Weak eMudhra certificate keys leave servers exposed even after browsers were fixed

Cloudflare has warned that customers may need to take action on their own servers after weak keys in eight eMudhra intermediate certificates left them open to forgery. Because the keys were weak, an attacker could create a forged certificate for any domain and use it to impersonate servers, intercepting or modifying traffic. Nothing had to be stolen: the private keys, generated in an April 2025 key ceremony, were integers below 256 rather than large random numbers, so anyone could recalculate them almost instantly. eMudhra told Mozilla's bug tracker that a code flaw during an approved test created a second, weaker set of keys, which were then used by mistake. eMudhra is India's largest licensed certification authority.

Browser users were protected quickly. Firefox and Chrome had blocked the certificates since 17 October, and Apple devices since 19 October. Software outside the browsers was harder to fix. Cloudflare said it learned of the compromise at about 17:00 UTC on 19 October and prioritised patching critical services before public disclosure. It found no active exploitation on its services, and released updates to its WARP, 1.1.1.1 and Tunnel software on 20 October, completing CDN and server-side Zero Trust mitigations by 19:20 UTC.

Cloudflare said there was "much work still to be done". Customers using mutual TLS should review which certificate authorities they trust to verify client certificates, and those running containers should check their images for affected trust stores. Developers face a subtler problem, because updating SDKs may not be enough: an API client can hand certificate checks to an HTTP library, a language runtime, the operating system or a custom configuration. A passing browser test does not prove a background service is protected, nor does a curl test if the application uses a different verifier.