CVE-2026-23527 is a high-severity (CVSS 8.9) CWE-444 vulnerability in h3. A fix is available for h3 — see the affected versions and patch details below.
h3 v1 has Request Smuggling (TE.TE) issue
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-23527.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-23527 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,636 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
h3npmDescription
I was digging into h3 v1 (specifically v1.15.4) and found a critical HTTP Request Smuggling vulnerability.
Basically, readRawBody is doing a strict case-sensitive check for the Transfer-Encoding header. It explicitly looks for "chunked", but per the RFC, this header should be case-insensitive.
The Bug: If I send a request with Transfer-Encoding: ChuNked (mixed case), h3 misses it. Since it doesn't see "chunked" and there's no Content-Length, it assumes the body is empty and processes the request immediately.
This leaves the actual body sitting on the socket, which triggers a classic TE.TE Desync (Request Smuggling) if the app is running behind a Layer 4 proxy or anything that doesn't normalize headers (like AWS NLB or Node proxies).
Vulnerable Code (src/utils/body.ts):
if (
!Number.parseInt(event.node.req.headers["content-length"] || "") &&
!String(event.node.req.headers["transfer-encoding"] ?? "")
.split(",")
.map((e) => e.trim())
.filter(Boolean)
.includes("chunked") // <--- This is the issue. "ChuNkEd" returns false here.
) {
return Promise.resolve(undefined);
}
I verified this locally:
- Sent a
Transfer-Encoding: ChunKedrequest without a closing 0 chunk. - Express hangs (correctly waiting for data).
- h3 responds immediately (vulnerable, thinks body is length 0).
Impact: Since H3/Nuxt/Nitro is often used in containerized setups behind TCP load balancers, an attacker can use this to smuggle requests past WAFs or desynchronize the socket to poison other users' connections.
Fix: Just need to normalize the header value before checking: .map((e) => e.trim().toLowerCase())
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | h3 | all versions | 1.15.5npm install h3@1.15.5 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for h3, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update h3 to 1.15.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-23527 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-23527 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-23527. 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-2026-23527 in your dependencies?
O3 Security finds CVE-2026-23527 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.