) / 10;

uint256 guaranteedMint = weaponShardChance / 1000;

if (weaponShardChance != 0) {
    uint256 chance = weaponShardChance - (guaranteedMint * 1000);
    randomNumber_ = uint256(keccak256(abi.encode(randomNumber_, requestId)));
    if ((randomNumber_ % 1000) < chance) {
        guaranteedMint += 1;
    }
}
if (guaranteedMint > 0) {
    items.mint(hero.owner, 1, guaranteedMint);
}

WEAPON_SHARD_TABLE is 0xD282321E140A. The shift for rarity 6 is the line above it marked @> VULN. The fix is a 56-bit constant, 0xD2D282321E140A, so rarity 6 reads 0xD2.

Root cause#

The packed word was sized for rarities 0 through 5. Rarity 6 shifts past every defined byte. weaponShardChance is 0, guaranteedMint is 0, and the random branch is skipped, so items.mint never runs.

Preconditions#

• The hero's weapon rarity is 6.
• Zone is 3, where byte 210 becomes exactly 2 guaranteed shards.
• The claim uses request id 0 and random 0, so the 100-unit remainder does not add a bonus shard.

Attack walkthrough#

1. Equip hero 1 with a Mythical weapon in zone 3.
2. Claim bonus rewards. The table shift produces byte 0. The mint amount is 0. The hero's balance stays 0.
3. The 56-bit table, same zone and same random, owes 2 shards.
4. Equip hero 2 with a Legendary weapon, rarity 5, in the same zone. That claim mints 2 shards. The short table still works for rarities 0-5.

Control: the same claim against 0xD2D282321E140A mints 2 shards to the Mythical hero.

Diagrams#

Impact#

The Mythical hero is paid 0 weapon shards on a claim that owes 2. Players who hold the rarest weapon receive nothing from this path. A Legendary hero in the same zone is paid the 2 shards the Mythical hero was denied.

Remediation#

Extend WEAPON_SHARD_TABLE from 48 bits to 56 and store the Mythic coefficient in the new byte. With 0xD2 in that byte, zone 3 pays 2 shards before the remainder roll.

Sources#

• AuditVault finding 61819
• Shieldify Onchain Heroes S2 Phase 2 review
• Vulnerable function: Dragma.sol at 6bc8f2a

Taxonomy#

• severity/high
• vuln/logic/reward-calculation

Sources & further analysis#

Reproductions & code

• Standalone PoC + full trace: 61819-h-02-hero-owner-with-mythical-weapon-can-never-receive-weapo_exp (evm-hack-registry mirror).
• AuditVault finding: 61819-h-02-hero-owner-with-mythical-weapon-can-never-receive-weapo.

Alerts & third-party analyses

• Web3Sec X hacked database: search.
• Rekt leaderboard: search.
• Solodit incident search: search.

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