11+ SSL Security Threats Every Business Should Know About

Whether you call it SSL or TLS (yes, everybody’s actually using Transport Layer Security nowadays, but SSL stuck around the way Kleenex did for tissues), the protocol that secures communications between a host and a server is critical for the modern web and yet it can fail in more ways than most people realize. 

In this post, we cover the different SSL security threats, their consequences, and how to reduce the risks.

The Most Obvious SSL Security Threat: SSL Certificate Expiry

Let’s get the most obvious out of the way first. Certificate expiry is seemingly the least dramatic threat on this list, yet it is also the most common. It happens when a digital certificate isn’t renewed before its validity period runs out, usually because renewal was tracked manually or there was a change in ownership, and no one noticed the expiration date.

According to Keyfactor, 86% of organizations experienced a disruption in one year due to certificate mismanagement. 

The renewal problem is about to get more pressing though, since the CA/Browser Forum has approved a plan to slowly shrink maximum certificate lifespans to 47 days by 2029. This is being done to improve online security by bypassing inefficient revocation status checks and reducing the time window attackers can exploit if a private key is ever stolen and misused. This reduced lifespan should indeed help address problems with stolen keys, but it makes manual tracking even less efficient. 

Consequences: For the person visiting your site, an expired certificate leads them to a browser warning that gives visitors a strong urge to leave, and most do. For the business, that translates into abandoned checkouts, broken API integrations, failed mobile app connections, and a dent in customer trust you spent years building. Search engines also treat HTTPS validity as a ranking signal, so an expired certificate can and will cost you traffic.

Prevention: Automating certificate renewal is the best way to avoid this threat. The ACME protocol (used by Let’s Encrypt and most modern certificate authorities) enables automation, and certificate management tools can track expiration dates across every domain and subdomain an organization owns. An SSL Certificate API, for example, can pull a domain’s full certificate chain, allowing you (or the systems you integrate it with) to flag certificates that are approaching or past their expiration date, which is very useful especially when you’re managing multiple certificates.

Current SSL-Based Attacks

Aside from certificate expiry, SSL security threats come in the form of attack types that target the SSL/TLS handshake or the cryptography behind it directly. TLS continues to evolve, which has already made some threats obsolete, but the six attack types below target how SSL/TLS is implemented and used today.

1. SSL/TLS Hijacking 

SSL/TLS hijacking is a man-in-the-middle attack in which the attacker generates a fake certificate for the site a victim is trying to reach. Pulling this off requires several steps. One possible approach looks like this:

• Something — for example, a phishing attack — tricks the victim into installing the attacker’s CA certificate, letting the attacker generate fake certificates the browser will trust.
• The attacker then sets up a transparent proxy that quietly relays traffic between the victim and the real site.
• The attacker performs DNS spoofing, manipulating DNS resolution or otherwise redirecting the victim’s traffic to infrastructure the attacker controls. 
• When the victim visits the site, the attacker presents a forged certificate so the victim’s browser believes it has a secure connection to the requested site. In reality, the attacker decrypts and logs everything before relaying it to the real site over a separate connection, so both the victim and the real server can stay unaware.

Consequences: For the user, everything sent during the session (passwords, payment details, personal messages) passes through the attacker in plain view, even though the browser shows an encrypted connection. For the business, stolen session cookies can enable follow-on session hijacking, and because the connection looks legitimate on both ends, the attack is hard to detect until stolen data appears elsewhere.

2. SSL Stripping 

Like SSL/TLS hijacking, SSL stripping is a man-in-the-middle attack in which the attacker downgrades a session from HTTPS to HTTP without the user noticing. The attack unfolds in a few steps:

• The attacker positions themselves as a man-in-the-middle between the victim and the real site, often on public WiFi or through ARP spoofing on a local network.
• When the victim’s browser tries to upgrade the connection to HTTPS, the attacker intercepts that redirect before it reaches the victim.
• The attacker maintains a legitimate HTTPS connection to the real server while keeping the victim on plain HTTP.
• The victim keeps browsing normally, but everything they send, including login credentials and form data, passes through the attacker in cleartext before being relayed to the real site.

