When thieves stole an estimated $190 million from U.S. crypto firm Nomad last week, it was the seventh hack of 2022 to target an increasingly important cog in the crypto machine: Blockchain "bridges" - strings of code that help move crypto coins between different applications.

So far this year, hackers have stolen crypto worth some $1.2 billion from bridges, data from London-based blockchain analysis firm Elliptic shows, already more than double last year's total.

"This is a war where the cybersecurity firm or the project can't be a winner," said Ronghui Hu, a professor of computer science at Columbia University in New York and co-founder of cybersecurity firm CertiK.

"We have to protect so many projects. For them (hackers) when they look at one project and there's no bugs, they can simply move on to the next one, until they find a one weak point."

At present, most digital tokens run on their own unique blockchain, essentially a public digital ledger that records crypto transactions. That risks projects using these coins becoming siloed, reducing their prospects for wide use.

Blockchain bridges aim to tear down these walls. Backers say they will play a fundamental role in "Web3" - the much-hyped vision of a digital future where crypto's enmeshed in online life and commerce.

Yet bridges can be the weakest link.

"Blockchain bridges are the most fertile ground for new vulnerabilities," said Steve Bassi, co-founder and CEO of malware detector PolySwarm.

Nomad did not respond to requests for comment, but it has said it is working with law enforcement agencies and a blockchain analysis firm to track the stolen funds. Late last week, it announced a bounty of up to 10% for the return of funds hacked from the bridge.

Nomad said on Saturday it had recovered over $32 million of the hacked funds so far.

"The most important thing in crypto is community, and our number one goal is restoring bridged user funds," Mohan said. "We will treat any party who returns 90% or more of exploited funds as a white hats. We will not prosecute white hats," he said, referring to so-called ethical hackers.

Several cyber security and blockchain experts told Reuters that the complexity of bridges meant they could represent an Achilles' heel for projects and applications that used them.

"A reason why hackers have targeted these cross-chain bridges of late is because of the immense technical sophistication involved in creating these kinds of services," said Ganesh Swami, CEO of blockchain data firm Covalent in Vancouver, which had some crypto stored on Nomad's bridge when it was hacked.

"Cross-chain bridges are an attractive target for hackers because they often leverage a centralized infrastructure, most of which lock up assets," said Victor Young, founder and chief architect at U.S. blockchain firm Analog.

So how best to address the problem?

Some experts say audits of smart contracts could help to guard against cyber thefts, as well as "bug bounty" programs that incentivize open-sourced reviews of smart contract code.

Others call for less concentration of control of the bridges by individual companies, something they say could bolster resiliency and transparency of code.