CVE-2026-22031VulnerabilityPublished 19 Jan 2026

CVE-2026-22031: fastify middie · Improper Handling of URL Encoding (Hex Encoding)

Affects fastify middieSource CVE Database V5

Not known to be exploited · CVSS 8.4 High : upgrade to 9.1.0 soon.

Data as of 26 Oct 2026, 06:26 UTC

CVSS 3.1 base score

8.4High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.5% chance of exploitation in 30 days – higher than 44% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 9.1.0

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

Attack Vector: Network
Attack Complexity: High
Privileges Required: Low
User Interaction: None
Scope: Changed
Confidentiality: High
Integrity: High
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 19 Jan 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 2026-10-26.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade middie and @fastify/middie to 9.1.0.

Affected software

Product	Affected	Fixed in
fastifymiddie	Affected <9.1.0	Fixed in 9.1.0
npm@fastify/middie`pkg:npm/@fastify/middie`Other threats	Affected <9.1.0	Fixed in 9.1.0

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

1. Upgrade @fastify/middie to version 9.1.0 or later immediately to apply the fix for this vulnerability. 2. Audit all Fastify applications to identify usage of @fastify/middie and verify middleware path prefixes for potential bypass. 3. Implement additional server-side validation and access controls at the route handler level to reduce reliance solely on middleware for critical security functions. 4. Employ Web Application Firewalls (WAFs) with rules to detect and block suspicious URL-encoded requests targeting sensitive paths. 5. Monitor application logs for unusual access patterns involving URL-encoded paths that could indicate exploitation attempts. 6. Conduct security testing including fuzzing and penetration testing focusing on URL encoding edge cases to uncover similar issues. 7. Educate development teams on secure middleware usage and the importance of consistent URL decoding in all request processing layers. 8. Maintain an inventory of all web applications using Fastify and middleware plugins to ensure timely patch management.

Description

CVE-2026-22031 is a high-severity vulnerability in the @fastify/middie plugin prior to version 9.1.0 that allows bypassing middleware protections via URL-encoded path prefixes. The middleware fails to decode URL-encoded characters when matching paths, while Fastify's router correctly decodes them, enabling attackers to access protected routes without middleware enforcement. This can lead to unauthorized access, compromising confidentiality and integrity of applications using vulnerable versions. The vulnerability requires network access and low privileges but no user interaction. Although no known exploits are reported, the impact on affected systems is significant. Upgrading to version 9.1.0 or later mitigates the issue.

Impact

For European organizations, this vulnerability poses a significant risk of unauthorized access to protected web application endpoints that rely on @fastify/middie for middleware enforcement. This can lead to exposure of sensitive data, unauthorized actions, and potential compromise of application integrity. Industries with strict data protection requirements such as finance, healthcare, and government are particularly at risk. The bypass of middleware could circumvent authentication, logging, or input validation middleware, increasing the risk of data breaches and regulatory non-compliance under GDPR. Since Fastify is popular in modern Node.js web applications, organizations using it in critical infrastructure or customer-facing services could face reputational damage and operational disruption. The lack of user interaction and low privilege requirements make exploitation feasible by remote attackers scanning for vulnerable endpoints. Although no exploits are currently known, the high CVSS score and nature of the vulnerability warrant urgent remediation to mitigate potential future attacks.

CWE-177Improper Handling of URL Encoding (Hex Encoding) (opens in a new tab)
Variant weakness
The product does not properly handle when all or part of an input has been URL encoded.
Architecture and Design · Input Validation
Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
Implementation · Input Validation
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.
Implementation · Input Validation
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).
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
Deserialization (opens in a new tab)
OWASP ASVS 4.0.3
V1.5.1 (opens in a new tab)
Attack patterns
Double Encoding (opens in a new tab)
Generic Cross-Browser Cross-Domain Theft (opens in a new tab)
Using Slashes and URL Encoding Combined to Bypass Validation Logic (opens in a new tab)
URL Encoding (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 26 Jan 2026, 20:11 UTC

WeaknessesCWE-177

Technical details

References

Community reviews

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