Certificates both protect TLS connections and digitally sign applications, and forged ones turn up in real-world attacks from time to time. We’ve seen forged or stolen certificates abused by the DarkHotel APT and the Flame malware, and in 2011 we saw attack attempts after the certificate authority Comodo was compromised. Pulling off this trick usually takes serious expertise. But attacking eMudhra certificates in 2026 doesn’t take much effort at all. An attacker just has to guess a number from 1 to 255. Private keys for intermediate certificate authorities are normally gigantic numbers. But in this case, eight of them were the following: 4, 56, 66, 115, 155, 156, 230, and 237. They could be guessed using the public keys (available in public certificates) in under a second. As a result, a hypothetical attacker could issue certificates for anything by signing them with one of the compromised private keys.

Six of these keys were meant for issuing TLS certificates, and the other two were for code signing. All eight intermediate certificate authorities (CAs) were also cross-signed by the Sectigo CA. This means clients with different trust chains could trust a forged certificate.

The major browsers have already blocked the dangerous certificate chains. But IT and security teams still need to assess and remediate the risk in many other systems across their organizations. Scripts for scheduled data-processing, corporate NGFWs, monitoring servers, applications with built-in certificate stores, and devices that don’t get regular updates may accept forged certificates and be vulnerable to man-in-the-middle (MitM) attacks. The keys needed for this are already built into publicly available interception tools.

Timeline of the eMudhra certificate authority incident

The problematic keys came from a batch of 24 intermediate CAs that eMudhra created on April 30, 2025. Instead of cryptographically random values for a 384-bit elliptic curve, eight of the keys turned out to be small numbers. According to eMudhra’s own description, a “key wrap/unwrap step” during key generation created a second, weak set of keys. Because keys are created and stored in a dedicated device called a hardware security module (HSM), a weak key is hard to notice on its own. It’s still unclear what error caused the keys to be generated twice and the weak set to be used; the investigation is ongoing.

In May 2025, the Sectigo CA signed all eight intermediate certificates. Then, on October 17, 2026, an independent researcher notified the security teams at Mozilla, Google, and Apple, as well as the CA itself, that the keys were recoverable. On October 20, Mozilla publicly disclosed the incident. This means that for almost a year and a half the keys could have been recovered and certificates forged.

On October 22, eMudhra announced that it was exiting the global SSL/TLS certificate business.

What an attacker can do

Using the vulnerable intermediate CAs, an attacker can sign a forged certificate for someone else’s domain. Clients that still trust that chain and have no other safeguards will take the forgery for the real server.

A certificate alone isn’t enough to intercept HTTPS traffic. The attacker also needs a way to tamper with the victim’s traffic or redirect the connection. That’s possible, for example, in a compromised office network, on a Wi-Fi access point the attacker controls, or by interfering with routing and DNS. And all of this does happen in the wild, as the DarkHotel attacks, among others, have shown.

There’s one more danger: two of the keys can be used for code signing. In theory, that would let an attacker deliver malware that appears to come from a trusted vendor and doesn’t trigger scary OS warnings during installation.

Was it exploited?

Experts from various companies who analyzed the after-effects of the incident haven’t found any malicious attacks that used these certificates. eMudhra and CERT-In have also reported finding no abuse. The Chrome team found no certificates from the six CAs in public Certificate Transparency logs.

This means that systems where browsers check certificates through Certificate Transparency (CT) were most likely not attacked. But many devices and applications — especially servers and IoT devices, don’t check CT at all, so we can’t say anything about them. Today, malicious actors no longer need to write their own hacking tools to pull off an attack like this. Version 12.3 of mitmproxy — a popular tool among security researchers — already has built-in support for compromised keys.

Where the risk has been eliminated, and where that isn’t guaranteed

Major platforms quickly revoked the compromised certificates. As early as October 17, Chrome did so through CRLSets, and Firefox — through OneCRL. On October 18, eMudhra itself revoked the certificates, citing “CA Compromise”. Apple took action on October 19, and the Sectigo CA also revoked the cross-signed copies. Microsoft revoked the certificates on October 20. Depending on the settings of a specific application and OS, revocation may have taken effect even without an application update, since blocklists are often updated separately.

At the same time, a more radical measure is under way: withdrawing trust from the root CA. Chrome has announced the planned removal of “emSign ECC Root CA – G3” from the browser. Certificates issued under the “emSign Root CA – G1” root, and the root itself, aren’t affected. According to eMudhra’s estimate, about 30 000 certificates will need to be migrated.

But certificate revocation only works where the client can find out about the revocation to then be able to actually act upon it. Many applications check the signature, server name, and certificate validity period, but don’t retrieve or update blocklists. Here are the typical cases:

Outdated and LTS systems. LTS (long-term support) status alone doesn’t mean a system is vulnerable. By definition, a system with this status receives support. But old containers, frozen OS images, and systems that are no longer updated can keep trusting revoked CAs for a long time. It all depends on updates to ca-certificates and TLS libraries, and the settings of the specific application.

