certifi bundles emSign root CAs with compromised intermediate CA keys

The certifi package, which provides a bundle of root CA certificates for Python applications, included the emSign ECC Root CA - G3 root certificate in versions before 2026.10.18. Eight intermediate CA certificates chaining to this root have compromised private keys and have been revoked by the CA. This compromise allows an attacker possessing one of these keys to issue fraudulent certificates for any hostname. If the attacker can perform a man-in-the-middle attack, they can intercept or alter TLS traffic from clients that rely on certifi's CA bundle, affecting tools such as requests, urllib3, pip, and the Python.org macOS installer. Certifi version 2026.10.18 removed all emSign root certificates, and version 2026.10.19 restored only those emSign roots not affected by the compromise. Users are advised to upgrade to 2026.10.19 or later and rebuild any container images that include certifi to mitigate the risk.

The compromised intermediate CA keys allow attackers to issue valid-looking TLS certificates for any hostname trusted by certifi bundles before 2026.10.18. This can enable interception or modification of TLS traffic (man-in-the-middle attacks) for affected Python applications and installers that use certifi's CA store. The trust in TLS connections relying on these certificates is undermined until the certifi bundle is updated.

A patch is available. Certifi version 2026.10.18 removed all emSign root certificates, and version 2026.10.19 restored only unaffected emSign roots. Users should upgrade to certifi 2026.10.19 or later. Additionally, any container images or environments that vendor certifi should be rebuilt to incorporate the updated CA bundle. No other mitigation is required as the vendor has addressed the issue.