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Reproduced Exploit

Majority Protocol — multi assertResults causes bond loss for all but first

assertResults is permissionless and accepts multiple bonds for the same sessionId. The first successful recordResults fills winners[sessionId]; the second reverts WinnersAlreadyRecorded inside the OO callback, so later asserters never recover their bonds (UMA docs: callbacks must not revert).

Jan 2026•Other•untagged•2 min read

Chain

Other

Category

untagged

Date

Jan 2026

Source

AuditVault

PoC & write-upPoC for your local evm-hack-analyzerAuditVault PoCs

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Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 65379-if-multiple-users-call-defaultsessionassertresults-all-but-t. Standalone Foundry PoC and full write-up: 65379-if-multiple-users-call-defaultsessionassertresults-all-but-t_exp in the evm-hack-registry mirror.

Reproduction: self-contained Foundry PoC (forge-std only) — no fork. Full trace: output.txt.

AuditVault taxonomy: lang/solidity · platform/cyfrin · sector/gaming · severity/high · genome: missing-modifier · direct-drain · dos-resistance

Key info#

ImpactHIGH — second (and later) assertResults callers permanently lose their UMA USDC bonds
ProtocolMajority Protocol
Bug classNo per-session uniqueness on assertDataFor; recordResults reverts on second resolve
FindingCyfrin (Dacian) · #65379
Reporthttps://github.com/solodit/solodit_content/blob/main/reports/Cyfrin/2026-01-27-cyfrin-majority-protocol-v2.0.md
SourceAuditVault
StatusAudit finding — reproduced as a standalone local synthetic
Compiler^0.8.24 (PoC)

TL;DR#

assertResults is permissionless and accepts multiple bonds for the same sessionId. The first successful recordResults fills winners[sessionId]; the second reverts WinnersAlreadyRecorded inside the OO callback, so later asserters never recover their bonds (UMA docs: callbacks must not revert).

HARM: second asserter loses 250 USDC bond permanently.

Root cause#

No session-level assertion uniqueness; callback path reverts instead of no-op when winners already recorded.

Preconditions#

Game in Ended state; two users independently call assertResults with bonds.

Attack walkthrough#

  1. User A asserts results (posts bond).
  2. User B asserts same session (posts bond).
  3. OO resolves A truthfully — winners set, A recovers bond.
  4. OO resolves B — recordResults reverts; B's bond stuck.

Diagrams#

sequenceDiagram participant A as UserA participant B as UserB participant DS as DefaultSession participant OO as OptimisticOracle A->>DS: assertResults bond B->>DS: assertResults bond OO->>DS: assertionResolvedCallback A Note over DS: winners recorded A bond returned OO->>DS: assertionResolvedCallback B Note over DS: WinnersAlreadyRecorded reverts Note over OO: B bond stuck

Impact#

All but the first successful asserter lose their minimum USDC bonds.

Sources#

Sources & further analysis#

Reproductions & code

Alerts & third-party analyses

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

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