CVE-2026-15157 is a medium-severity (CVSS 4.2) CWE-93 vulnerability in undici. A fix is available for undici — see the affected versions and patch details below.
undici vulnerable to CRLF Injection via blob-like body 'type' property
Exploitation Status
No confirmed exploitation observed yet
- 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-2026-15157.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-15157 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 383,485 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
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.
undicinpmDescription
Impact
When an application passes a duck-typed blob-like body to undici's HTTP/1.1 dispatcher (via request(), stream(), pipeline(), or dispatch()) with a .type derived from untrusted input, an attacker can inject CRLF sequences (\r\n) to append arbitrary HTTP headers and potentially smuggle a second request past the upstream.
The vulnerable branch in lib/dispatcher/client-h1.js pushes body.type directly into the outgoing headers with no validation, while every other header path in undici goes through isValidHeaderValue():
} else if (util.isBlobLike(body) && request.contentType == null && body.type) {
headers.push('content-type', body.type) // bypasses isValidHeaderValue()
}
The bug requires a hand-rolled duck-typed blob object or a Blob subclass with a controlled .type. Native Blob is safe because its constructor strips CRLF from .type. fetch() is unaffected because it validates via the Headers class. Ecosystem consumers that build duck-typed blob shapes from user input include form-data-encoder, formdata-polyfill, and formdata-node.
Same defect class as CVE-2022-35948 (explicit content-type sink, fixed in undici 5.8.2) and CVE-2026-1527 (upgrade option sink, fixed in 6.24.0 / 7.24.0), both closed by adding isValidHeaderValue() on their respective sinks. This branch was missed.
Patches
Patched in undici v6.28.0, v7.29.0, and v8.9.0. Users should upgrade to one of these versions or later.
Workarounds
- Set an explicit, validated
content-typeheader on the request options (skips the vulnerable branch). - Use a native
Blob(orfetch-blob) instead of a hand-rolled duck-typed object. - Reject control characters in the MIME type before assigning it to
.type. - Use
fetch()instead of the non-fetchAPIs.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | undici | all versions | 6.28.0npm install undici@6.28.0 |
| 📦npm | undici | ≥ 7.0.0&&< 7.29.0 | 7.29.0npm install undici@7.29.0 |
| 📦npm | undici | ≥ 8.0.0&&< 8.9.0 | 8.9.0npm install undici@8.9.0 |
Affected Products
undicinodejsDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for undici, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update undici to 6.28.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-15157 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.
Frequently Asked Questions
Is CVE-2026-15157 in your dependencies?
Find it across npm, including transitive dependencies.