In a blog post, researchers working in Google's Project Zero team said that after they reported their findings to Apple, the Cupertino-based tech giant patched the vulnerabilities.
The malicious websites may have compromised personal files, messages, and real time location data of iPhone users.
"Earlier this year, Google's Threat Analysis Group (TAG) discovered a small collection of hacked websites. The hacked sites were being used in indiscriminate watering hole attacks against their visitors, using iPhone 0-day," said Ian Beer, Project Zero.
There was no target discrimination as simply visiting the hacked site was enough for the exploit server to attack the iPhone, and if it was successful, install a monitoring implant.
"We estimate that these sites receive thousands of visitors per week," said the Google blog post.
The researchers were able to collect five separate, complete and unique iPhone exploit chains, covering almost every version from iOS 10 through to the latest version of iOS 12.
"This indicated a group making a sustained effort to hack the users of iPhones in certain communities over a period of at least two years," said Beer.
"I will not get into a discussion of whether these exploits cost $1 million, $2 million, or $20 million. I will instead suggest that all of those price tags seem low for the capability to target and monitor the private activities of entire populations in real time," the researcher noted.
The websites delivered their malware indiscriminately and were operational for years, said Google.
Apple was yet to issue a comment on Google's blog post.
( With inputs from IANS )