CVE-2026-73261 Memory corruption in mongoose
An unauthenticated remote attacker can corrupt TCP MSS values via malformed options, causing silent data truncation and denial of service in Mongoose.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-73261?
CVE-2026-73261 is a high-severity memory corruption vulnerability in cesanta mongoose, published on 14 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-73261?
How do I fix CVE-2026-73261?
Upgrade to a fixed version.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2026-73261 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-73261 exists.
Sources
Also known as DEBIAN-CVE-2026-73261, GHSA-3fmr-92g4-wchx, UBUNTU-CVE-2026-73261.