Automated systems and servers. On emudhra.we-have-a-problem.com — a site created by enthusiasts where you can check any client software for misplaced trust — curl and Zabbix dominate the list of the most vulnerable systems. For HTTP checks, Zabbix usually uses libcurl and the certificate store configured for it. Similar behavior is also documented in the built-in curl in macOS. At the OS level and in Safari, the certificate is revoked, but curl reads a plain /etc/ssl/cert.pem file, and that’s precisely where the fix has to be made. This is a good example: even when system-level trust and revocation mechanisms work, individual applications — including ones that ship with the OS by default — can remain vulnerable.

Python, Ruby, Go, Java, and containers. An application may use its own set of root CAs, which isn’t updated along with the OS. Endor Labs found more than 25 popular packages that contain the affected root.

Slack, GitHub Desktop, and other apps built on the Electron framework. These apps ship with their own Electron/Chromium, so updating a separately installed Chrome doesn’t update Slack. The certificate revocations are already in the new Electron version, but now every developer has to rebuild and distribute their app, and every user has to install the update. That said, not every Electron app is necessarily vulnerable. The outcome depends on the version, how the app updates its blocklists, and which network stack it uses.

Mobile browsers and IoT devices. Mobile Chrome, Firefox, and Safari get certificate revocation data as quickly as their desktop counterparts do. With the stock browsers from smartphone makers, this isn’t the case. In a published test, Samsung Browser 31.0.0.64 accepted all six test chains from emSign while rejecting the ones from Sectigo. For HarmonyOS 6 and 7, researchers found the affected CAs in Huawei’s published list of root CAs, and those CAs can’t be revoked manually.

For unsupported devices, home appliances, and old firmware, a certificate store update may never be released. In those cases, the problem could linger for years.

Impact on TLS inspection in a corporate network

When an NGFW or web gateway inspects encrypted traffic, it maintains two connections: one with the client and the other with the external server. The client receives a certificate signed by the corporate CA, while the gateway itself verifies the authenticity of the external server.

If the gateway accepts a forged eMudhra certificate, even an updated browser on an employee’s computer won’t see the dangerous chain. Instead, it will receive the familiar corporate certificate. In this scenario, the security tool itself hides the problem from the client. That said, carrying out such an attack would also require intercepting traffic.

To fix the validation policy and certificate lists on the inspection device itself, follow the vendor’s instructions — if the vendor has published any, that is.

At Cisco, the status of the issue is documented in detail for all products. Most of them are still listed as “under investigation”, but mitigation measures are recommended for vulnerable devices.

Sophos and Check Point promise device software updates and, in the meantime, advise removing the certificates manually. Neither company has published a security advisory, but employees of each posted recommendations on its support forum.

For Fortinet devices, instructions for checking and removing emSign certificates can be found here. Palo Alto offered only general advice instead of specifics for its own products.

Of course, a properly configured gateway can indeed temporarily protect outdated client devices that trust the dangerous certificates.

To check specific devices, services, containers, and virtual machines, you can use a dedicated website. One possible scenario for proactively hunting for threats that exploit or have previously exploited weak keys is described here.

Contrary to a common misconception, people haven’t stopped enjoying books in the digital age. The truth is that for a while now, reading on paper hasn’t been the only way to enjoy the written word.

At first glance, what cybersecurity risks could there possibly be? But then, since there are more than a billion e-book readers in the world, cybercriminals are bound to try to cash in. We spoke with Andrey Sidenko, Head of Child Online Safety at Kaspersky, to learn how criminals exploit interest in e-books, and what to look out for so you don’t become a victim yourself.

Malware instead of a book

Most often, scammers target computer and smartphone users who try to download an e-book from a pirate site. For example, our experts recently discovered a miner spreading this way: it was tapping the processing power of infected computers to generate various cryptocurrency tokens. Several of these sites let visitors read e-book excerpts for free. In reality, the attackers were stealing passwords and using a malicious script to swap links: users got an archive with malware named the same as the book they wanted. What’s more, links to these sites sat at the top of search results, so the risk of taking the bait was very high.

A year ago, we also wrote about fake e-books in Turkish or Arabic. These files were actually malicious executables disguised as ordinary PDFs. Once launched, they installed infostealers on the victim’s device: malware that sent chat-app session tokens, browser files, saved passwords, and crypto wallet data to the cybercriminals.

Traps in search results

Sites that engage in search spam pose a danger of their own. Picture this: you search for “The Lord of the Rings free download” — and land on a page that repeats your search phrase word for word and supposedly offers that very book for download. But instead of the book you were hoping for, you may be redirected to yet another sketchy site and treated to a malicious file.

By the way, this scam can work even if you search for a made-up author and a made-up book. Such sites can automatically generate plausible download pages for just about any query.

