Symbiotic exploit exposes Bitcoin Bridge risks without touching Bitcoin itself

Synchronization protocol shuts down native Bitcoin bridges, highlighting cross-chain infrastructure vulnerability

On Friday, Symbiosis shut down its native Bitcoin bridge service after attackers used its BridgeV2 contract to forge a large number of unsecured synthetic BTC. However, the attackers only extracted approximately $336,000 in actual value. The importance of this huge difference cannot be ignored. It must be emphasized that the security of Bitcoin itself has not been compromised. What really exposed the vulnerability is the introduction of BTC into the decentralized finance (DeFi) infrastructure.

The incident occurred just days after the Liquid Network suffered a breach that resulted in $320 million in losses, raising questions: Although Bitcoin itself is secure, its bridging service has repeatedly failed. How secure is the cross-chain infrastructure?

BTC routing is suspended and the rest of the network remains open

Symbiosis revealed on the X platform that it discovered evidence of a Bitcoin bridging attack at around 04:28 am UTC on September 11 and immediately stopped BTC routing services. However, other routing protocols continue to function normally.

In the Delta Incident Archive, this event is classified as DCI-2026-304. The file states that BridgeV2 processed an error message that resulted in the generation of more than 2^62 syBTC on BNB Chain and Ethereum. As a result, the criminals converted part of the illegal balance into approximately 4.39 WBTC on Ethereum, making a profit of approximately $336,000. DeFiLlama classified the incident as "unsecured cross-chain casting."

How cross-chain trust enters the system

Symbiosis documentation shows that the bridging service relies heavily on the secure transmission and authentication of cross-chain messages. BridgeV2 connects the protocol's Portal and Synthesis contracts to its offline relay network. These relay nodes submit signed transactions through multi-party computing (MPC) keys stored in the contract.

Native Bitcoin (BTC) is protected in the Portal by using MPC threshold signatures. This allows relay nodes to create syBTC on a separate blockchain and then convert it into user-preferred assets. Symbiosis claims that its native BTC bridging service has passed a Decurity audit.

The model relies on ensuring that instructions sent across chains are accurately authenticated, but this premise did not hold true in this incident.

Huge amounts of synthetic coins minted and small actual losses

Huge amounts of synthetic coins minted should not be confused with the total amount stolen. Although generating more than 2^62 raw syBTC created a huge imbalance in accounting, hackers were only able to convert a small portion of them into real assets. Total losses are estimated at approximately $336,000.

This incident brings Symbiosis closer to the bottom of the major hacking incidents in 2026. TRM Labs reported that there were 207 cryptocurrency hacking incidents in the first half of the year, setting its highest half-year total in history, with an average loss of $219,000. Total losses dropped significantly, from $2.3 billion in the first half of 2025 to $972 million in the first half of 2026.

Bridging vulnerabilities occur repeatedly

A more worrying issue is the frequency of bridging service failures. Currently, DeFiLlama estimates that the total bridge losses are at least US$3.68 billion. Symbiosis points out that a common reason for bridging attacks is the lack of strong message authentication.

The consequences may go beyond the bridge itself. For example, the Policy Institute's analysis of the KelpDAO intrusion showed that unsecured rsETH due to poor cross-chain verification exacerbated the overall pressure on Aave. A total of $5 billion worth of stablecoins were withdrawn, and borrowing rates climbed to 10%.

Symbiosis BTC bridging vulnerability highlights growing cross-chain risks

Aftermath of the $320 million Liquid Network hack

Symbiosis incident follows the more serious failure of Liquid Network. Chainalysis said hackers who called themselves white-hats took advantage of flaws in cached transaction verification certificates to steal 4,000 of Liquid's 4,200 BTC's, worth approximately $320 million. This flaw led to the creation of unsecured L-BTC, which was eventually exchanged for real Bitcoin.

Cryptopolitan reported earlier this week that hackers had returned 3,400 BTC, or about 85% of the stolen funds.

Neither vulnerability compromised Bitcoin itself, but rather pointed to flaws in the system around which it was designed.

The problem is not limited to Symbiosis. According to DeFiLlama, the total lock-in value (TVL) in the cross-chain bridging field of Bitcoin is only about US$1.32 million, of which the TVL of the Symbiosis platform is 0. If such incidents continue to occur, investors will be prevented from putting BTC into DeFi. This can trap liquidity in isolated ecosystems and make cross-chain options appear riskier and less attractive.

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