CVE-2026-18165: @fastify/oauth2 is an OAuth 2.0 plugin for Fastify

Q: How do I fix CVE-2026-18165?

fastify\/oauth2: upgrade to 8.3.0. Exact commands for each OS are on this page.

Q: Is CVE-2026-18165 being exploited?

CVE-2026-18165 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-18165 affect?

CVE-2026-18165 affects fastify\/oauth2. Severity is medium (CVSS 4.2). @fastify/oauth2 is an OAuth 2.0 plugin for Fastify. In versions from 7.2.0 up to but not including 8.3.0, the plugin validates the OAuth state, and with PKCE the code verifier, by comparing the callback query parameter against an unprefixed

Q: How urgent is CVE-2026-18165?

Priority 15/100. No elevated risk signals yet.

Routine

MEDIUM 4.2UPGRADE

CVE-2026-18165 is a medium-severity flaw (CVSS 4.2) affecting fastify\/oauth2. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

@fastify/oauth2 is an OAuth 2.0 plugin for Fastify. In versions from 7.2.0 up to but not including 8.3.0, the plugin validates the OAuth state, and with PKCE the code verifier, by comparing the callback query parameter against an unprefixed, predictable cookie, with no server-side binding to the browser that began the flow. Any party able to write a cookie for the application's host, such as a sibling subdomain under the same registrable domain,

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	4.2 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.1% (top 98%)
CISA exploited list	No
Weakness	CWE-352
Published	2026-08-15
Last modified	2026-09-04

Publisher (CNA): openjs · Vendor: fastify

Last updated 2026-09-04 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-08-15 14:17 UTCPublished CVE record published
2026-09-04 13:16 UTCUpdated Record last modified

fastify fastify\/oauth2 UPGRADE

Affected: >= 7.2.0 and < 8.3.0

Fixed in: 8.3.0

Safe to downgrade to: a release before 7.2.0

1. Check installed version

npm ls fastifyoauth2

2. UPGRADE to 8.3.0

npm install fastifyoauth2@">=8.3.0"

3. If no update is available yet, DOWNGRADE to a release before 7.2.0

npm view fastifyoauth2 versions npm install fastifyoauth2@"<7.2.0"

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-18165?

fastify\/oauth2: upgrade to 8.3.0. Exact commands for each OS are on this page.

Is CVE-2026-18165 being exploited?

CVE-2026-18165 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-18165 affect?

CVE-2026-18165 affects fastify\/oauth2. Severity is medium (CVSS 4.2). @fastify/oauth2 is an OAuth 2.0 plugin for Fastify. In versions from 7.2.0 up to but not including 8.3.0, the plugin validates the OAuth state, and with PKCE the code verifier, by comparing the callback query parameter against an unprefixed

How urgent is CVE-2026-18165?

Priority 15/100. No elevated risk signals yet.

References

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