Viral marketing, in which low-profile ads spread quickly online as people share them, has become an intriguing strategy for many marketers trying to look beyond traditional 30-second television commercials.

But marketers are not the only ones taking the viral approach.

Because the Web and cheap video technology keep making it easier for anyone with some knowledge and equipment to produce spots that look almost like real commercials, brand managers may find their carefully calibrated marketing messages increasingly being tweaked, teased, spoiled or entirely undermined.



Consumers, on the other hand, can now wonder whether each supposed hoax is an authorized, but deniable, below-the-radar marketing ploy.



The hoax at hand has set off a particularly sharp bout of distress since its appearance last week, because it looks almost exactly like a real commercial for the Volkswagen Polo, a model sold outside the United States, except that it portrays the Polo driver as a suicide bomber.



In the commercial, the driver pulls up to a busy outdoor cafe, exposes explosives strapped to his chest and pushes a detonator. His car, however, contains the explosion without cracking a window. The spot ends with the Volkswagen logo and the actual Polo ad theme: “Small but tough.”



The spot was sent to the London office of DDB Worldwide, a Volkswagen roster agency, by two people known as Lee and Dan. “We had no part in disseminating it,” said Annouchka Behrmann, public relations director at DDB London, part of the DDB Worldwide division of the Omnicom Group. “We think it’s absolutely disgusting.”



Mr. Buckett said yesterday that his company was about to deliver Lee and Dan a letter demanding the source material, an admission that they created the ad without authorization, a public apology in language to be worked out with the automaker and a public promise to neither infringe on Volkswagen’s trademarks nor create or distribute any more Volkswagen work.



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