Sanral's e-toll Web site suffered a denial of service (DoS) attack on Friday, according to the agency.
"Some users complained of slow site performance, and our service provider traced the problem to a denial of service attack of international origin," said Sanral spokesman Vusi Mona.
No further details of the attack were available, but Alex van Niekerk, project manager for the Gauteng Freeway Improvement Project, said the site has come under repeated attack since going live, but suffered only minor performance degradation.
DoS attacks, particularly distributed denial of service (DDoS) attacks, are a popular technique used to knock sites offline, overwhelming them with traffic until they are unable to service their clients. Activist group Anonymous frequently uses DDoS to attack targets, using its wide base of supporters to generate traffic.
Botnets often launch DDoS attacks from their installed base of zombie PCs. And last year, anti-spam service Spamhaus suffered one of the largest DDoS attacks in history, with incoming traffic peaking at 300Gbps, launched by a Dutch Web host known for harbouring spammers.
Sanral's Web site has been the target of several attacks lately, including a hack which may have leaked personal information, a flaw which allowed motorists to be tracked in real-time, and a session fixation attack which allowed login sessions to be hijacked.