GoAhead Web Server Flaw Let Revoked Certificates Log In, With No Warning to Admins

GoAhead is a small web server that device makers build into firmware to run management pages on routers, printers, gateways and industrial controllers. Its maker, EmbedThis, says it runs in hundreds of millions of devices. Researcher Sébastien Féry of FoxIO found that builds using the OpenSSL backend loaded a certificate revocation list at startup but never switched revocation checking on. The server reported no error, so an operator had every reason to believe revoked certificates were being blocked.

In practice, a client whose certificate had been revoked, such as a stolen or retired device, could complete the connection and get whatever access that identity allowed. The problem affects only a deliberate setup: mutual-TLS with client certificates required, a trusted certificate authority bundle, issuer checking and a revocation list. An attacker would also need a certificate signed by a trusted authority and listed as revoked. Builds using the Mbed TLS backend are unaffected. EmbedThis rates the flaw 9.1 (its own rating), and GoAhead 6.1.1 fixes it.

As of October 27, the CVE identifier had been requested but was not yet live in the public CVE and NVD databases. Researchers also described two further TLS problems, each tied to a specific TLS build. This summary is based on the opening of the source article and does not cover those in detail.