The key was 4: what the eMudhra certificate failure means for India

The short version

• What happened: eight eMudhra “intermediate” certificates, the kind that vouch for websites, were created with private keys that were tiny numbers. One was 4. Anyone could work them out from public information.
• Why it matters: whoever has such a key can make a fake certificate for any website, which lets them impersonate it to software that still trusts eMudhra.
• Browsers are protected: Chrome, Firefox, Apple and Microsoft blocked the certificates between 17 and 20 October.
• Other software may not be: apps, servers and devices that do not get browser-style updates can keep trusting them until they are updated.
• Our check: none of the 100 Indian bank, insurer, market, payment and government sites that responded uses the compromised certificates. Seven use other eMudhra certificates.

First, what a certificate does

When you open your bank’s website, the padlock means your browser checked an ID card for that site: a certificate. The card is signed by a certificate authority that your browser already trusts, usually through a middle layer called an intermediate. The signature is made with a private key, a secret number that only the authority should know.

If someone learns an intermediate’s private key, they can sign an ID card for any site they like, and software that trusts that intermediate will believe it.

How big a key should be

These were elliptic-curve keys of a standard type called P-384. A proper key is a random number up to 116 digits long, so many possibilities that nobody could ever search them. Security researcher Scott Helme recovered all eight eMudhra keys from their public certificates. This is what he found:

“One of eMudhra’s certificate authority private keys is the number 4. … I recovered all of them from their public certificates on my laptop just now, and it took less than a second.”Scott Helme, security researcher, 20 October 2026

Every key fitted in a single byte, so there were at most 255 numbers to try. Nothing in the certificates looked wrong: the only way to notice was to try.

How it unfolded

1. 30 Apr 2025
   The keys are made. eMudhra creates the intermediates in a key ceremony on hardware security modules. By its own account, a flaw in the process produced a second set of weak keys, and the weak ones were the ones published.
2. May 2025
   Trust spreads. Sectigo, another certificate authority, cross-signs all eight, so they also chain to its older root.
3. 17 Oct, evening UTC
   The report. A researcher credited as Wayne reports to Apple, Google, Microsoft, Mozilla and eMudhra that the keys are compromised, signing the message “We have a problem here” with each key as proof. Chrome blocks the six website intermediates within two hours; Firefox follows the same night.
4. 18 Oct
   eMudhra revokes the certificates.
5. 19–20 Oct
   Everyone else moves. Apple blocks them on the 19th, Microsoft on the 20th. Mozilla makes the incident public on the 20th and removes eMudhra’s emSign ECC Root CA - G3 from Firefox’s trust list; Chrome says the same root will go. Over the following days Go, Python, Node.js, Linux distributions and many others ship updates.
6. 21 Oct, 10:57 am IST
   eMudhra tells investors. A press release filed with the NSE says its infrastructure was not compromised, no data was breached, and its Indian digital signature (DSC) and eSign services are unaffected. MeitY, on X, says it has formed a special team to investigate, that the impact on Indian websites and applications has been “very limited and for a very short interval”, and that CERT-In has issued an advisory with mitigation measures.

Was anyone actually attacked?

Nobody knows, and that is the uncomfortable part. Normally a stolen key has a thief and a moment it was taken. Here the keys could be worked out by anyone, at any time, from April 2025 onwards.

Cloudflare said its review found no active exploitation on its own services. eMudhra says only one of the intermediates was ever put into production and none was used to issue customer certificates.

We checked 108 Indian websites

On 21 October we looked at which certificate authority signs the websites of 108 Indian banks, insurers, stock-market institutions, regulators, payment companies and government services, by connecting to each one and reading the certificate it sends.

The seven on eMudhra’s older chain are LIC (licindia.in), Bandhan Bank (bandhanbank.com), two UIDAI sites (uidai.gov.in and myaadhaar.uidai.gov.in, run by the Aadhaar authority), the national cybercrime reporting portal (cybercrime.gov.in), the National Crime Records Bureau (ncrb.gov.in) and the Controller of Certifying Authorities (cca.gov.in), the government body that licenses certificate authorities in India, including eMudhra. Their certificates are safe today. Chrome has said it is reviewing that older root as well; if it is removed, these sites would need new certificates to keep working in Chrome.

Read the statements like an analyst

What we don’t know yet

• The full root cause. eMudhra says it is still establishing it and will recheck every key ceremony it has run.
• Whether anyone used the keys against software that does not check Certificate Transparency, during the 17 months they were exposed.
• What Chrome decides about eMudhra’s other root, emSign Root CA - G1, which the seven Indian sites above rely on.
• The details of the CERT-In advisory that MeitY says was issued. We had not found its text published by the time of writing.