CVE-2026-25223 is a high-severity (CVSS 7.5) CWE-436 vulnerability in fastify. A fix is available for fastify — see the affected versions and patch details below.
Fastify's Content-Type header tab character allows body validation bypass
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-2026-25223.
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-2026-25223 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 378,567 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.
fastifynpmDescription
Impact
A validation bypass vulnerability exists in Fastify where request body validation schemas specified by Content-Type can be completely circumvented. By appending a tab character (\t) followed by arbitrary content to the Content-Type header, attackers can bypass body validation while the server still processes the body as the original content type.
For example, a request with Content-Type: application/json\ta will bypass JSON schema validation but still be parsed as JSON.
This vulnerability affects all Fastify users who rely on Content-Type-based body validation schemas to enforce data integrity or security constraints. The concrete impact depends on the handler implementation and the level of trust placed in the validated request body, but at the library level, this allows complete bypass of body validation for any handler using Content-Type-discriminated schemas.
This issue is a regression or missed edge case from the fix for a previously reported vulnerability.
Patches
This vulnerability has been patched in Fastify v5.7.2. All users should upgrade to this version or later immediately.
Workarounds
If upgrading is not immediately possible, user can implement a custom onRequest hook to reject requests containing tab characters in the Content-Type header:
fastify.addHook('onRequest', async (request, reply) => {
const contentType = request.headers['content-type']
if (contentType && contentType.includes('\t')) {
reply.code(400).send({ error: 'Invalid Content-Type header' })
}
})
Resources
- https://github.com/fastify/fastify/blob/759e9787b5669abf953068e42a17bffba7521348/lib/validation.js#L272
- https://github.com/fastify/fastify/blob/759e9787b5669abf953068e42a17bffba7521348/lib/content-type-parser.js#L125
- Fastify Validation and Serialization Documentation
- https://hackerone.com/reports/3464114
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | fastify | all versions | 5.7.2npm install fastify@5.7.2 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastify, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fastify to 5.7.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-25223 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-2026-25223 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-25223. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This IMPORTANT vulnerability in Fastify, a Node.js web framework, allows remote attackers to bypass request body validation by manipulating the Content-Type header. This can lead to unexpected data processing and integrity issues in applications. Red Hat products such as Red Hat Enterprise Linux AI, Red Hat OpenShift…
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat OpenShift AI 2.16 | rhoai/odh-dashboard-rhel8:1774282136 | RHSA-2026:5807 |
| Red Hat OpenShift AI 2.25 | rhoai/odh-dashboard-rhel9:1776742021 | RHSA-2026:10184 |
| Red Hat OpenShift Dev Spaces 3.27 | devspaces/dashboard-rhel9:1774476526 | RHSA-2026:6192 |
Frequently Asked Questions
Is CVE-2026-25223 in your dependencies?
O3 Security finds CVE-2026-25223 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.