PrivDog, has been described by several experts as being "worse than Superfish".
A particular concern is its links to the Israeli security firm Comodo, which issues a third of the secure certificates used on the web.
PrivDog was developed by the founder of Comodo, Melih Abdulhayogulu, and some versions of it are packaged with Comodo's own software.
But Comodo told the BBC that the affected versions "had never been distributed" by it.
"PrivDog is in every sense as malicious as Superfish," added Simon Crosby, co-founder of security firm Bromium.
"It intercepts and decrypts supposedly secure communication between the browser and a remote site - such as the user's bank - ostensibly to insert its own advertising into pages in the browser.
"It is substantially more scary, though, because PrivDog effectively turns your browser into one that just accepts every https certificate out there without checking its validity, increasing vulnerability to phishing attacks, for example."
http://www.bbc.com/news/technology-31586610