CVE-2026-66296: Reflected XSS in oaskit’s default HTML error handler

What it is

Improper Neutralization of Input During Web Page Generation (XSS) vulnerability in lud oaskit allows reflected cross-site scripting via the default HTML error handler. Oaskit.ErrorHandler.Default.format_reason/4 and Oaskit.ErrorHandler.Default.reason_to_html/1 in lib/oaskit/error_handler/default.ex render request-validation failures as an HTML page whenever the request’s Accept header contains html, interpolating request-controlled strings into that page without HTML escaping. The unescaped values are object keys taken from a request body or from an object or deepObject query parameter, which appear in the JSON Schema error’s instance path when a schema rejects them (for example under additionalProperties: false), and the raw Content-Type header, reflected in unsupported-media-type errors when it fails to parse. Because browsers send Accept: text/html on ordinary top-level navigation, a crafted GET link is sufficient to trigger the error page; no form submission, custom Content-Type, or attacker-controlled script on the victim’s side is required. A payload such as filter[</code></h2><script>alert(document.domain)</script>]=x terminates the enclosing markup and the injected script executes in the origin of the application using oaskit, giving it access to that origin’s cookies, session, and same-origin responses. Both HTML error rendering and the vulnerable handler are enabled by default: Oaskit.Plugs.ValidateRequest defaults :html_errors to true and :error_handler to Oaskit.ErrorHandler.Default, so applications following the documented usage are affected without any opt-in. This issue affects oaskit: from 0.1.0 before 0.14.1.

Description quoted from the CVE record (assigned by EEF, published 2026-08-03).

Weakness: CWE-79 Improper Neutralization of Input During Web Page Generation (‘Cross-site Scripting’)

How it can be exploited

Exploitable over the network with low attack complexity with no special attack requirements needing no privileges and a user must actively take an action. Impact: confidentiality none, integrity none, availability none. CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

CISA assessment (SSVC): CISA assesses that a public proof of concept exists. Automatable: no. Technical impact: partial.

Public exploit evidence (Exploit Observer)

Exploit Observer found 5 code repositories and 1 research write-up naming this CVE.

Code repositories that name a CVE are often scanners, copies or fakes, and some are malware. Read the code before running anything.

See all 11 references on Exploit Observer

Live scores

EPSS

0.004
low predicted chance of attack

VEDAS

0.986
strong exploit evidence

On CISA KEV

Not listed

Detection rule

Nuclei · Suricata

Who is affected

Version ranges exactly as the CVE record states them. “Before X” means X is the first fixed version.

How to fix it

Fixed versions stated in the CVE record: 0.14.1.

Vendor guidance:

• https://github.com/lud/oaskit/security/advisories/GHSA-h7xw-x8wr-xpcc (vendor-advisory, related)
• https://github.com/lud/oaskit/commit/b70c6b2eaf0b11bdd0bbb21b8a87dbb3d46918a1 (patch)

How to detect it

Nuclei template (active check)

Oaskit - Reflected Cross-Site Scripting

Oaskit (Elixir OpenAPI request-validation toolkit) before 0.14.1 renders request-validation failures as an HTML page whenever the request Accept header contains “html”. The default error handler (Oaskit.ErrorHandler.Default.format_reason/4 and reason_to_html/1) interpolates request-controlled strings into that page without HTML escaping. The unescaped values include object keys taken from an object or deepObject query parameter, which appear in the JSON Schema error’s instance path when a schema rejects them (for example under additionalProperties: false). Because browsers send Accept: text/html on ordinary navigation, a crafted GET link whose query key breaks out of the surrounding markup causes the injected script to execute in the origin of the application using oaskit. Both HTML error rendering (html_errors: true) and the default handler are enabled by default, so applications following the documented usage are affected.

Name and description quoted from the template, written by VEDAS.

