# CVE-2026-66729: facil.io 0.6.0 - 0.7.6 Integer Underflow DoS via Multipart MIME Body Parser

> Remote attackers can crash the server via integer underflow in the multipart MIME parser, requiring an upgrade to facil.io.

- **Severity:** High 8.7 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 27 Jul 2026
- **Updated:** 31 Jul 2026
- **Also known as:** GHSA-g2qx-6r8w-37px

## What is CVE-2026-66729?

CVE-2026-66729 is a high-severity vulnerability in boazsegev facil.io, published on 27 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

facil.io 0.6.0 through 0.7.6 contains an integer underflow vulnerability in the multipart MIME body parser that allows unauthenticated remote attackers to crash the server process by sending a crafted Content-Disposition header with an empty field name. Attackers can trigger a uint32_t wraparound in http_mime_parser.h causing an out-of-bounds memory read past the name pointer, resulting in a bus fault that crashes the handling worker with a single POST request.

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2026-66729?


## How do I fix CVE-2026-66729?

No fix has been published for CVE-2026-66729 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-66729 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-66729 exists.

Public exploit code:

- Security Advisory: Out-of-Bounds Read in facil.io MIME Parser leads to Server crash (github_poc): https://github.com/theopaid/CVE-2026-66729-Out-of-Bounds-Read-in-facil.io-MIME-Parser-leads-to-Server-Crash

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-66729 and fix it if it is affected.

What it is: CVE-2026-66729 is a high-severity vulnerability in boazsegev facil.io, published on 27 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-66729 exists.

Affected product: boazsegev facil.io. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2026-66729/ (Markdown: https://insights.sbomify.com/CVE-2026-66729/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-66729
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-66729
- OSV: https://osv.dev/vulnerability/CVE-2026-66729

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-66729/
Summaries are generated and may contain mistakes; the official description above is the record.