Tumblr founder David Karp has loosened his hardline "no ads allowed" stance, but the young blog mogul has promised that any ads on Tumblr will be artful and "make you feel something for the brand." So it was a surprise when a fraction of users suddenly started seeing typically spammy-looking banner ads peppered throughout their dashboards.

The ads appear to be showing up for only a small fraction of Tumblr's massive userbase. Still, dozens of users are griping publicly. "Tumblr has started with ads now? Ugh," read a typical tweet.

But the ads aren't coming from Tumblr.

The fine print under the ads reads "ads not by this site," suggesting a third party is responsible through a browser extension or plug-in. Jeremy Cutler, the developer behind the popular Tumblr plug-in Missing e, said it's not his app. "I do know that it is possible for browser extensions to inject ads into pages, but this is one of the more nefarious things a browser extension can do," he said in an email. "If it isn't Tumblr, I think it is inappropriate."

One user said she'd fixed the problem by uninstalling a swath of extensions at once, so she was unsure which one was responsible. Another user said the only Chrome extension that was installed was Google Translate, an unlikely culprit.

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The ads may be caused by third-party browser extension IWantThis!, Tumblr spokesperson Katherine Barna said in an email. "Users seeing these ads should check if they are running IWantThis! and remove if they are," she said. (As it turns out, complaints about IWantThis! are well-documented.)

Karp is fond of saying that Tumblr "could be profitable tomorrow" if it placed just one ad on users' dashboards. But the ads Tumblr users are seeing are crude, and the products they're pushing are small, off-brand services that probably couldn't afford to advertise with Tumblr. The ads are appearing multiple times in users' dashboards, reportedly slowing down the service. Although Tumblr has been experimenting with advertising, this is exactly the kind of advertising the startup has sworn it will never do.

Flummoxed Tumblr users are threatening to quit, and many are relying on AdBlocker to get rid of the offending ads. Given the backlash over these banner ads, it looks like Karp is right to be cautious about using traditional web advertising within Tumblr—one wrong move and users could revolt.