CVE-2026-76169: fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers
Noroxi assessment
Priority: Monitor (action score 30/100)
- The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
- Probability of exploitation in the next 30 days: 0.53% (FIRST EPSS).
- Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
- According to CISA the attack is automatable: targets can be found by mass scanning.
- Fixed version: fastify fastify 5.12.2.
Low priority for now. Upgrade to fastify fastify 5.12.2 or later.
Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it
Primary sources: NVD · CVE.org · FIRST EPSS
fastify versions >= 4.0.0 and before 5.12.2 can route a malformed URL sent under one plugin prefix to the custom not-found handler of a different sibling plugin, and invoke it without the preHandler hook declared for that handler. The internal not-found router for encapsulated handlers dispatches malformed paths through a single shared handler pointer before URL decoding, ignoring the prefix and skipping the selected handler's normal lifecycle. An unauthenticated attacker can therefore reach an authentication-protected private fallback through an unrelated public prefix and read its full response, bypassing the authentication hook and breaking prefix encapsulation. Users should upgrade to fastify 5.12.2 or later.
- Published
- Sep 4, 2026
- Updated
- Sep 15, 2026
- EPSS
- 0.5% · 44th percentile
- CWE
- CWE-288
Report tools
Action score
30
Monitor
Low priority for now.
- CVSS
- 30 / 40 · 7.5 / 10
- CISA KEV
- 0 / 30 · Not listed
- EPSS
- 0 / 30 · 0.5%
CISA SSVC decision
- Exploitation
- none
- Automatable
- yes
- Technical impact
- partial
Vulnrichment: CISA's decision-tree inputs.
CNA vs NVD score
- NVD
- —
- CNA · openjs
- 7.5
- NVD has not scored this yet; the score shown is the CNA’s.
The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.
Get told when this record changes
- Not in KEV: no record of exploitation in the wild
- No public exploit code on record
- A fix has been released
If any of these changes, we'll tell you in the daily email digest.
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Noroxi analysis
What does it mean in your environment?
The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.
We use this product, ask for helpAffected systems
Is your version affected?
Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.
| Vendor | Product | CPE |
|---|---|---|
| fastify | fastify | cpe:2.3:a:fastify:fastify |
Affected versions
NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.
- fastify fastify4.0.0 and later · before 5.12.2
Versions reported by the vendor
Affected version ranges reported by the assigning authority (openjs). Independent of NVD's CPE analysis and usually ahead of it.
fastify fastify
- 5.12.2not affected
- 4.0.0 and later · before 5.12.2affected · semver
Package-level exposure
OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.
| Ecosystem | Package | Affected range | Fix |
|---|---|---|---|
| npm | fastify | from 4.0.0 · before 5.12.2 | 5.12.2 |
Same product
fastify: all recordsOther highest-scoring records for the same primary product.
- CVE-2022-39288Denial of service in Fastify via Content-Type header48Plan
- CVE-2022-41919Fastify vulnerable to Cross-Site Request Forgery (CSRF) attack via incorrect content type35Monitor
- CVE-2026-84504fastify vulnerable to request body replacement via an async validation result collision32Monitor
Published before and after this one in the same product
- ← EarlierCVE-2026-16732Aug 18, 2026 · fastify vulnerable to X-Forwarded-* spoofing under trustProxy hop-count24Monitor
- ← EarlierCVE-2026-18504Aug 18, 2026 · fastify vulnerable to schema validation bypass via root primitive coercion mismatch21Monitor
- Later →CVE-2026-84469Sep 4, 2026 · fastify vulnerable to request validation bypass via skipped boolean false schemas30Monitor
Remediation
Which version to upgrade to
Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.
| Product / package | Fixed version | Source |
|---|---|---|
| fastify fastify | 5.12.2 | Vendor (CNA) |
| npm:fastify | 5.12.2 | Package registry (OSV) |
Exploit status
No known public exploit
No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.
Research context
For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.
Timeline
From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.
No dated events beyond publication.
FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).
Patch and commit links
Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.
No commit or PR link among the references.
Credits
All researchersFinders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.
- vvvvvvvvvvitelreporter
- mcollinaremediation developer
- UlisesGasconremediation reviewer
- schecthellraiser606finder
- B1gN0Sefinder
Variant candidates
Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.
No nightly-computed relations.
Chain candidates
An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.
—
Bug bounty scope
No known public program.
Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).
Technical details
Attack conditions
- Anyone who can reach it over the internet can trigger it.
- No account or password is required.
- No user action is required.
- No special conditions are required; it is repeatable.
If successful
- Confidentiality
- high · data can be read
- Integrity
- none
- Availability
- none
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality impact
- High
- Integrity impact
- None
- Availability impact
- None
Weakness class (CWE)
CWE-288 · Authentication Bypass Using an Alternate Path or ChannelCVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
nvd-secondary
Attack context
MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.
Attack patterns (CAPEC)
Change log
- Fix✗ → ✓
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References
Show all references (1 more)Show less
Vendor advisories and official records. Exploit/PoC links are deliberately left out.