CVE-2026-73251 is a critical (CVSS 9.1) certificate validation flaw in the Mongoose embedded web server library. Clients using the built-in TLS stack with a multi-certificate CA bundle accept a forged certificate chain. Fixed in Mongoose 7.23, so rebuild affected firmware and applications.
What's affected
Mongoose before 7.23. The GitHub advisory names version 7.22 as affected and 7.23 as patched.
The trigger is narrow. It applies to Mongoose acting as a TLS client with the built-in TLS implementation, when the CA option holds a bundle with more than one certificate. A client given a single CA certificate follows a different code path that does verify the signature. Servers, and clients using an external TLS library, are not described as affected.
Because Mongoose is embedded in other products, you may be exposed through firmware or an application you did not write. Mongoose is source code that gets compiled in, so updating means a new build, not a package update.
How to check if you're exposed
If you build with Mongoose, find the version in your source tree.
grep MG_VERSION mongoose.h
Then check three things in your code. Whether you use the built-in TLS (not an external library such as OpenSSL or mbedTLS), whether the connection is a client connection (HTTPS, MQTTS or WSS to a server), and whether the CA option holds more than one certificate.
If you ship devices, look through your bill of materials for Mongoose and ask vendors of embedded products whether they bundle it.
What to do now
- Update Mongoose to 7.23 or later and rebuild.
- Ship updated firmware or applications to the field, with a plan for devices that update rarely.
- Until then, where you can, configure a single CA certificate for the connection, which uses the checked code path.
- Consider certificate pinning for connections to your own servers.
- Add Mongoose to your software inventory so future advisories are easy to match.
Why it matters
The record maps this to CWE-295, improper certificate validation. In the advisory's description, the bundle path searched the CA bundle for an entry whose common name matched the issuer name of the last certificate in the chain, and then did nothing with the result. The signature check that proves the chain really ends at that CA was guarded by a condition that was never true in bundle mode.
So trust came down to comparing two names as text. A certificate an attacker makes up, which names a well known CA as its issuer, can pass. Combined with a network position such as hostile Wi-Fi or DNS spoofing, that gives interception of data the client believes is protected.
TLS client bugs of this kind are quiet. Nothing fails, the connection simply goes to the wrong party. The record's KEV flag is false.
Finding Mongoose in firmware and products
Mongoose is typically compiled directly into a program, so you will not see it as a separate package on a device. Look for the library name in your source tree and in vendor documentation, and check your software bill of materials. For products you buy, ask the vendor whether Mongoose is bundled and whether the built-in TLS client with a CA bundle is used. The advisory's conditions make most devices unaffected, but only the vendor can tell you which ones are.
If you do use that configuration, remember that the fix requires a new build and a firmware release. Plan the rollout, including devices that rarely connect or update.
Why the failure is silent
The tricky thing about certificate validation bugs is that nothing visibly breaks. Connections succeed, data flows and no log line complains. The only symptom is that the peer might be someone else. That is why tests for TLS clients should include negative cases: a certificate signed by an unknown CA, an expired one, one for the wrong name, and a self-signed one that claims a trusted issuer by name. The last case is the one this advisory describes, and it is a good addition to any test suite for code that handles certificate chains.
The free in-browser dependency scan at gold.safeguard.sh/scan can show whether a project you maintain pulls in an affected package, and the Gold vulnerability directory keeps the record for this CVE.
Quick check
Are Mongoose servers affected?
The advisory describes the TLS client path with a CA bundle. It does not describe server side exposure.
What version is safe?
7.23 or later.
Sources
Last reviewed: 19 October 2026.
