"The digital signing of CryptoWall samples is likely an attempt to evade antivirus detection. The success of this approach is debatable since this practice is no longer uncommon among malware developers and many security products account for it. However, there might be cases where signing malware with certificates stolen from trusted developers might bypass some application whitelisting rules.
The new CryptoWall samples were not detected by any of the 55 antivirus products used on the VirusTotal website when they were discovered Sunday, the Barracuda researchers said. The detection rate has slightly increased since then, they said."
From another story: http://threatpost.com/new-signed-version-of-cryptowall-ransomware-on-the-loose