Cross-user cache poisoning in SSG and ISR is the item to act on, and the 15.x line backports only some of the set.
Next.js 16.3.8, released September 30, carries six security fixes. The ones to act on are the cache bugs, not the one rated highest.
Two of the medium-severity items are cross-user cache poisoning in SSG and ISR rendering. A poisoned page is served to a different user than the one who triggered the render, and held open it becomes a persistent denial of service. If you self-host with ISR or use cache, this is act-now: the poisoned entry stays in your cache, so the attacker does not need to return. A related pair leaks Draft Mode content into regular responses while a use cache fill is pending, which is how unannounced preview content becomes retrievable by ordinary visitors, and leaks across root parameter values in nested use cache functions, rendering one route param's content for another.
The high-severity item is a server-side request forgery in Image Optimization. Assess it in one line: if you serve the optimizer at the default /_next/image endpoint on a public host, you accept attacker-influenced image URLs by design and the conditional applies. The last medium, disclosure in App Router metadata image routes via a dynamicParams bypass, needs no separate action.
Go to 16.3.8. The 15.x line gets 15.5.27, which backports three of the mediums but not the SSRF, so confirm your line before assuming coverage.
Next.js moved its August security release forward to Aug 25, shipping 16.3.3 and 15.5.24 to close two critical RCEs, a Windows-only path traversal and a crafted-AVIF image optimization exploit.
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sharp below 0.35.4 inherits two libheif flaws rated critical upstream that can reach remote code execution on glibc-based Linux. The fix is a version bump or a one-line AVIF block.
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The Aug 31 patch brings back the AVIF output that the critical Aug 25 security release disabled, but only when your installed sharp resolves to 0.35.4 or newer.
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