Who Is Harry Kalodner? Offchain Labs Co-Founder

Harry Kalodner is a co-founder of Offchain Labs, the company that built Arbitrum, and his name comes first on a June 2024 Stylus security audit that warned attackers could use program activation to slow the chain. In early October 2026, Arbitrum's Security Council turned that same activation charge into the switch that paused new Stylus programs.

The audit's first finding said the gas for activating WASM programs "is not charged early enough in the activation code to prevent denial-of-service attacks." Just under 28 months later, the Council answered "sophisticated attacks on hand-crafted WASM programs" by raising that charge from zero to about 18.45 quintillion gas, the 64-bit maximum. Kalodner's company commissioned the review, and the Security Council made the 2026 decision.

Harry Kalodner at a Glance

Who Is Harry Kalodner?

Kalodner's speaker profile for Devcon SEA, the November 2024 Ethereum conference in Bangkok, named him "Co-Founder and Chief Technology Officer at Offchain Labs where he leads the engineering team."

The same profile says Kalodner attended Princeton as a PhD candidate and researched the economics and anonymity of cryptocurrencies before the company existed. That research trail runs straight into the network he co-founded, the Ethereum scaling chain explained in what Arbitrum is as a Layer 2.

The company Kalodner co-founded goes by Offchain on its website, and the site's footer gives the legal name Offchain Labs, Inc.

Kalodner put his name on Arbitrum's first paper at Princeton and, as an Offchain Labs co-founder, on the Nitro design that runs it. Kalodner's name also leads the cover of the Stylus security review, whose first finding flagged the activation gas charge that became the switch for the October 2026 pause.

Harry Kalodner's Career

Kalodner's published research starts at Princeton. In 2016 he co-wrote On the Instability of Bitcoin Without the Block Reward for the ACM CCS conference with three Princeton colleagues. The paper found that once transaction fees replace the block reward, forking a "wealthy" block to "steal" its rewards starts to pay.

Kalodner posted BlockSci to arXiv in September 2017 as first author. BlockSci is an open-source platform for blockchain analysis, and the full paper ran at the 29th USENIX Security Symposium in 2020 with Kalodner still listed first.

Arbitrum itself began as a Princeton paper. Kalodner was the first of five authors on the Arbitrum paper presented at the 27th USENIX Security Symposium in August 2018. It described contracts whose parties agree off-chain on what a virtual machine will do, so miners "need only verify digital signatures" when everyone agrees.

Kalodner co-founded Offchain Labs in 2018, and the company built that design into a live network. Kalodner is one of 16 Offchain Labs authors on the Arbitrum Nitro whitepaper, which describes the second-generation rollup that has run on Arbitrum One since August 31, 2022.

The whitepaper has Nitro settling Arbitrum One's results on Ethereum through an optimistic rollup built on interactive fraud proofs, which puts Arbitrum among the Ethereum Layer 2 solutions.

Who Founded Arbitrum?

Ed Felten, Steven Goldfeder and Harry Kalodner founded Arbitrum, and Offchain Labs' founder pages profile all three as co-founders. Felten was a Princeton computer science professor and served as Deputy US Chief Technology Officer at the White House from 2015 to 2017.

Steven Goldfeder earned his PhD at Princeton, working where cryptography meets cryptocurrencies. Offchain Labs' founder pages list Goldfeder as chief executive and Felten as chief scientist, which makes Kalodner the Arbitrum founder whose 2024 profile put him in charge of engineering.

What Is Arbitrum Stylus?

Arbitrum Stylus is an upgrade to Arbitrum Nitro that adds a second virtual machine beside the EVM, one that runs WebAssembly (WASM) programs written in languages such as Rust. A Stylus program has to go through activation before it can run. The Stylus audit calls activation "a computationally expensive operation that involves decompressing bytecode," and the chain charges gas for it.

Offchain Labs hired Trail of Bits to review Stylus. The firm dated its security assessment of Stylus June 2024 and addressed it to five Offchain Labs people, with Kalodner first among them.

The review ran from late October 2023 to early May 2024, took 47 engineer-weeks and logged 41 findings. Two of the 41 rated High and six rated Medium, and the other 33 fell to Low, Informational or Undetermined.

What Did the Stylus Audit Warn About, and What Happened in October 2026?

The first finding, TOB-STYLUS-1, warned that Stylus collected activation gas too late. Activation decompresses the program's bytecode first, and if that decompression fails, the user isn't charged for work that "can be expensive."

