balancer just got drained for ~$116M
this wasn't sophisticated. it was a basic access control bug in their vault contract
here’s how it happened, what it reveals - and why it should scare every defi protocol alive:
1/
the exploit hit balancer v2 today across ethereum, arbitrum, polygon, base, optimism, and more
over $116M gone. 6,590 WETH. 6,851 osETH. 4,260 wstETH
all pulled from the core vault at 0xBA1...BF2C8
2/
the bug was in "manageUserBalance" - a function that's supposed to validate who can move funds
instead, it confused msg.sender with a user-supplied op.sender field
attackers used WITHDRAW_INTERNAL operations to drain tokens they never deposited
3/
what makes this worse?
balancer V2 uses a single vault for everything. every pool, every chain.
hit the vault, hit them all.
many forks are at risk as well.
4/
this is balancer's third major hack in five years
2021, 2023, and now 2025: $116M+ and counting
5/
let's zoom out.
balancer isn't some experiment. $750M TVL. audited.
live for years.and yet: a basic access control flaw sat in prod, audits missed it, no proper sender validation, funds mixed in one central vault.
6/
this is balancer's third major hack in five years
2021: millions lost
2023: $238K after being warned
2025: $116M+
this isn't just a Balancer problem. it's a defi illusion problem
"audited" =/= safe. "battle-tested" =/= secure
7/
takeaway:
basic access control bugs are still destroying blue-chip protocols
if you're building: review every permission check twice.
if you're a user: "audited" means someone looked once, not that it's bulletproof
this wasn't advanced. we're just careless
8/
defi can do better. but first, we need to admit: the basics still matter more than the hype.
i’ll post more as the onchain trail evolves
9/
there are a few reports that the attacker didn't just exploit permissions. they manipulated BPT pricing through precision loss in the StableSwap math.
- drain one token to a rounding edge
- exploit rounding errors to deflate BPT price
- buy back BPT cheap, profit
/10
more details here about how price manipulation was done[1]
still waiting for the balancer’s official response. will keep updating the thread as things unfold.
[1]
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