Western security has been undercut by the avarice of two companies, Toshiba of Japan and Kongsberg Vaapenfabrikk of Norway, and by their Governments' lackadaisical supervision of militarily sensitive exports. Under false pretenses, the companies shipped machinery to the Soviet Union that will enable the Russians to fabricate quieter submarine propellers. Soviet submarines will now become much harder to detect, all so that Toshiba and Kongsberg could turn some extra profit.

Both Norway and Japan concede the seriousness of the diversion. Neither seems yet to appreciate the sentiment building in Congress for sterner reprisal than the diplomatic tut-tuts being uttered by the Administration. The House last week voted 415 to 1 to demand compensation. In the Senate, Jake Garn suggests that ''We ought to really hurt Toshiba.'' Norway and Japan would be smart to volunteer some more substantial remedy than the wrist slaps so far given to their errant companies.

No technology can be kept a Western monopoly forever, but there's every reason to try slowing the rate of seepage of militarily useful technology. Under rules set by their coordinating committee, Cocom, NATO countries and Japan therefore restrict export of such products. Though Cocom rules are administered seriously by the United States, France and Britain, other countries have lax enforcement and mild penalties.

Kongsberg and Toshiba evidently believed they had little to fear in their conspiracy to sell propeller-milling equipment to the Soviet Union. Toshiba provided the milling machines, Kongsberg the numerical-control computer and software to drive them. Both companies lied to their export control authorities about the sophistication of the machines. The Defense Department believes the Russians supplied the design of the propellers to Kongsberg, which wrote the software. The Norwegian Embassy asserts software was provided only for the computer's operating system.