Attackers can exploit the flaw to install malicious code on a user's machine if they open a specially crafted QuickTime file, US-CERT said Wednesday evening.
The researcher who reportedly discovered the vulneraiblity, Petko Petkov of the Gnucitizen think tank, could not immediately be reached for comment.
"US-CERT encourages users to use caution when opening QuickTime files, and apply best security practices...to help mitigate the risks," according to the alert.
So far this year, Apple has delivered three security updates to remedy bugs in QuickTime.
Vulnerable client-side software is nothing new, as researchers have reported similar problems in other offerings.
An Apple spokeswoman did not respond to a request for comment on Thursday.