Samsung explains the "serious vulnerability" in KNOX is actually a common MitM network attack5
About three weeks later, Samsung's KNOX team has published an official statement in its blog, regarding the revelation. According to the post, Samsung has collaborated with the Israeli researchers in a further investigation, which hasn't discovered any flaws or bugs in KNOX that could cause this particular vulnerability.
Apparently, the security gap in question is not due to a weakness in KNOX. The alleged “exploit” is actually a common attack which makes illicit use of "legitimate Android network functions" to intercept unencrypted network connections from and to apps on a mobile device. This is also known as the "Man in the Middle (MitM)" approach, which makes it possible to “eavesdrop” on a network and hijack messages sent between its points.
Samsung points out that KNOX is equipped with additional protections against this type of network attack - Mobile Device Management, Per-APP VPN, and FIPS 140-2. According to professor Patrick Traynor at the Georgia Institute of Technology, who was involved in research for KNOX, users should be encouraged to take advantage of those mechanisms to avoid this and other common security issues.