CVE-2026-106491: Backstage Improper Input Validation Vulnerability

MODERATE
Advisory Identifier: GHSA-9ghw-48h5-v2w5
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CVSS 6.4
CWE-20 CWE-22
Published
Advisory Summary

CVE-2026-106491 is a MODERATE (CVSS 6.4) improper input validation vulnerability (CWE-20: Improper Input Validation) affecting Backstage libraries in the npm ecosystem. The issue involves improper input validation in proxy-backend. Remediation is available by upgrading affected packages to version 0.6.17.

CVE-2026-106491 at a Glance

CVE Identifier CVE-2026-106491
GHSA Identifier GHSA-9ghw-48h5-v2w5
Severity MODERATE
CVSS v3.1 Base Score 6.4
CWE Classification CWE-20: Improper Input Validation
Affected Product Backstage
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 6.4

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

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

How to Fix CVE-2026-106491

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

@backstage/plugin-proxy-backend fixed: 0.6.17 
npm install @backstage/plugin-proxy-backend@0.6.17
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 @backstage/plugin-proxy-backend

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-106491, inspect your project dependencies using npm:

npm ls @backstage/plugin-proxy-backend

Analyze the terminal output to locate any version of Backstage in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 0.6.17), your application is affected.

Note: The npm ls @backstage/plugin-proxy-backend 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-20: Improper Input Validation, where the product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. In Backstage, 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 An authenticated Backstage user could craft a request URL that causes the proxy-backend to forward the request to a path outside the configured base path on the target server. This is limited to target servers already configured as proxy endpoints and requires Backstage authentication by default. ### Patches Patched in `@backstage/plugin-proxy-backend` version `0.6.17` ### Workarounds - Deploy a reverse proxy or WAF in front of Backstage that normalizes request paths before they reach the backend.

CVE-2026-106491 Affected Backstage Versions

1 component(s)
Fixed Versions:
0.6.17
Product / Package Ecosystem Affected Versions Fixed Version
@backstage/plugin-proxy-backend
npm/@backstage/plugin-proxy-backend
npm
< 0.6.17
0.6.17

References

GitHub Advisory / Wiki ADVISORY
https://github.com/backstage/backstage/security/advisories/GHSA-9ghw-48h5-v2w5
NVD — CVE-2026-106491 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-106491
Security Fix Commit FIX COMMIT
https://github.com/backstage/backstage/commit/233287d0ce3133e41549bf92a7470c940a41753c
Backstage Source Repository REPOSITORY
https://github.com/backstage/backstage
Backstage Release RELEASE
https://github.com/backstage/backstage/releases/tag/v1.54.6

Frequently Asked Questions

What is CVE-2026-106491?
CVE-2026-106491 is a security vulnerability in Backstage involving Improper input validation in proxy-backend under CWE-20: Improper Input Validation.
Which Backstage versions are affected?
Affected versions include releases prior to 0.6.17 across affected packages: @backstage/plugin-proxy-backend.
What version fixes CVE-2026-106491?
Upgrading to 0.6.17 (or the latest patched release for your respective branch) resolves the vulnerability.
Is @backstage/plugin-proxy-backend affected?
Yes, @backstage/plugin-proxy-backend is affected in releases prior to 0.6.17. Upgrade your dependency to 0.6.17 or later.
What is the CVSS score of CVE-2026-106491?
CVE-2026-106491 has a CVSS v3.1 base score of 6.4 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N.
What CWE is associated with CVE-2026-106491?
CVE-2026-106491 is classified under CWE-20: Improper Input Validation (The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.).
How can I check npm dependencies for CVE-2026-106491?
Run the dependency tree command: 'npm ls @backstage/plugin-proxy-backend'. 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-9ghw-48h5-v2w5",
  "modified" : "2026-10-07T17:59:49Z",
  "published" : "2026-10-07T17:59:49Z",
  "aliases" : [ "CVE-2026-106491" ],
  "summary" : "Backstage: Improper input validation in proxy-backend",
  "details" : "### Impact\n\nAn authenticated Backstage user could craft a request URL that causes the proxy-backend to forward the request to a path outside the configured base path on the target server. This is limited to target servers already configured as proxy endpoints and requires Backstage authentication by default.\n\n### Patches\n\nPatched in `@backstage/plugin-proxy-backend` version `0.6.17`\n\n### Workarounds\n\n- Deploy a reverse proxy or WAF in front of Backstage that normalizes request paths before they reach the backend.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "@backstage/plugin-proxy-backend"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.6.17"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/backstage/backstage/security/advisories/GHSA-9ghw-48h5-v2w5"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-106491"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/backstage/backstage/commit/233287d0ce3133e41549bf92a7470c940a41753c"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/backstage/backstage"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/backstage/backstage/releases/tag/v1.54.6"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-20", "CWE-22" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-07T17:59:49Z",
    "nvd_published_at" : "2026-10-06T21:17:18Z"
  }
}