Eight eMudhra keys, trusted to vouch for websites, could be guessed in 256 tries

One of the keys was worth 4. Not a number of about 116 digits, as expected from a key on the P-384 curve used here: just 4. On October 17, a researcher known as Wayne alerted browser vendors that the private keys of eight certificates from the Indian certificate authority eMudhra (emSign brand) could be found in a handful of tries.

These "intermediate" certificates are used to sign website certificates. Six of them, intended to secure connections (TLS), were trusted by Chrome, Firefox, Safari and Windows. The other two were used to sign software and were trusted by Microsoft. According to Mozilla, the keys in the affected series were integers below 256, "in practice only zeros except for the last byte." Anyone who held one of the six TLS keys could issue a certificate for any website.

How is such a mistake possible?

The keys were created in April 2025, during a ceremony that produced 24 intermediate certificates. In the case opened by Mozilla, eMudhra says they were generated in Utimaco hardware security modules (HSMs). According to the authority, its script mistakenly produced a second set of weak keys, which was used instead of the right ones because their labels looked alike. eMudhra acknowledges "human errors at several stages" and says its investigation is continuing.

The keys were therefore exposed for about eighteen months. eMudhra says most of these certificates were never used in production and that it found no trace of abuse. According to the alert site run by researcher Dennis Jackson, browsers that require certificate transparency (Chrome, Firefox, Edge, Safari) could not have been attacked this way. Other software remains an unknown.

Browsers blocked the certificates within four days

Firefox and Chrome blocked the certificates on October 17, Apple on the 19th and Microsoft on the 20th. eMudhra revoked them on the 18th. Sectigo, which had signed some of them, revoked its signatures. Mozilla is going further: it is removing its trust in the emSign ECC Root CA - G3 root, and Chrome plans to do the same. eMudhra estimates that about 30,000 customer certificates need to be migrated.

What should you do?

Updating browsers and systems is not always enough: many programs ship their own certificate list and do not check revocations. The site emudhra.we-have-a-problem.com keeps a list of fixed versions, including:

• the ca-certificates packages for Ubuntu, Debian, Fedora, Alpine and Arch;
• Go 1.27.3 and 1.26.10 (CVE-2026-111943), Python certifi 2026.10.19, pip 26.2.2, rustls-webpki 0.103.16, Electron and the curl certificate bundle;
• Node.js, Bun and FreeBSD, still pending as of October 20.

On macOS, the system's built-in curl still trusts these certificates: the site provides a command to remove them. It also offers test addresses, which should trigger a certificate error on a protected machine.

As Let's Encrypt shortens the lifetime of its certificates, the case is a reminder that trust across the whole Web also depends on the rigor of each authority. The findings of eMudhra's investigation, and the checks of its other key ceremonies promised within the week, will need to be followed.