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Response snapshot Oct 27, 2026, 03:07 UTC

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VulnerabilityCVE-2026-107832

fastify is vulnerable to header validation bypass via case-sensitive propertyNames

Fastify lowercases header names in a `headers` schema before compilation, because Node.js stores request header names in lowercase. This normalization does not reach the header names held in a `propertyNames` subschema's `enum` and `const`.

At a glance

Exploited
Not known
Severity
3.7 Low (CVSS 3.1)
Chance of exploitation
0.3% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: fastify: 5.12.6. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-389g-gv7q-r4x8 (opens a new tab)

What happened

### Impact

Fastify lowercases header names in a `headers` schema before compilation, because Node.js stores request header names in lowercase. This normalization does not reach the header names held in a `propertyNames` subschema's `enum` and `const`.

A header-name deny-list written in the canonical header case, such as `propertyNames: { not: { enum: ['X-Debug-Override'] } }`, therefore never matches the lowercased request headers, and the forbidden header reaches the handler. Any application that uses a `propertyNames` deny-list or allow-list to constrain request header names does not get the constraint it expects.

The same rule written in lowercase works, and the `not: { required: [...] }` form is unaffected, so this is a gap in the built-in normalization rather than a wrong schema. This is an incomplete fix of CVE-2026-84428 ([GHSA-9q9j-q6p8-xq58](https://github.

Continue reading

com/fastify/fastify/security/advisories/GHSA-9q9j-q6p8-xq58)), whose normalization covered header names used as property keys but not those used as values inside `propertyNames`.

### Patches

Users should upgrade to fastify `5.12.6` or later, which lowercases header names held in `enum` and `const` inside a `propertyNames` subschema so they match the lowercased request headers.

Header-name constraints that cannot be normalized automatically, regular expressions (`propertyNames` `pattern` and `patternProperties` keys) and a `$ref` used inside `propertyNames`, are documented and, for the `$ref` case, warned at startup (`FSTSEC003`). Author those to match the lowercased request headers.

### Workarounds

Write the `propertyNames` header-name constraint in lowercase (for example `enum: ['x-debug-override']`), or enforce the header check in an `onRequest` or `preValidation` hook instead of the schema.

Affected package ranges reported by GitHub: fastify: >= 5.12.2, < 5.12.6

Scores and how they were measured

CVSS base severity

3.7

Version 3.1As of

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.3%

Scored Percentile 17.2%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-389g-gv7q-r4x8 (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-389g-gv7q-r4x8, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-389g-gv7q-r4x8, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  4. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-389g-gv7q-r4x8, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  5. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-389g-gv7q-r4x8, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  6. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-389g-gv7q-r4x8, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  7. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-389g-gv7q-r4x8, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  8. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-389g-gv7q-r4x8, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  9. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-389g-gv7q-r4x8, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  10. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-389g-gv7q-r4x8, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  11. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-389g-gv7q-r4x8, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  12. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-107832, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 22). fastify is vulnerable to header validation bypass via case-sensitive propertyNames (CVE-2026-107832). https://kevwire.com/record/CVE-2026-107832

MLA

"fastify is vulnerable to header validation bypass via case-sensitive propertyNames (CVE-2026-107832)." KEVWire, 22 Oct. 2026, kevwire.com/record/CVE-2026-107832.

Chicago

KEVWire. "fastify is vulnerable to header validation bypass via case-sensitive propertyNames (CVE-2026-107832)." Last modified October 22, 2026. https://kevwire.com/record/CVE-2026-107832.

Evidence summary (plain text)

fastify is vulnerable to header validation bypass via case-sensitive propertyNames (Oct 22, 2026).
Fastify lowercases header names in a `headers` schema before compilation, because Node.js stores request header names in lowercase. This normalization does not reach the header names held in a `propertyNames` subschema's `enum` and `const`. A header-name deny-list written in the canonical header case, such as `propertyNames: { not: { enum: ['X-Debug-Override'] } }`, therefore never matches the lowercased request headers, and the forbidden header reaches the handler. Any application that uses a `propertyNames` deny-list or allow-list to constrain request header names does not get the constraint it expects. The same rule written in lowercase works, and the `not: { required: [...] }` form is unaffected, so this is a gap in the built-in normalization rather than a wrong schema. This is an incomplete fix of CVE-2026-84428 ([GHSA-9q9j-q6p8-xq58](https://github.com/fastify/fastify/security/advisories/GHSA-9q9j-q6p8-xq58)), whose normalization covered header names used as property keys but not those used as values inside `propertyNames`.

Users should upgrade to fastify `5.12.6` or later, which lowercases header names held in `enum` and `const` inside a `propertyNames` subschema so they match the lowercased request headers. Header-name constraints that cannot be normalized automatically, regular expressions (`propertyNames` `pattern` and `patternProperties` keys) and a `$ref` used inside `propertyNames`, are documented and, for the `$ref` case, warned at startup (`FSTSEC003`). Author those to match the lowercased request headers.
CVSS 3.7 (version 3.1; evidence as of Oct 22, 2026; https://github.com/advisories/GHSA-389g-gv7q-r4x8).
EPSS 0.3% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-107832&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-389g-gv7q-r4x8: https://github.com/advisories/GHSA-389g-gv7q-r4x8
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo1MTozMFrOAAf0Mg%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0OToxMlrOAAf0LQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NzoxN1rOAAf0Kg%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NTozNlrOAAf0Jw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NTowNVrOAAf0JQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0Mzo0NVrOAAf0Iw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0MzozMVrOAAf0Ig%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODozNTowMVrOAAZRcw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODozMDoyN1rOAAY_5Q%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODoyOTowNVrOAAfz8g%3D%3D
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-107832&date=2026-10-26
Record ID: CVE-2026-107832.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 01:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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