The developers of Ethereum's Geth software release a new version just days before the Byzantium hard fork due to the discovery of a denial-of-service (DoS) attack vulnerability.
Ethereum's Geth 1.7.2 urgent update
With Geth comprising about 75% of all Ethereum nodes, the vulnerability could leave nodes running the previous Byzantium-compatible release more susceptible to DoS attacks after the hard fork.
Casey Detrio, Ethereum developer, explains on reddit.com that the vulnerability stems from an oversight in one of the new Byzantium features. The risk is that this bug could be exploited by an attacker who wants to take ethereum nodes offline – a form of attack that the Ethereum community has dealt with in the past.
The second concern is that there may be more undiscovered consensus bugs that could be found after the activation block, which would then result in needing to perform emergency client updates.
It's also worth mentioning, that yesterday, the team behind Parity issued a new release of its software that corrected a "consensus bug" – an error which could have caused the network to partition during the hard fork.