Cloudflare logged about 22,000 attempted TLS handshakes involving eMudhra’s weak intermediate certificate authority keys in the week after the flaw became public, and its measurements suggest many older browsers and embedded clients still accept the compromised certificate chain.
In a follow-up published October 30, Cloudflare said it found evidence that the keys had been used to create forged certificates, although it has not seen evidence that the certificates were used to intercept its own connections. The incident began when researchers discovered that eMudhra’s eight intermediate CA private keys were trivially recoverable from public certificates.
The keys were expected to contain large, unpredictable values, but instead used numbers including 4, 56, 66, 115, 155, 156, 230 and 237. Anyone who recovered the keys could issue certificates for arbitrary domains that unpatched clients might trust. Abijita’s earlier report on the compromised TLS certificates explains the original disclosure and browser response.
These were not 22,000 confirmed successful attacks. Cloudflare said roughly two-thirds of the handshakes were connections to test sites. Nearly all the remaining handshakes involved servers that appeared to have issued certificates for themselves using the weak intermediates. About 160 handshakes presented certificates whose Subject Alternative Names named domains unrelated to the requested hostname. Cloudflare said those certificates looked like default certificates served by the originating servers, not evidence of interception, and every handshake configured to verify the certificate was rejected.
One forged certificate was dated October 20 at 18:19 UTC, less than an hour after Mozilla’s public report, showing that the weak keys were used to create certificates quickly after disclosure. Cloudflare stressed that this does not rule out targeted interception elsewhere, but it has not observed evidence that its own connections were intercepted.
To estimate how many clients still trusted the affected chain, Cloudflare ran about 50 million probes through challenge pages. The company cautioned that the sample is skewed by bots and includes only clients that execute JavaScript. Even with those limitations, its results showed that up-to-date browsers were mostly patched while older browsers and embedded clients lagged. Cloudflare estimated that 30% to 40% of Android WebView clients in its sample still accepted the chain, and more than half of probes across large networks continued to accept it.
Cloudflare said all of its high-priority systems have been patched, with a small number of lower-impact remediation tasks still underway. eMudhra has also announced that it will exit the public SSL/TLS certificate business; Abijita has covered the company’s planned withdrawal and certificate transition.
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What users and administrators should do
Install the latest updates for browsers, operating systems, Android WebView and applications that maintain their own certificate stores. Organizations should identify certificates that chain to the affected eMudhra roots and replace them where needed, and Windows administrators should apply Microsoft’s emergency trust-list updates. Developers should also update embedded browsers and TLS libraries rather than relying only on desktop browser updates. For background, see Abijita’s SSL/TLS and HTTPS explainer.
