Blend v2 — Steal other users' emissions via vulnerable claim

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 62062-h-02-user-can-steal-other-users-emissions-due-to-vulnerable. Standalone Foundry PoC and full write-up: 62062-h-02-user-can-steal-other-users-emissions-due-to-vulnerable_exp in the evm-hack-registry mirror.

Vulnerability classes: reward-theft · lockup · missing emission checkpoint

Reproduction: self-contained Foundry PoC (Solidity reduction of Soroban backstop emissions accounting) with only forge-std — no fork. Full trace: output.txt.

Key info#


Impact	HIGH — claim-to-third-party deposits shares without emission init → inflated claim steals inventory
Protocol	Blend v2 — backstop emissions
Vulnerable code	`execute_claim` adds shares to `to` without `update_emissions(to)`
Finding	Code4rena — 2025-02-blend · #62062 · [H-02] · reporter oakcobalt
Report	code4rena.com/reports/2025-02-blend-v2-audit-certora-formal-verification
Source	AuditVault
Compiler	`^0.8.24` (PoC synthetic; original is Soroban/Rust)

TL;DR#

execute_claim can deposit exchanged backstop LP to a different to address.
It updates to's share balance without update_emissions(to).
A fresh to later claims with default index 0 while holding shares → accrues full historical index.
Stolen inventory leaves other users unable to claim.

The vulnerable code#

SOLIDITY

// Deposit LP tokens into pool backstop
let to_mint = pool_balance.convert_to_shares(deposit_amount);
pool_balance.deposit(deposit_amount, to_mint);
// @audit missing update_emissions(to)
user_balance.add_shares(to_mint); // @> VULN

Root cause#

Share balance and emission index must be checkpointed atomically. Skipping the checkpoint on the deposit recipient leaves them able to claim unearned historical emissions.

Diagrams#

sequenceDiagram participant A as userA participant C as addressC participant B as Backstop participant V as userB A->>B: execute_claim(to=addressC) Note over B: shares to C without update_emissions(C) C->>B: claim_to_wallet (index default 0) Note over B: inflated accrual drains inventory V->>B: claim - reverts insufficient inventory

Impact#

Direct theft of other depositors' emissions budget; victim claims fail for lack of inventory.

Sources#

AuditVault: https://github.com/Auditware/AuditVault/blob/main/findings/62062-h-02-user-can-steal-other-users-emissions-due-to-vulnerable.md
Report: https://code4rena.com/reports/2025-02-blend-v2-audit-certora-formal-verification
Repo@commit: code-423n4/2025-02-blend@f23b3260763488f365ef6a95bfb139c95b0ed0f9 blend-contracts-v2/backstop/src/emissions/claim.rs

Taxonomy: [[direct-drain]] · [[lockup]] · severity/high · sector/lending

Sources & further analysis#

Reproductions & code

Standalone PoC + full trace: 62062-h-02-user-can-steal-other-users-emissions-due-to-vulnerable_exp (evm-hack-registry mirror).
AuditVault finding: 62062-h-02-user-can-steal-other-users-emissions-due-to-vulnerable.

Alerts & third-party analyses

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

These dashboards index community alerts tweets, post-mortems, and independent write-ups. Reach them through the protocol name above to cross-check this reproduction against other analyses.

Source-level debugger — step opcodes and Solidity in sync