Yahoo said Monday it has fixed a bug that was mistaken for the Shellshock flaw, but no user data was affected.
Three of the company's servers with APIs (application programming interfaces) that provide live streaming for its Sports service "had malicious code executed on them this weekend by attackers looking for vulnerable Shellshock servers," wrote Alex Stamos, Yahoo's chief information security officer.
Stamos wrote on the Hacker News website that the servers had been patched after the Shellshock vulnerability was disclosed.
Yahoo was notified by Jonathan Hall, senior engineer and president of Future South Technologies, a security consulting firm. Hall wrote on his blog that he uncovered a vulnerability in at least two Yahoo servers.
Hall wrote he found evidence that a group of what appears to be Romanian hackers had struck Yahoo, Lycos and WinZip, using the Shellshock vulnerability to infect servers and build a botnet, the term for a network of infected machines.
Shellshock, first identified late last month, is the nickname for a flaw in a form of software known as Bash, a command-line shell processor on Unix and Linux systems. The security hole could let attackers insert extra code into computers running Bash, allowing them to take control of servers remotely.
In a statement released earlier on Monday, Yahoo appeared to confirm Hall's finding that Shellshock was to blame. But Stamos later published a post on the Hacker News saying that further investigation showed Shellshock was not the cause.
The attackers, Stamos wrote, had "mutated" their exploit and ended up taking advantage of a different bug that was in a monitoring script being run by Yahoo's developers to parse and debug Web logs. That bug was only specific to a small number of machines, he wrote.
"As you can imagine this episode caused some confusion in our team, since the servers in question had been successfully patched (twice!!) immediately after the Bash issue became public," he wrote.
Hall wrote that he sent an email warning of his findings to WinZip, a division of Canada-based Corel. WinZip is a file compression utility.
In an email statement on Monday, WinZip spokeswoman Jessica Gould didn't directly address Hall's findings but said "we were contacted by Mr. Hall almost a week after we began our patching process. We've since replied to Mr. Hall directly to thank him for contacting us."
Hall wrote on his blog that it appeared WinZip's servers had been compromised using Shellshock. Those servers were then being used to search for other vulnerable Web servers. The malicious code on WinZip's servers connected to an IRC server, where it awaits commands from the hackers.
(Zach Miners in San Francisco contributed to this report.)