Harris found the problem wasn't limited to Google, but also Microsoft and Yahoo, all of whom appeared to have fixed the issue as of two days ago, according to US-CERT. Harris told Wired he found either 512-bit or 768-bit keys in use at PayPal, Yahoo, Amazon, eBay, Apple, Dell, LinkedIn, Twitter, SBCGlobal, US Bank, HP, Match.com and HSBC.

Weak signing keys are a boon for cybercriminals. They selectively target people with emails containing malicious links in an attempt to exploit a computer's software and install malware, a style of attack known as spear phishing. If an email contains the correct DKIM signature, it's more likely to end up in a recipient's inbox.

US-CERT also warned of another problem. The DKIM specification allows a sender to flag that it is testing DKIM in messages. Some recipients will "accept DKIM messages in testing mode when the messages should be treated as if they were not DKIM signed," US-CERT said.