Computer users are being urged to update the Firefox web browser to close serious security holes in it.
Some of the security lapses in Mozilla software, which Firefox is based on, could allow malicious hackers to hijack computers.
There have been a total of 21 security flaws in various versions of Firefox, according to security firm Secunia.
Users are urged to download the latest versions of all Mozilla programs to protect their computers from attack.
The US Computer Emergency Readiness Team (Cert) warn that other Mozilla products including e-mail client, Thunderbird, and the internet application, Seamonkey, may also be affected.