Its exploit scenario has an attacker submit crafted WASM "with the purpose of slowing down the Arbitrum chain" and pay less than full price. Trail of Bits told Kalodner's team to charge gas "as early as possible" and to charge it "even if activation fails for any reason."

In early October 2026 the Arbitrum Foundation notified the Security Council of the need for an emergency action, and the Council bundled two fixes into one payload. One added a permissionless guard that lets anyone pause Arbitrum One's settlement on Ethereum if the one-step proof of BoLD accepts two conflicting answers.

The other paused new Stylus activations on Arbitrum One and Nova. The Security Council's emergency action reportblamed "AI-assisted tooling" for a rising number of attacks through hand-crafted programs built to degrade the chain's performance. It said no known attack allowed theft of user funds.

The Council's lever was the charge the audit had flagged. It raised the up-front gas for activating a Stylus program from zero to the 64-bit maximum, which the report says makes activation "prohibitively expensive." In mid-October 2026 the setting still read that maximum on both chains, so the pause was still in place.

The bug itself differs. TOB-STYLUS-1 dealt with failed decompression dodging the fee, while the 2026 attacks came through hand-crafted programs built without the standard Stylus compiler toolchain. Both pass through the same gate, the step that turns uploaded WASM into a running program, and in 2026 the price of that step became the off switch.

Programs active under the current Stylus version keep running. The report said no active contract on Arbitrum One would expire before August 20, 2027. The last program activated before the pause, in early October 2026, can run until early October 2027. Older programs fared worse. Stylus moved from version 2 to version 3 on Arbitrum One in August 2026.

Of the 254 program addresses activated between late February 2026 and the pause, at least 188 were still on version 2 in mid-October 2026 and rejected calls with an upgrade error. Upgrading means activating again, and the Council's report lists reactivation after a Stylus version change among the steps the pause blocks.

Method: Arbitrum One's ArbWasm precompile gives the activation charge through the public RPC, checked at the blocks either side of the Council's transaction and again in mid-October 2026. The program count comes from ArbWasm's activation events since late February 2026, with each program then called directly.

How to Trade Arbitrum (ARB) on Phemex

Phemex's ARBUSDT perpetual closed mid-October at 0.187 USDT, 3.26% above the previous day's close of 0.1811. The Arbitrum blog has no post and the Arbitrum Foundation forum no new topic from that day, and posts on X fall outside this page's sources.

Arbitrum's own channels did carry two other dated items before that close. A late-September 2026 Arbitrum guide describes Priority Gas Auctions replacing Timeboost as Arbitrum One's transaction-ordering policy. In mid-October 2026 the Arbitrum Stablecoin Growth Program opened with a 2 million ARB prize pool.

On Phemex, ARB trades as the ARBUSDT perpetual and the ARB/USDT spot pair, both listed since March 2023. The perpetual gives leverage up to 50x and settles funding every 8 hours. Futures use leverage, and a leveraged position is closed automatically if losses reach its margin.

Price levels belong to the forecast side, and our Arbitrum price prediction for September 2026 maps them.

1. Open a Phemex account and deposit the USDT you plan to trade.

2. For spot, search ARB and choose the ARB/USDT pair, following the steps in how to buy Arbitrum on Phemex.

3. For futures, open the ARBUSDT perpetual, pick a margin mode and set your leverage.

4. Set a stop-loss before you confirm, so you know your exit in advance.

Frequently Asked Questions

What does Offchain Labs do?

Kalodner's company Offchain Labs built Arbitrum, and its engineers wrote the Nitro design that runs Arbitrum One. It also commissioned the 2024 Stylus security audit. Its website uses the brand Offchain and pitches enterprise blockchain infrastructure for what it calls the programmable economy.

Is Offchain Labs Arbitrum?

No. Offchain Labs is the company that built Arbitrum, founded in 2018 by Kalodner and two fellow co-founders. Arbitrum is the network itself. The early October 2026 emergency change to the network came from the Arbitrum Security Council, acting on an Arbitrum Foundation notice.

What did Harry Kalodner research at Princeton?

Kalodner co-wrote a 2016 paper on how Bitcoin's mining incentives break down once fees replace the block reward. He was also first author of BlockSci, an open-source blockchain analysis platform, and of the 2018 Arbitrum paper.

Are Arbitrum Stylus contracts still running?

Programs activated under Stylus version 3 keep running until they expire, and none on Arbitrum One expires before August 2027. In mid-October 2026 at least 188 programs activated under version 2 rejected calls, and the pause still blocked their reactivation.

Disclaimer: This article is for informational purposes only and is not financial advice. Cryptocurrency trading involves substantial risk. Always do your own research before making investment decisions.