Euler Finance — donateToReserves() Missing Health Check

I. Incident Overview

Root cause: donateToReserves() transfers EToken shares out of the caller's position without triggering a health check. No checkLiquidity() or equivalent insolvency gate is executed post-donation. An attacker can donate borrowed assets to reserves, rendering their own position artificially insolvent, then self-liquidate to capture the soft-liquidation bonus of up to 20% introduced in eIP-14. No external price oracle was manipulated — the failure is purely internal to the EToken accounting.

Attack vector: internal accounting invariant — donateToReserves() without insolvency gate  |  Layer: smart_contract  |  Attacker: UNKNOWN — funds returned progressively, no attribution published

Key Facts

• donateToReserves() moves EToken shares out of a position without triggering a health check — no checkLiquidity() gate post-donation [Omniscia]
• The attacker donates borrowed assets to reserves, rendering their own position artificially insolvent, then self-liquidates [Omniscia]
• eIP-14 soft liquidation permits a liquidator bonus of up to 20% on underwater positions — the attacker captures it against themselves [Omniscia]
• The self-liquidation call occurs in the same transaction as the donation [BlockSec]
• A flash loan is used to accumulate the large borrowed position before the donation [BlockSec]
• No external price oracle was manipulated — the vulnerability is purely internal to EToken accounting [BlockSec]
• The vulnerability was live on-chain for approximately 8 months since eIP-14 deployment [Omniscia]
• Approximately $197M drained across DAI, USDC, USDT, stETH and wBTC positions [Omniscia]
• Sherlock and Cyfrin independently confirmed the root cause as a missing insolvency check after donateToReserves() — not a consequence of eIP-14 alone [BlockSec]
• BlockSec detected the exploit in real time on 2023-03-13 [BlockSec]
• The attacker returned funds progressively; Euler Labs confirmed complete recovery on 2023-04-04 [Omniscia]
• Elapsed time exploit to post-mortem: 22 days [FC corpus — t_interval_primary]
• Per-asset breakdown: $135.8M stETH, $33.8M USDC, $18.5M WBTC, $8.7M DAI [BlockSec]
• The attack used a series of six distinct flash loans [BlockSec]
• donateToReserves() was vulnerable across five separate pools [CertiK]
• donateToReserves() was introduced to fix a smaller 'first depositor' bug, missed by all prior auditors, reported by a white hat via Immunefi roughly one year before the 2023 exploit [Euler Labs]

Timeline

Affected Protocols

Evidence Classes & Policies

Every figure in this report is traceable to a named source. Statements attributed to Euler Labs or to the attacker's on-chain messages are marked as primary; everything else is Forensic Capital reconstruction from public chain data. Where the public record is silent, this report says so rather than closing the gap by inference. Sources are listed in full at the end of this document.

II. TVL Impact Analysis

Baseline TVL (J−7 to J−1): $0
Impact J0: N/A  |  Impact J+7: N/A

Source: DefiLlama API — historical TVL, protocol slug: N/A

III. Attack Vector — EToken Accounting Invariant

The Euler exploit required no compromise of any trust boundary. It exploited a single missing gate inside the protocol's own accounting:

FC-CLASS-005-A — Internal Invariant Failure donateToReserves() writes EToken state without re-deriving the solvency invariant. No external dependency. No oracle. No key. The protocol violated its own accounting constraint from within.

Primary sources: Omniscia post-mortem, BlockSec flashloan attack analysis. FC classification: health_check_not_enforced_before_debtWrite, unique across 38 corpus incidents.

Sources: Omniscia post-mortem (omniscia.io/blog/euler-finance-incident-post-mortem), BlockSec flashloan attack analysis (blocksec.com/blog/euler-finance-flashloan-attack).

Technical Detail — EToken Accounting — Invariant Failure Path

IV. Intelligence Analysis — FC Findings

FC Analysis — Structural Findings

Derived from cross-protocol signal analysis (FC signal_inventory [2026-05-28]) and public incident data. 5 structural observations:

V. Forward Scenarios

Scenario 1 is realised — full recovery confirmed 2023-04-04. Scenarios 2 and 3 are structural observations drawn from the FC corpus, not predictive.

Attribution

The exploit was executed on 13 March 2023 against a donation path that bypassed the health check. Attacker addresses and transaction hashes are on the public record. Approximately $197M was drained; the majority was subsequently returned following on-chain negotiation. No party has been identified. The returning address described itself as acting in good faith, a claim no independent verification supports or refutes. Forensic Capital attributes the mechanism, not the actor.

VI. Unknown Declarations

Declarations below apply at confidence threshold <70%.

VII. Sources — Public Record

Position in the corpus