# CVE-2026-126075: Active Support: Possible XSS vulnerability in Active Support MessagePack serializer


- **Severity:** Medium 4.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 10 Nov 2026
- **Updated:** 12 Nov 2026
- **Also known as:** GHSA-2w9f-fxwj-w2qf, UBUNTU-CVE-2026-126075

## What is CVE-2026-126075?

CVE-2026-126075 is a medium-severity vulnerability in rails, published on 10 Nov 2026. 

### Official description

Active Support is a collection of utility classes and standard library extensions used by Rails and other Ruby projects. From 8.1 until 8.1.4.1, ActiveSupport::MessagePack serializes an ActiveSupport::SafeBuffer as its bytes alone and deserializes it as a new SafeBuffer, so a buffer that is no longer html_safe? (for example, one mutated in place with gsub!) is restored as html_safe. If such a buffer holds untrusted input and is stored through a MessagePack serializer, rendering it after it is loaded bypasses ERB auto-escaping, possibly leading to XSS. Applications on Rails 8.1 are affected if they use a MessagePack serializer, such as config.action_dispatch.cookies_serializer = :message_pack or :message_pack_allow_marshal, or serializer: :message_pack for Rails.cache, ActiveSupport::MessageVerifier or ActiveSupport::MessageEncryptor, and store a SafeBuffer that was mutated in place with untrusted input. Rails 8.0 and earlier do not serialize SafeBuffer with MessagePack and are not affected. This issue is fixed in version 8.1.4.1.

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

## Which versions are affected by CVE-2026-126075?

1 open source package ship an affected version, including activesupport.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `activesupport` | RubyGems | From 8.1.0.beta1 | 8.1.4.1 |

## How do I fix CVE-2026-126075?

Upgrade activesupport to 8.1.4.1 or later.

## Is CVE-2026-126075 being exploited?

There is no known exploitation of CVE-2026-126075 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-126075 and fix it if it is affected.

What it is: CVE-2026-126075 is a medium-severity vulnerability in rails, published on 10 Nov 2026. 
Exploitation: There is no known exploitation of CVE-2026-126075 in the wild and no public exploit code.

Affected packages and fixed versions:
- activesupport (RubyGems): affected from 8.1.0.beta1; fixed in 8.1.4.1

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-2026-126075/ (Markdown: https://insights.sbomify.com/CVE-2026-126075/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-126075
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-126075
- OSV: https://osv.dev/vulnerability/CVE-2026-126075

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-126075/
Summaries are generated and may contain mistakes; the official description above is the record.