CVE-2026-86818: Fast uri Remote Code Execution Vulnerability

MODERATE
Advisory Identifier: GHSA-jvvf-x445-j334
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CVSS 4.8
CWE-436 CWE-172
Published
Advisory Summary

CVE-2026-86818 is a MODERATE (CVSS 4.8) remote code execution vulnerability (CWE-436: Interpretation Conflict) affecting Fast uri libraries in the npm ecosystem. The issue involves fast-uri vulnerable to mailto header injection via percent-encoded field-name desynchronization. Remediation is available by upgrading affected packages to version 4.1.5.

CVE-2026-86818 at a Glance

CVE Identifier CVE-2026-86818
GHSA Identifier GHSA-jvvf-x445-j334
Severity MODERATE
CVSS v3.1 Base Score 4.8
CWE Classification CWE-436: Interpretation Conflict
Affected Product Fast uri
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 4.8

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N).

Attack Vector Network (AV:N)
Attack Complexity High (AC:H)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality Low (C:L)
Integrity Low (I:L)
Availability None (A:N)

How to Fix CVE-2026-86818

To remediate this vulnerability, upgrade all affected Fast uri dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

fast-uri fixed: 4.1.5 
npm install fast-uri@4.1.5
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

npm ls fast-uri

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-86818, inspect your project dependencies using npm:

npm ls fast-uri

Analyze the terminal output to locate any version of Fast uri in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 4.1.5), your application is affected.

Note: The npm ls fast-uri command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-436: Interpretation Conflict, where product a handles inputs or steps differently than product b, which causes a to perform incorrect actions based on its perception of b's state. In Fast uri, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### Impact `fast-uri`'s `mailto` scheme parser compares each query field name to the reserved names (`to`, `subject`, `body`) while the name is still percent-encoded, and only percent-decodes it when storing it as a generic header. On serialize, the decoded name is re-emitted, so a field name such as `%74o` (percent-encoded `to`) is not recognized as a recipient at parse time (`parse().to` shows only the legitimate recipient) but materializes as a literal `to=` field after `serialize()`, and reparsing then treats it as a recipient. The same technique smuggles `subject` and `body` through `%73ubject` and `%62ody`. An application that parses an untrusted mailto URI, makes a display, allowlist, or logging decision on `parse().to`, then re-serializes the result and passes the serialized string to a mail client or an outbound send path can gain an attacker-chosen recipient, subject, or body that was not visible when the recipient list was checked. A scanner inspecting the raw input for an extra `to=` sees nothing, because the injected field appears only after `fast-uri` serializes. ### Patches Upgrade to `fast-uri` 4.1.5. ### Workarounds Percent-decode and compare mailto field names case-insensitively before trusting `parse().to`, or re-check the recipient list on the serialized output rather than only on the initial parse, until upgrading.

CVE-2026-86818 Affected Fast uri Versions

1 component(s)
Fixed Versions:
4.1.5
Product / Package Ecosystem Affected Versions Fixed Version
fast-uri
npm/fast-uri
npm
≥ 4.1.3
< 4.1.5
4.1.5

References

GitHub Advisory / Wiki ADVISORY
https://github.com/fastify/fast-uri/security/advisories/GHSA-jvvf-x445-j334
NVD — CVE-2026-86818 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-86818
Security Fix Commit FIX COMMIT
https://github.com/fastify/fast-uri/commit/f40a88f33e684a46faec3f5b820bcbb1e85add64
Fast uri Security Information SECURITY
https://cna.openjsf.org/security-advisories.html
Fast uri Source Repository REPOSITORY
https://github.com/fastify/fast-uri
Fast uri Release RELEASE
https://github.com/fastify/fast-uri/releases/tag/v4.1.5

Frequently Asked Questions

What is CVE-2026-86818?
CVE-2026-86818 is a security vulnerability in Fast uri involving fast-uri vulnerable to mailto header injection via percent-encoded field-name desynchronization under CWE-436: Interpretation Conflict.
Which Fast uri versions are affected?
Affected versions include releases prior to 4.1.5 across affected packages: fast-uri.
What version fixes CVE-2026-86818?
Upgrading to 4.1.5 (or the latest patched release for your respective branch) resolves the vulnerability.
Is fast-uri affected?
Yes, fast-uri is affected in releases prior to 4.1.5. Upgrade your dependency to 4.1.5 or later.
What is the CVSS score of CVE-2026-86818?
CVE-2026-86818 has a CVSS v3.1 base score of 4.8 (MODERATE) with vector string CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N.
What CWE is associated with CVE-2026-86818?
CVE-2026-86818 is classified under CWE-436: Interpretation Conflict (Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.).
How can I check npm dependencies for CVE-2026-86818?
Run the dependency tree command: 'npm ls fast-uri'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-jvvf-x445-j334",
  "modified" : "2026-09-29T23:51:16Z",
  "published" : "2026-09-29T23:51:16Z",
  "aliases" : [ "CVE-2026-86818" ],
  "summary" : "fast-uri vulnerable to mailto header injection via percent-encoded field-name desynchronization",
  "details" : "### Impact\n\n`fast-uri`'s `mailto` scheme parser compares each query field name to the reserved names (`to`, `subject`, `body`) while the name is still percent-encoded, and only percent-decodes it when storing it as a generic header. On serialize, the decoded name is re-emitted, so a field name such as `%74o` (percent-encoded `to`) is not recognized as a recipient at parse time (`parse().to` shows only the legitimate recipient) but materializes as a literal `to=` field after `serialize()`, and reparsing then treats it as a recipient. The same technique smuggles `subject` and `body` through `%73ubject` and `%62ody`.\n\nAn application that parses an untrusted mailto URI, makes a display, allowlist, or logging decision on `parse().to`, then re-serializes the result and passes the serialized string to a mail client or an outbound send path can gain an attacker-chosen recipient, subject, or body that was not visible when the recipient list was checked. A scanner inspecting the raw input for an extra `to=` sees nothing, because the injected field appears only after `fast-uri` serializes.\n\n### Patches\n\nUpgrade to `fast-uri` 4.1.5.\n\n### Workarounds\n\nPercent-decode and compare mailto field names case-insensitively before trusting `parse().to`, or re-check the recipient list on the serialized output rather than only on the initial parse, until upgrading.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "fast-uri"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.1.3"
      }, {
        "fixed" : "4.1.5"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/security/advisories/GHSA-jvvf-x445-j334"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-86818"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/commit/f40a88f33e684a46faec3f5b820bcbb1e85add64"
  }, {
    "type" : "WEB",
    "url" : "https://cna.openjsf.org/security-advisories.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/fastify/fast-uri"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/releases/tag/v4.1.5"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-172", "CWE-436" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-29T23:51:16Z",
    "nvd_published_at" : "2026-09-15T11:17:12Z"
  }
}