CVE-2026-18165

MEDIUMPre-NVD 4.24.2—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 4.2 under a secondary CVSS source (NVD's own analysis pending). EPSS exploit probability: 0.1%, top 99% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, secondary
4.2EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 4.2Exploit: None knownExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

@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, can plant matching state and verifier cookies and complete an attacker-owned OAuth flow inside a victim's browser, silently signing the victim in to the attacker's account (login CSRF). It does not expose the victim's own account, credentials, or tokens. The issue is fixed in @fastify/oauth2 8.3.0, which adds an opt-in hostPrefixedCookies option. Users should upgrade to 8.3.0 and enable it, or bind state to a server-side session.

CVSS v3
4.2
EG Score
4.2(medium)
EG Risk
19(Track)
EG Risk 19/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity42% × 45%
Exploitation0% × 40%
Automatability0% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
1%
KEV
Not listed

Published

August 15, 2026

Last Modified

August 28, 2026

Advisory Details (2)

Auto-updated Aug 15, 2026
Patch available. Sources: github.
generic

Security Advisories | OpenJS Foundation CVE Numbering Authority

https://cna.openjsf.org/security-advisories.html
github Patch Available

@fastify/oauth2 vulnerable to Login CSRF via plantable OAuth state cookies · Advisory · fastify/fastify-oauth2 · GitHub

https://github.com/fastify/fastify-oauth2/security/advisories/GHSA-p8h8-rj28-m8q9

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 7× in last 7d / 23× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-08-29 22:03 UTCEG score recompute
  2. 2026-08-28 21:41 UTCEPSS rescore
  3. 2026-08-27 16:09 UTCEG score recompute
  4. 2026-08-27 14:25 UTCEPSS rescore
  5. 2026-08-26 14:46 UTCEPSS rescore
  6. 2026-08-25 10:59 UTCEG score recompute
  7. 2026-08-23 06:51 UTCEG score recompute
  8. 2026-08-23 00:19 UTCEPSS rescore
  9. 2026-08-22 04:48 UTCEG score recompute
  10. 2026-08-21 23:49 UTCEPSS rescore
  11. 2026-08-21 02:44 UTCEG score recompute
  12. 2026-08-20 22:55 UTCEPSS rescore
  13. 2026-08-20 00:39 UTCEG score recompute
  14. 2026-08-19 17:03 UTCEPSS rescore
  15. 2026-08-17 20:30 UTCEG score recompute
  16. 2026-08-17 19:50 UTCEG score recompute
  17. 2026-08-17 18:53 UTCEG score recompute
  18. 2026-08-17 13:47 UTCEPSS rescore
  19. 2026-08-16 16:32 UTCEG score recompute
  20. 2026-08-16 14:56 UTCEPSS rescore
  21. 2026-08-15 14:29 UTCEG score recompute
  22. 2026-08-15 13:53 UTCEG score recompute
  23. 2026-08-15 13:51 UTCMITRE cvelistV5first tracked

Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2026-18165?
CVE-2026-18165 is a medium vulnerability published on August 15, 2026. @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…
When was CVE-2026-18165 disclosed?
CVE-2026-18165 was first published in the National Vulnerability Database on August 15, 2026, with the most recent update on August 28, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-18165 actively exploited?
CVE-2026-18165 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 99.1% of all scored CVEs.
What is the CVSS score of CVE-2026-18165?
CVE-2026-18165 has a CVSS v3 base score of 4.2 (NVD).
How do I remediate CVE-2026-18165?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-18165, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

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