# CVE-2026-66731: facil.io 0.7.5 - 0.7.6 HTTP/1.1 Chunked Transfer Encoding Parser Crash DoS

> Remote attackers can crash the server by sending a malformed chunked request with a negative size, causing a memory fault in the parser.

- **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-vg6c-748h-g68q

## What is CVE-2026-66731?

CVE-2026-66731 is a high-severity denial of service 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 0.7.5 through 0.7.6 contains a denial-of-service vulnerability in the HTTP/1.1 chunked transfer encoding parser that allows unauthenticated remote attackers to crash the server by sending a negative chunk size value. Attackers can send a single POST request with a Transfer-Encoding: chunked header containing a leading minus sign in the chunk size field, causing the parser in http1_parser.h to compute a large positive integer from the negated value, corrupting internal state and moving the read pointer into unmapped memory resulting in a fault.

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

## Which versions are affected by CVE-2026-66731?


## How do I fix CVE-2026-66731?

No fix has been published for CVE-2026-66731 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-66731 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-66731 exists.

Public exploit code:

- Security Advisory: Negative Chunk-Size Parsing Causes Memory Corruption in facil.io (github_poc): https://github.com/theopaid/CVE-2026-66731-Negative-Chunk-Size-Parsing-Causes-Memory-Corruption-leading-to-Server-Crash

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-66731 and fix it if it is affected.

What it is: CVE-2026-66731 is a high-severity denial of service 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-2026-66731 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-2026-66731/ (Markdown: https://insights.sbomify.com/CVE-2026-66731/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-66731
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-66731
- OSV: https://osv.dev/vulnerability/CVE-2026-66731

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-66731/
Summaries are generated and may contain mistakes; the official description above is the record.