Summary: Erroneous implementation of BLOCKHASH can trigger a chain reorganisation leading to consensus problems
Affected configurations: All geth versions up to 1.1.3 and 1.2.2. All eth versions prior to 1.0.0.
Details: Both C++ (eth) and Go (geth) clients have an erroneous implementation of an edge case in the Ethereum virtual machine, specifically which chain the BLOCKHASH instruction uses for retrieving a block hash. This edge case is very unlikely to happen on a live network as it would only be triggered in certain types of chain reorganisations (a contract executing BLOCKHASH(N - 1) where N is the head of a non-canonical subchain that is not-yet reorganised to become the canonical (best/longest) chain but will be after the block is processed).
pyethereum is unaffected.
Effects on expected chain reorganisation depth: none
Remedial action taken by Ethereum: Provision of hotfixes as below.
Geth:PPA: sudo apt-get update then sudo apt-get upgrade
Brew: brew update then brew reinstall ethereum
Windows: download the updated binary from https://github.com/ethereum/go-ethereum/releases/tag/v1.2.3
Building from source:
git fetch origin && git checkout origin/master
Window and Mac OS X: https://build.ethdev.com/cpp-binaries-data/
Building from source: https://github.com/ethereum/webthree-umbrella/wiki