# CVE-2026-125111: Rack: Multipart parsing allows DoS via unbounded scan buffer growth


- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 9 Nov 2026
- **Updated:** 12 Nov 2026
- **Also known as:** DEBIAN-CVE-2026-125111, GHSA-5v64-555j-wjxm, UBUNTU-CVE-2026-125111

## What is CVE-2026-125111?

CVE-2026-125111 is a high-severity vulnerability in rack, published on 9 Nov 2026. 

### Official description

Rack is a modular Ruby web server interface. Prior to 2.2.25, 3.1.23, and 3.2.8, Rack::Multipart::Parser can retain parsed data in its internal StringScanner buffer when reads end inside or immediately after multipart headers. An unauthenticated attacker can align many small parts with rack.multipart.buffer_size boundaries so the buffer grows with the request despite per-field, header, and preamble limits, causing a worker to exhaust memory. The attack requires only a well-formed multipart body and affects applications that parse multipart requests. This issue is fixed in versions 2.2.25, 3.1.23, and 3.2.8.

Weakness: CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2026-125111?

3 open source packages ship an affected version, including rack.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `rack` | RubyGems | All versions | 2.2.25 |
| `rack` | RubyGems | From 3.0.0 | 3.1.23 |
| `rack` | RubyGems | From 3.2.0 | 3.2.8 |

## How do I fix CVE-2026-125111?

Upgrade rack to a fixed version for your release line: 2.2.25, 3.1.23 or 3.2.8.

## Is CVE-2026-125111 being exploited?

There is no known exploitation of CVE-2026-125111 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-125111 and fix it if it is affected.

What it is: CVE-2026-125111 is a high-severity vulnerability in rack, published on 9 Nov 2026. 
Exploitation: There is no known exploitation of CVE-2026-125111 in the wild and no public exploit code.

Affected packages and fixed versions:
- rack (RubyGems): affected all versions; fixed in 2.2.25
- rack (RubyGems): affected from 3.0.0; fixed in 3.1.23
- rack (RubyGems): affected from 3.2.0; fixed in 3.2.8

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-2026-125111/ (Markdown: https://insights.sbomify.com/CVE-2026-125111/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-125111
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-125111
- OSV: https://osv.dev/vulnerability/CVE-2026-125111

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-125111/
Summaries are generated and may contain mistakes; the official description above is the record.