CVE-2026-75931 — fast-uri vulnerable to host confusion via skipped IDN canonicalization on scheme-relative references
fast-uri is a URI parser for Node.js. It canonicalizes a host to its ASCII form only when the input carries an explicit scheme, so a scheme-relative reference such as a host preceded by two slashes is returned with its host verbatim and no error set. As a result fast-uri's own entry points disagree with each other: parse, resolve, normalize, and equal can yield different hosts for the same input depending only on whether a scheme is written out, and equal can return opposite verdicts for the same pair of hosts. An application that extracts a host with fast-uri to check it against a policy list and then resolves the same reference can make its decision on one host while the destination is another, enabling host confusion and policy bypass. The affected versions are 2.4.2 up to but not including 2.4.5, 3.1.3 up to but not including 3.1.6, and 4.0.1 up to but not including 4.1.3. The issue is fixed in 2.4.5, 3.1.6, and 4.1.3, which canonicalize the host consistently across the resolve path. Users should upgrade to a patched version.
Who exploits it and how urgent it is
A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.
How the score was calculated
- EPSS 0.4% (30-day exploitation probability)+5
- Technical severity (high)+6
- Automatable exploitation (SSVC)+3
The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).
Triage
Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.
CISA SSVC assessment CISA
Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.
Remediation
Fixes and mitigations documented by the sources — linked to the original advisory.
- Documented mitigation — github.com (NIST National Vulnerability Database)
- Npm Fast-uri: fixed in version 2.4.5 — github.com (GitHub Security Advisories)
- Npm Fast-uri: fixed in version 3.1.6 — github.com (GitHub Security Advisories)
- Npm Fast-uri: fixed in version 4.1.3 — github.com (GitHub Security Advisories)
First 24 hours
Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.
- Identify exposed assets running the affected vendor/product/version combinations (Fast-uri Fast-uri, Openjsf Fast-uri, Npm Fast-uri).
- The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
- Apply the fixed version or the patch documented under Remediation.
- Search available logs for exploit probes, errors and authentication anomalies.
Affected scope
Vendor, product and versions as the sources word them.
| Vendor | Product | Versions | Status | Source |
|---|---|---|---|---|
| Fast-uri | Fast-uri | ≥ 2.4.2 and < 2.4.5≥ 3.1.3 and < 3.1.6≥ 4.0.1 and < 4.1.3 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Openjsf | Fast-uri | ≥ 2.4.2 and < 2.4.5≥ 3.1.3 and < 3.1.6≥ 4.0.1 and < 4.1.3 | Vulnerable | NIST National Vulnerability Database |
| Npm | Fast-uri | ≥ 2.4.2 and < 2.4.5≥ 3.1.3 and < 3.1.6≥ 4.0.1 and < 4.1.3 | Vulnerable | GitHub Security Advisories |
Attack characteristics
The CVSS vector, decoded. It describes the attack, not your exposure to it.
Every base score collected
Sources score independently and may disagree; each row says who scored it and under which version.
| Score | Version | Severity | Expl. | Impact | Scored by |
|---|---|---|---|---|---|
| 7.5 | CVSS 3.1 | High | 3.9 | 3.6 | ce714d77-add3-4f53-aff5-83d477b104bb via NIST National Vulnerability Database |
| 7.5 | CVSS 3.1 | High | — | — | GitHub, Inc. via GitHub Security Advisories |
| 7.5 | CVSS 3.1 | High | — | — | openjs via CVE Program (cve.org) e CISA ADP |
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N (score shown at the top: ce714d77-add3-4f53-aff5-83d477b104bb) Weakness MITRE
Weakness class according to MITRE’s CWE catalog.
EPSS over time FIRST
Oct 13, 2026 → Oct 15, 2026 · 3 reading(s) · max 0.40%
Detection
Read off the CVSS vector and the weakness class. Starting points, not tested rules.
- Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
Timeline
What happened to this CVE, newest first.
- 2026
- Fix documented
- Affected products added · npm/fast-uri
- Affected products added · fast-uri/fast-uri
- SSVC assessment updated
- Record published
- Fix documented
- CVSS changed · 7.5
- Source status changed · → Analyzed
- Record created in this database
- CVE published
Source activity
Repeated readings are counted rather than listed.
- 1× References added · Oct 14, 2026 – Oct 14, 2026
- 1× Official description updated · Oct 14, 2026 – Oct 14, 2026
References
10 on the record
- github.com/fastify/fast-uri/security/advisories/GHSA-5jgf-p345-68v8
- cna.openjsf.org/security-advisories.html
- github.com/fastify/fast-uri/releases/tag/v4.1.3
- github.com/fastify/fast-uri/commit/4e4ebd8b245f6ca16e448203559eea3e545453a3
- github.com/fastify/fast-uri/commit/0256bc8d1f28b5d0ac657faf67e2411a189dfcb5
- nvd.nist.gov/vuln/detail/CVE-2026-75931
- github.com/advisories/GHSA-5jgf-p345-68v8
- github.com/fastify/fast-uri/releases/tag/v2.4.5
- github.com/fastify/fast-uri/releases/tag/v3.1.6
- github.com/fastify/fast-uri/commit/444ecdad447db2cc23c4d422acc6f0daa6fa8eef
Elsewhere on this site
Related CVEs (same products)
- Medium CVE-2026-86472 fast-uri vulnerable to inconsistent host case normalization via percent-encoded octets Sep 15, 2026
- Medium CVE-2026-86818 fast-uri vulnerable to mailto header injection via percent-encoded field-name desynchronization Sep 15, 2026
- High CVE-2026-84394 fast-uri vulnerable to host confusion via an unclosed bracket in the URI authority Sep 3, 2026
- High CVE-2026-84292 fast-uri vulnerable to authority injection via an unvalidated port in serialize Sep 2, 2026
- High CVE-2026-76172 fast-uri vulnerable to host confusion via percent-encoded scheme normalization Aug 24, 2026
- High CVE-2026-75975 fast-uri vulnerable to server-side request forgery via malformed IPv6 normalization Aug 24, 2026
Not in any source we poll
Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.
- No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
- No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
- No exploit packet captures, log samples, or incident case IDs supplied.
Answered from this record
Text assembled only from the fields above — no additional claims.
What should defenders know first?
CVE-2026-75931 is "fast-uri vulnerable to host confusion via skipped IDN canonicalization on scheme-relative references", rated High. It affects Fast-uri (Fast-uri); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 7.5 describes technical severity. EPSS 0.40% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.
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