Mastodon - Open Redirect

mediumseverity

4.3CVSS score

0.5%likelihood of exploitation, next 30 days

41%of all CVEs are less likely

Readycheck to confirm it

Mastodon version < 4.5.8, < 4.4.15, < 4.3.21 is vulnerable to unauthenticated Open Redirect vulnerability (CWE-601) exists in the /web/* route due to improper handling of URL-encoded path segments.

Impact

Redirect users to external domain.

Fix

Update Mastodon to versions 4.5.8, 4.4.15, 4.3.21.

How to confirm it

Send: GET {{BaseURL}}/web/%2Finteract.sh:443
Positive when: matches in header /(?m)^(?:Location\s*?:\s*?)(?:https?:\/\/|\/\/)?(?:[a-zA-Z0-9\-_\.@]*)interact\.…/ AND status 302 or 301

Run it with nuclei: nuclei -id CVE-2026-33868 -u <target>

The full check

id: CVE-2026-33868

info:
  name: Mastodon - Open Redirect
  author: theamanrawat
  severity: medium
  description: |
    Mastodon version < 4.5.8, < 4.4.15, < 4.3.21 is vulnerable to unauthenticated Open Redirect vulnerability (CWE-601) exists in the /web/* route due to improper handling of URL-encoded path segments.
  impact: |
    Redirect users to external domain.
  remediation: |
    Update Mastodon to versions 4.5.8, 4.4.15, 4.3.21.
  reference:
    - https://github.com/mastodon/mastodon/security/advisories/GHSA-xqw8-4j56-5hj6
    - https://nvd.nist.gov/vuln/detail/CVE-2026-33868
  classification:
    cvss-metrics: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
    cvss-score: 4.3
    cve-id: CVE-2026-33868
    epss-score: 0.00473
    epss-percentile: 0.41265
    cwe-id: CWE-601
  metadata:
    verified: true
    vendor: mastodon
    product: mastodon
    shodan-query: html:"mastodon-"
  tags: cve,cve2026,mastodon,open-redirect,vuln,unauth

http:
  - method: GET
    path:
      - "{{BaseURL}}/web/%2Finteract.sh:443"

    matchers-condition: and
    matchers:
      - type: regex
        regex:
          - '(?m)^(?:Location\s*?:\s*?)(?:https?:\/\/|\/\/)?(?:[a-zA-Z0-9\-_\.@]*)interact\.sh.*$'
        part: header

      - type: status
        condition: or
        status:
          - 302
          - 301

References

Use this check only on systems you are authorised to test. Where the data comes from