Book phishing

There are also phishing scams aimed squarely at book lovers. For example, one group of scammers sent out fake emails to users pretending to be from the popular book community Goodreads, offering various bogus (paid) services, which, alas, some recipients fell for: becoming a moderator, taking part in book events, or buying a paid promotion.

Here’s another example: ads supposedly from Orell Füssli, a well-known Swiss bookstore chain, circulated on social media. The ads offered books at hefty discounts. To place an order, users had to follow a link and enter their bank card details. Our regular readers will have no doubt guessed that the link led to a fake site, which in this case simply set up a paid subscription billed to the victim’s card.

Finally, scammers often create phishing sites that pose as pirate libraries. These libraries no longer cater only to fiction readers: readers go there for textbooks, academic papers, studies, rare editions, and other material that’s hard to find publicly. Because such libraries get blocked regularly and have to keep changing domains, it can be hard for users to tell the real site from a fake one.

Attackers take advantage of precisely this by promoting their own sites as official mirrors. These fake sites are often fully functional: you can search for and download books just like on the real thing, and the design is almost indistinguishable from the original. In 2024, for example, data on almost 10 million users of a phishing site mimicking Z-Library, a platform providing free downloads of access-restricted books and publications, was leaked online.

The leak included names, email addresses, countries of residence, password hashes, purchase details, and even crypto wallet addresses. With data this detailed, it’s easy to plan a follow-up attack: say, posing as a police officer to accuse the person of visiting an illegal site and demand money to drop the criminal case.

Can an e-reader get infected?

For attackers, e-readers aren’t the juiciest target: it’s hard to predict what device someone uses, or whether that person even has internet access. Besides, most e-readers are basic models — essentially a flash drive with a screen.

But there are more advanced devices — with internet access, browsers, and the ability to download third-party apps and use cloud storage. These are more likely targets, but let’s be honest: most of the time, it’s cybersecurity researchers who hack them — not criminals. For example, in 2025, researchers discovered a vulnerability in the Kindle that could let an attacker hijack the linked Amazon account.

Data leaks and other less obvious risks

Some e-readers let you highlight quotes, take notes on books you like, and share either with other users. Attackers can use this data for doxing, especially if a person wants to keep quiet about reading a particular book — something like, say, Fifty Shades of Grey, or anything personal that isn’t meant for the public.

In 2020, a Guardian journalist requested a copy of her personal data from Amazon, and received more than 20 000 records regarding how she’d used her e-reader. These included which books she read and when, how long she spent reading them, which passages she highlighted or copied, and which words she looked up in the dictionary.

So it’s worth treating your reading history as personal data too. If a book service leaks data, or your account gets hacked, a bad actor could learn not just your email address but far more personal details.

So how do you read e-books safely?

One of the simplest tips is to use official bookstores — and not just for copyright reasons. Since these platforms value their reputation, their content is vetted — meaning the files are more trustworthy.

Another essential tip is to update all your devices regularly — even the most basic e-reader: if it runs an operating system and any software — keep those updated. Timely updates close vulnerabilities that attackers could exploit, which lowers the risk of being hacked. The same goes for apps for reading and cataloging e-books: this year, researchers found serious vulnerabilities in both Calibre and Koodo Reader.

Note that installing unofficial firmware on your devices — for example, closed-ecosystem e-readers like Kindle — may not be the best idea. Risks include bricking the device and losing your warranty. The device may die or lock up during flashing or later on — when an automatic over-the-air update installs official firmware. The manufacturer may also block your account if it detects the modified firmware when the device goes online. And, naturally, the firmware itself may carry malware, such as stealers that siphon off your personal data and passwords. The fact that there’s no way to install an antivirus on your e-reader only exaggerates the potential problem.

It’s also a good idea to back up your digital library from time to time. Buying a book doesn’t guarantee you’ll keep access to it in your account forever. A title may become unavailable because of changes to licensing terms, platform policies, or other restrictions. What’s more, some formats can only be bought and read inside the e-reader’s closed ecosystem. So we recommend keeping a backup copy on your computer or another device, where you can get to it quickly offline. That way, you’ll keep access to your content even if something goes wrong with either the book service or your device.

Every time you download an e-book, be sure to check the file extension. A regular e-book filename should certainly not end in .exe or .bat. You can read more about the most common extensions of malicious files here.

With scammers, alas, getting better and better at disguising malware, perhaps the best all-around advice is this: don’t visit dubious websites, and don’t download unknown files. This is where Kaspersky Premium comes in: one of its main features is protection against phishing and malware. The application warns you if you try to visit an untrustworthy site, or download a potentially dangerous file. This keeps your data and money safe, and you can download and read your favorite books with peace of mind.

Love the written word? Check out our other posts about books:

• Into the rabbit hole: AI slop is flooding Amazon
• Books on cybersecurity: 5+ recommendations from our experts
• Information security in Dune