96% of Advertisers Know What Principal Media Is. It Didn't Matter.

When Richard Foster, a 17-year GroupM veteran, raised concerns about the media-buying arm's rebate practices with WPP's general counsel Nicola McCormick, she reportedly answered with one word: existential. That word now sits inside an amended New York State Supreme Court filing, dated 13 August 2026, built substantially on an independent investigation Sony Pictures reportedly ran into WPP's media-buying practices after a 2023 kickback scandal in China. Foster was fired on 10 July 2025 and filed his original wrongful-termination suit that November, seeking at least $100 million.

According to the complaint, WPP's media arm, GroupM, now rebranded WPP Media, advertises a discount on ad inventory that isn't really a discount: WPP allegedly pays a fraction of what the inventory costs, covers the remaining balance from a rebate pool contractually belonging to clients, and keeps the spread as what the filing calls "near pure profit shielded from audits." Sony's investigators reportedly found WPP retained $350 million this way in China in 2024 alone, passing just $110 million through to the clients it was owed to. The complaint alleges internal KPIs called "extraction rates" measured how much proprietary inventory a market secured relative to client spend and tied those metrics to chief investment officers' compensation, that margin caps were used to disguise the numbers as ordinary margins, and that GroupM held two versions of at least one vendor contract, a sanitised one shown to the client and a separate internal one recording the real terms. WPP calls the claims baseless and has moved to seal parts of the filing.

Here is the allegation that should worry every client currently telling themselves they are protected because they never opted into a proprietary media deal. As of late 2024, roughly 62% of GroupM's largest clients, reportedly including Coca-Cola, Unilever, Amazon and Adidas, had declined to enter proprietary media agreements, which should have meant their spend sat outside the rebate mechanism entirely. According to Foster's complaint, it didn't: GroupM's trading desks allegedly pooled all client spend, participating or not, to pressure vendors into steep discounts that could be reclassified as proprietary media, with vendors that refused, reportedly including Meta itself in 2023, having spend redirected elsewhere. The value generated from that pooled leverage was allegedly never credited back to the clients whose spend helped create it, "all while maintaining a public posture of '100% transparency,'" the filing states.

If that specific allegation holds up, it collapses the premise clients have been operating on, that declining a principal-media relationship is itself a form of protection. None of this is conceptually new. Jared Belsky, CEO of independent agency Acadia, traces the practice to legitimate, disclosed principal trading in linear TV and radio decades ago, and estimates unreported rebates now pad agency margins by up to $40 billion industry-wide: "You don't get to double and triple, quadruple dip, and then say you're a trusted advisor." Nick Manning, former Ebiquity chief strategy officer and now a retained expert witness for Foster's legal team, a disclosed conflict worth noting, separately estimated GroupM's 2023 principal-trading revenue at roughly $1bn, more than half of WPP's total profit that year, while finding clients who did opt in saw "virtually no benefit."

The uncomfortable finding from the ANA's own ten-year retrospective on this exact issue is that awareness was never the bottleneck. 96% of advertisers now say they understand what principal media is. Only 58% actually use any control mechanism around it, and of those, more than half apply it to less than 10% of their budget. Ninety percent remain unsure whether their own agency's recommendations on the subject serve their interests. As one industry analyst summarised it: "The problem was not solved. It was repackaged."

Manning's explanation for why this keeps recurring is structural, not moral: "You have a situation where the agency that you see day to day has two bosses. The people behind the agency [and] the people who are pulling those strings in the holding company groups... it's very difficult to serve two masters equally." Belsky's own agency ran a rebate experiment in 2026, kept the money that came in briefly, then redistributed it back to clients pro rata, because he could not justify keeping it, evidence that opting out of the model is a genuine choice, not an impossibility. His read on why the practice persists elsewhere isn't cynicism about agencies, it is an information gap: "CMOs and marketers generally don't understand the problem. And if they understood the problem, I think they would be more passionate about why it would rub them the wrong way."

What is genuinely different this time is who did the finding, not the mechanism. AdExchanger columnist Alessandro De Zanche's framing cuts to it: "A disclosed fee is still unauditable when the entity charging it is anonymous or unverifiable. An audit needs a name, directors, a registered address." Sony could commission a criminal-grade forensic audit because it was big enough, reportedly WPP's then-fifteenth-largest global client, to have the leverage and in-house resources to do it. Most advertisers never will be.

The practical fix isn't more disclosure language in a contract, it is financial governance applied to media the way it is already applied to every other large corporate expenditure. Name principal media inventory explicitly, line by line, in the media plan rather than leaving it aggregated. Designate one internal owner accountable for tracking every non-transparent service your agency provides. Reduce the number of programmatic supply partners to a range that can actually be audited rather than an unmanageable sprawl. Treat independent verification as a standing financial control, not a one-off trust exercise, the same discipline this platform has already covered regarding the verification industry's own consolidation.

Ask your own agency directly whether your contract would survive the kind of investigation Sony reportedly ran, not because you expect the same answer, but because 90% of advertisers in the ANA's own study couldn't honestly answer that question either. The uncomfortable truth this story surfaces isn't whether Foster's specific allegations hold up in court. It's whether you would need someone else's fifteenth-place ranking on a global client list to ever find out what your own agency is actually doing with your rebate pool.