High watermark (computer security)
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High-water mark was introduced in Clark Weissmann's Security controls in the ADEPT-50 timesharing system published in the AFIPS Conference Proceedings, volume 35, pages 119--133. FJCC, 1969. It pre-dates the BLP security model, whose first volume appeared in 1972.
Under high-water mark, any object less than the user's security level can be opened, but the object is relabeled to reflect the highest security level currently open. Hence the name.
The practical effect of the high-water mark was a gradual movement of all objects towards the highest security level in the system. If user A is writing a CONFIDENTIAL document, and checks the unclassified dictionary, the dictionary becomes CONFIDENTIAL. Then when user B is writing an SECRET report and checks the spelling of a word, the dictionary become SECRET. If user C is assigned to assemble the daily intelligence briefing at TOP SECRET, reference to the dictionary makes the dictionary TOP SECRET.
