The IX Multilevel-Secure Unix System (1992)
cs.dartmouth.edu
cs.dartmouth.edu
> Historically few implementations have been certified capable of MLS processing with a security range of Unclassified through Top Secret. Among them were Honeywell's SCOMP, USAF SACDIN, NSA's Blacker, and Boeing's MLS LAN, all under TCSEC, 1980s vintage and Intel 80386-based. Currently, MLS products are evaluated under the Common Criteria. In late 2008, the first operating system (more below) was certified to a high evaluated assurance level: Evaluation Assurance Level (EAL) - EAL 6+ / High Robustness, under the auspices of a U.S. government program requiring multilevel security in a high threat environment.
* https://en.wikipedia.org/wiki/Multilevel_security
On the networking side, see IPSO/CIPSO/CALIPSO security labels:
I'm clearly missing it, but which OS is it talking about here? The two sources from 2008 are not related to EAL6+