An expert on cryptography explains why the situation could have turned out much differently. Every key that had been generated by the vulnerable versions compromised, so was every key used on systems running those vulnerable versions of the OpenSSL package. This is important because an attacker can retroactively seek out DSA signatures generated during the vulnerable period and use them to recover your private key. The impact of this attack is that every signature generated on a vulnerable system reveals the signer s private key. An attacker can find old signatures by crawling your website, examining signed email, analyzing saved packet captures of an SSL exchange, etc.
Source: https://threatpost.com/how-debian-openssl-bug-almost-spawned-disaster-051809/72669/