CVE-2026-16728
MEDIUMCVE-2026-16728 is a medium-severity (CVSS 4.8) CWE-444 vulnerability in undici. O3 Security confirms whether CVE-2026-16728 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
undici vulnerable to downstream response desynchronization via retry interceptor
Blast Radius
undici📦undici📦undiciReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects npm packages — download data is not available via public APIs for these ecosystems.
Description
undici's retry interceptor can deliver a response whose body length does not match the Content-Length header exposed to the application after a retry or resume of a partial response. In undici before 6.28.0, from 7.0.0 up to before 7.29.0, and from 8.0.0 up to before 8.9.0, a malicious or faulty upstream can return a partial response with a mismatched framing header, close the socket early, and have the retry interceptor assemble a body of a different length while the original Content-Length stays attached. Applications that use the retry interceptor and forward upstream headers and bodies downstream, such as proxies or gateways, may then emit an invalid HTTP response with a stale Content-Length, leading to downstream response desynchronization, connection hangs, or response corruption. Exploitation requires the retry interceptor enabled, an upstream returning a mismatched partial response, and a downstream forwarder that does not remove or recalculate Content-Length. The issue is fixed in undici 6.28.0, 7.29.0, and 8.9.0.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | undici | all versions | 6.28.0 |
| 📦npm | undici | ≥ 7.0.0&&< 7.29.0 | 7.29.0 |
| 📦npm | undici | ≥ 8.0.0&&< 8.9.0 | 8.9.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for undici. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update undici to 6.28.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-16728 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-16728 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-16728. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-16728 in your dependencies?
O3 detects CVE-2026-16728 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.