GHSA-9q9j-q6p8-xq58 CVE-2026-84428
fastify vulnerable to header validation bypass via incomplete schema case normalization
- Severity
- 7.5 / 10
- High · CVSS 3.1
- Exploit chance (EPSS)
- 0.52%
- Chance of exploitation in the next 30 days. Higher than 44.49% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-178 Improper Handling of Case Sensitivity
- Fix
5.12.2- Upgrade fastify to this version or later
Description
Impact
Fastify lowercases header-schema property names before compiling the schema, because Node.js stores request header names in lowercase. That normalization was incomplete: it lowercased only top-level properties keys and the root required array, and did not lowercase the JSON Schema Draft 7 dependencies keyword (its trigger keys and dependent property names) or names in nested subschemas. As a result, a header schema that uses dependencies to require one header when another is present (for example X-Admin requiring X-Admin-Token) never matches the lowercased request headers, so the dependency assertion is silently skipped. An unauthenticated remote client can send the header that activates a privileged path while omitting the header the dependency was meant to require, bypassing a schema-enforced security control. The header schema is idiomatic, valid JSON Schema Draft 7, and no custom validator, malformed request, or misconfiguration is required.
Patches
Header-schema names are now normalized across all schema positions (properties, required, dependencies, dependentRequired, dependentSchemas, and nested subschemas). Patched in fastify 5.12.2. The fix is also included in the 6.0.0 release. Header schemas referenced through an external shared $ref (registered with addSchema) are not reached by this normalization and now emit an FSTSEC002 startup warning; inline the header schema to keep case-insensitive assertions in effect.
Workarounds
If upgrading is not immediately possible, write header-schema names in lowercase so the dependencies and other case-sensitive assertions match Node's lowercased request headers, or enforce the cross-header requirement in an onRequest or preValidation hook instead of the schema.
Affected versions
npm
| Package | Vulnerable | Fixed in |
|---|---|---|
| fastify | < 5.12.2 |
5.12.2 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
fastify npm → 5.12.2
- npm
npm install fastify@5.12.2- yarn
yarn add fastify@5.12.2- pnpm
pnpm add fastify@5.12.2
Severity breakdown
CVSS 3.1 7.5
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- High
- Availability
- None
Weakness
CWE-178: Improper Handling of Case Sensitivity
The product does not properly account for differences in case sensitivity when accessing or determining the properties of a resource, leading to inconsistent results.
How to prevent it
- Architecture and Design: Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
- Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- Implementation: Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.
References
Advisories
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-389g-gv7q-r4x8 fastify is vulnerable to header validation bypass via case-sensitive propertyNames | Low | Also affects fastify |
| GHSA-4mh8-r7rc-xpvc fastify vulnerable to Denial of Service via unhandled exception on HTTP/2 trailer responses | Medium | Also affects fastify |
| GHSA-667r-xxjv-c9mm fastify vulnerable to request body replacement via an async validation result collision | High | Also affects fastify |
| GHSA-hwr6-493r-vm6h fastify vulnerable to request validation bypass via skipped boolean false schemas | High | Also affects fastify |
| GHSA-p68q-wchp-6fh7 fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers | High | Also affects fastify |
| GHSA-w2qp-rph6-63g4 fastify vulnerable to schema validation bypass via root primitive coercion mismatch | Medium | Also affects fastify |
| GHSA-3m5p-2c4r-xxw2 fastify vulnerable to X-Forwarded-* spoofing under trustProxy hop-count | Medium | Also affects fastify |
| GHSA-444r-cwp2-x5xf fastify: request.protocol and request.host Spoofable via X-Forwarded-Proto/Host from Untrusted Connections | Medium | Also affects fastify |
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