crt.sh

Certificate Transparency Logs Fail to Block Rogue CA Man-in-the-Middle Attacks

A detailed technical analysis reveals that Certificate Transparency logs do not effectively protect users from man-in-the-middle attacks carried out with certificates issued by compromised or sloppy certificate authorities. The author attempted to verify SCT entries from real browser certificates using public CT log APIs such as get-proof-by-hash but consistently received 404 Not Found responses. Tools including Windows certutil also reported every SCT as not found in the logs. A practical proof-of-concept attack was then constructed by creating a self-signed CA that exactly replicated the certificate chain of habr.com and serving it through nginx with matching extensions and attributes. The resulting MITM connection was accepted by browsers without warnings despite HSTS, because the pre-certificate in the log differs from the presented certificate only in public key bytes. A publicly accessible demonstration was deployed on Yandex Cloud allowing anyone to test the bypass by overriding DNS for habr.com.