Earlier this week, there was a war of words between Yahoo, which had newly rejected yet another Microsoft buy-out bid, and the computing giant itself.
But now, in another letter published on its website, Yahoo has now turned its attention on billionaire investor and Yahoo shareholder Carl Icahn.
Icahn is pro the Microsoft buy-out but anti the current Yahoo administration, and has gone as far as to make it clear he will attempt to oust the board come Yahoo's annual meeting on 1 August.
On 7 July, "sources" reported that Yahoo was going to try and appease Icahn by offering him two board seats, but Icahn has openly said that he wants a minimum of four.
But now Yahoo has declared all out war and is urging shareholders not to back Icahn.
It warned them that Icahn's agenda presents a "significant risk" to their investment.
In the letter, Yahoo said the alliance between Icahn and Microsoft "continues to make misleading statements about their plans for Yahoo".
"Your board of directors believes strongly that the Icahn-Microsoft agenda - as presented to us jointly last week - will destroy stockholder value at Yahoo, serving only their very narrow special interests, clearly not your interests", Yahoo said.
Yahoo added that it is "preparing to implement (its) recently signed commercial agreement with Google that will increase cash flow".
The company said Microsoft and Icahn's latest proposal to buy its search advertising business represented "somewhat of an improvement" but was still not a good offer.
"The Icahn/Microsoft proposal was more 'smoke and mirrors' than objective reality", the letter said.
Yahoo repeated earlier statements that it was still open to selling the entire company to Microsoft for $33 per share.