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

> Reflected XSS in oaskit's default HTML error handler allows attackers to execute scripts in the victim's browser via crafted links, potentially stealing sessions.

- **Severity:** Medium 5.1 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 3 Aug 2026
- **Updated:** 3 Aug 2026
- **Also known as:** EEF-CVE-2026-66296, GHSA-5rgx-mcq5-r7wv, GHSA-h7xw-x8wr-xpcc

## What is CVE-2026-66296?

CVE-2026-66296 is a medium-severity cross-site scripting vulnerability in lud oaskit, published on 3 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

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.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-66296?

1 open source package ship an affected version, including oaskit. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `oaskit` | Hex | From 0.1.0 | 0.14.1 |

Products: Lud Oaskit.

## How do I fix CVE-2026-66296?

Upgrade oaskit to 0.14.1 or later.

## Is CVE-2026-66296 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-66296 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-66296 and fix it if it is affected.

What it is: CVE-2026-66296 is a medium-severity cross-site scripting vulnerability in lud oaskit, published on 3 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-66296 exists.

Affected packages and fixed versions:
- oaskit (Hex): affected from 0.1.0; fixed in 0.14.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-66296/ (Markdown: https://insights.sbomify.com/CVE-2026-66296/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-66296
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-66296
- OSV: https://osv.dev/vulnerability/CVE-2026-66296

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-66296/
Summaries are generated and may contain mistakes; the official description above is the record.