The template sends 1 request (GET /?filter[</code></h2><script>alert(vd66296xss)</script>]=1); it decides using word matchers. Severity in the template: medium. Marked as not yet verified against a live vulnerable target.

Run it against a host you own or are authorised to test:

nuclei -u https://your-host -t CVE-2026-66296.yaml

id: CVE-2026-66296

info:
  name: "Oaskit - Reflected Cross-Site Scripting"
  author: vedas-arpsyndicate
  severity: medium
  description: |
    Oaskit (Elixir OpenAPI request-validation toolkit) before 0.14.1 renders request-validation
    failures as an HTML page whenever the request Accept header contains "html". The default error
    handler (Oaskit.ErrorHandler.Default.format_reason/4 and reason_to_html/1) interpolates
    request-controlled strings into that page without HTML escaping. The unescaped values include
    object keys taken from an object or deepObject query parameter, which appear in the JSON Schema
    error's instance path when a schema rejects them (for example under additionalProperties: false).
    Because browsers send Accept: text/html on ordinary navigation, a crafted GET link whose query
    key breaks out of the surrounding markup causes the injected script to execute in the origin of
    the application using oaskit. Both HTML error rendering (html_errors: true) and the default
    handler are enabled by default, so applications following the documented usage are affected.
  reference:
    - https://www.exploit.observer/?keyword=CVE-2026-66296&match=exact
    - https://subdomain.center/?engine=ammonites&keyword=oaskit&match=exact
  classification:
    cvss-metrics: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N
    cvss-score: 6.9
    cve-id: CVE-2026-66296
    cwe-id: CWE-79
  metadata:
    verified: false
  tags: cve,cve2026,oaskit,xss
http:
  - raw:
      - |
        GET /?filter[</code></h2><script>alert(vd66296xss)</script>]=1 HTTP/1.1
        Host: {{Hostname}}
        Accept: text/html,application/xhtml+xml

    matchers-condition: and
    matchers:
      - type: word
        part: body
        words:
          - "<script>alert(vd66296xss)</script>"
      - type: word
        part: header
        words:
          - "text/html"

CVE-2026-66296.yaml on GitHub

Suricata rules (network detection)

1 rule that alert on traffic matching this exploit: VEDAS CVE-2026-66296 Oaskit - Reflected Cross-Site Scripting. Add them to your Suricata rule set and reload.

alert http $EXTERNAL_NET any -> $HTTP_SERVERS $HTTP_PORTS (msg:"VEDAS CVE-2026-66296 Oaskit - Reflected Cross-Site Scripting"; flow:established,to_server; http.uri; content:"script"; nocase; fast_pattern; pcre:"/(\[|%5b)[^=&]*(%3c|<)[^=&]*script/i"; classtype:web-application-attack; reference:cve,CVE-2026-66296; reference:url,https://www.exploit.observer/?keyword=CVE-2026-66296&match=exact; reference:url,https://subdomain.center/?engine=ammonites&keyword=oaskit&match=exact; sid:1079200; rev:1;)

CVE-2026-66296.rules on GitHub

References

Vendor advisories and patches

• https://github.com/lud/oaskit/security/advisories/GHSA-h7xw-x8wr-xpcc (vendor-advisory, related)
• https://github.com/lud/oaskit/commit/b70c6b2eaf0b11bdd0bbb21b8a87dbb3d46918a1 (patch)

Other references

• https://cna.erlef.org/cves/CVE-2026-66296.html (related)
• https://osv.dev/vulnerability/EEF-CVE-2026-66296 (related)

Other databases

• CVE record (cve.org)
• NVD
• Exploit Observer

Where this comes from

This advisory is generated automatically by VEDAS from: CVE.org record, cve-scores (EPSS and VEDAS), Exploit Observer, vedas-signatures. Text in quotes is reproduced from the source; everything else is a fixed template filled with source fields. Nothing here is written by a language model. Check the vendor advisory before acting, and read how VEDAS and Exploit Observer work, and their limits.