OpenSSL 1.0.0p / 1.0.1k was recently released and is being pushed out by various operating system maintainers. My review determined that this update is incompatible with the Bitcoin system and could lead to consensus forks. Bitcoin Core released binaries from Bitcoin.org are unaffected, as are any built with the gitian deterministic build system. If you are running third-party or self-compiled Bitcoin Core or an alternative implementation using OpenSSL you must not update OpenSSL or must run a Bitcoin software containing a workaround: https://github.com/bitcoin/bitcoin/commit/488ed32f2ada1d1dd108fc245d025c4d5f252783 (versions of this will be backported to other stable branches soon) The tests included with Bitcoin Core in the test_bitcoin utility already detect this condition and fail. (_Do not ignore or disable the tests in order to run or distribute software which fails_) The incompatibility is due to the OpenSSL update changing the behavior of ECDSA validation to reject any signature which is not encoded in a very rigid manner. This was a result of OpenSSL's change for CVE-2014-8275 "Certificate fingerprints can be modified". While for most applications it is generally acceptable to eagerly reject some signatures, Bitcoin is a consensus system where all participants must generally agree on the exact validity or invalidity of the input data. In a sense, consistency is more important than "correctness". As a result, an uncontrolled 'fix' can constitute a security vulnerability for the Bitcoin system. The Bitcoin Core developers have been aware of this class of risk for a long time and have taken measures to mitigate it generally; e.g., shipping static binaries, internalizing the Leveldb library... etc. It was somewhat surprising, however, to see this kind of change show up as a "low" priority fix in a security update and pushed out live onto large numbers of systems within hours. We were specifically aware of potential hard-forks due to signature encoding handling and had been hoping to close them via BIP62 in 0.10. BIP62's purpose is to improve transaction malleability handling and as a side effect rigidly defines the encoding for signatures, but the overall scope of BIP62 has made it take longer than we'd like to deploy. (Coincidentally, I wrote about this concern and our unique demands on cryptographic software as part of a comment on Reddit shortly before discovering that part of this OpenSSL update was actually incompatible with Bitcoin: https://www.reddit.com/r/Bitcoin/comments/2rrxq7/on_why_010s_release_notes_say_we_have_reason_to/cnitbz3 ) The patches above, however, only fix one symptom of the general problem: relying on software not designed or distributed for consensus use (in particular OpenSSL) for consensus-normative behavior. Therefore, as an incremental improvement, I propose a targeted soft-fork to enforce strict DER compliance soon, utilizing a subset of BIP62. Adding a blockchain rule for strict DER will reduce the risk of consensus inconsistencies from alternative implementations of signature parsing or signature verification, simplify BIP62, and better isolate the cryptographic validation code from the consensus algorithm. A failure to do so will likely leave us in this situation, or possibly worse, again in the future. The relevant incompatible transactions are already non-standard on the network since 0.8.0's release in February 2013, although there was seemingly a single miner still mining incompatible transactions. That miner has been contacted and has fixed their software, so a soft-fork with no chain forking should be possible.