Improper Validation of Specified Quantity in Input Affecting cesanta/mongoose package, versions [,7.23)
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Start learning- Snyk IDSNYK-UNMANAGED-CESANTAMONGOOSE-20948700
- published15 Oct 2026
- disclosed14 Oct 2026
- creditlihnucs
Introduced: 14 Oct 2026
NewCVE-2026-73261 (opens in a new tab)How to fix?
Upgrade cesanta/mongoose to version 7.23 or higher.
Overview
cesanta/mongoose is a networking library for C/C++. It implements event-driven non-blocking APIs for TCP, UDP, HTTP, WebSocket, MQTT.
Affected versions of this package are vulnerable to Improper Validation of Specified Quantity in Input in the handle_opt function in src/net_builtin.c, whose guard rejects len < 2 and opts[1] == 0 but never checks opts[1] > len, so an MSS option declaring length 4 with only three option bytes remaining reads opts[3] past the declared options area. An attacker can silently suppress or truncate all application data on a connection by sending a SYN with Data Offset 6 and the options [0x01, 0x02, 0x04, 0x00], which places the out of bounds byte into the low half of s->dmss, and tx_tcp then clamps every send to that value, so a zero byte there makes each mg_send deliver no payload while the connection continues to complete its handshake, acknowledgements, and teardown normally. This requires a build with MG_ENABLE_TCPIP=1 using the bundled TCP/IP stack, and it is reachable in both directions, since a client connecting to a server under the attacker's control processes the same options from the SYN-ACK.