Consequences: Users may hand over login credentials and form data in plaintext. For a business, the risk extends past the customer-facing site since admin portals, API keys, and enrollment tokens pasted or displayed during a downgraded session can be seen by attackers, too. 

HSTS (HTTP Strict Transport Security) solves this problem, forcing browsers to use HTTPS and preventing users from bypassing certificate warnings.

3. SSL/TLS Exhaustion  

An SSL/TLS exhaustion attack is a denial-of-service technique that targets the resource-intensive part of the handshake rather than the encryption itself. It works like this:

• The attacker, often using a botnet, opens large numbers of TLS connections and pushes them far enough into the handshake to force the target server to perform expensive cryptographic operations or maintain connection state.
• Against older TLS deployments, attackers can also repeatedly renegotiate connections to increase the server’s workload.
• The server allocates memory and CPU cycles to each pending handshake while it waits.
• With enough simultaneous requests, the server exhausts its available resources and can no longer process legitimate connections.

Consequences: Users simply can’t reach the site during an attack. For the business, that means downtime, business disruption, lost transactions, and incident response costs.

4. Browser Reconnaissance and Exfiltration via Adaptive Compression of Hypertext (BREACH)

BREACH is a side-channel attack that extracts secrets from an encrypted HTTP response without breaking the underlying encryption. Here’s how it plays out:

• The victim visits a malicious page or loads a hidden element controlled by the attacker while already logged into the target site.
• A script on that page makes the victim’s browser send repeated requests to the target site, each carrying a different guessed character reflected back in the response.
• The target site compresses its response with gzip before encrypting it, and that response includes both the reflected guess and a secret, like a CSRF token.
• The attacker measures the size of each encrypted response. A guess that overlaps with the real secret compresses slightly smaller than one that doesn’t.
• Repeating this process, character by character, enables attackers to reconstruct the full secret. Under favorable conditions, this can be done in well under a minute.

Consequences: For users, this can expose secrets such as CSRF tokens or session data while a session is still active, without any warning sign. For enterprises, a successful attack can result in compromising user accounts, enabling unauthorized actions and severe data privacy violations.

5. TLS 1.3 0-RTT Replay

TLS 1.3 introduced a feature called 0-RTT (zero round-trip time) that lets a returning client skip a full round trip of latency by sending data in its very first message to the server. Cloudflare, one of the first providers to roll this out, has been candid that the speed gain comes with weaker replay protection than the rest of TLS. Here’s how an attacker exploits that gap:

• A client that previously connected to a server keeps a session ticket for faster reconnection later.
• On a later visit, instead of waiting to complete the handshake, the client sends its first HTTP request immediately, encrypted with a key derived from that earlier ticket rather than a freshly negotiated one.
• An attacker with access to the encrypted connection captures a copy of that early request.
• The attacker resends the captured request to the server later, impersonating the original client.
• If the server can’t tell the replay apart from the original, it processes the request a second time.

Consequences: For users, a successfully replayed request could duplicate an action — resubmit a payment, repeat a form submission, or trigger any action that isn’t safe to run more than once. For businesses, a replay attack can cause duplicate transactions, data corruption, resource exhaustion, and loss of trust if customers experience double charges. 

6. Application Layer Protocol Confusion (ALPACA)

ALPACA targets organizations that use wildcard or multi-domain certificates (aka SAN certificates) across servers running different protocols, for example, a website and a mail or FTP server on the same domain. Here’s how it works:

• TLS secures the data flowing over a connection, but it doesn’t confirm which application protocol that connection is actually supposed to be using, so an attacker positioned as a man-in-the-middle can redirect a victim’s browser traffic, meant for an HTTPS website, to a different service on the same domain, like an FTP or mail server, that happens to share the same SAN certificate or wildcard certificate. 
• Since the SSL certificate still checks out, the browser accepts the connection without complaint.
• The mismatched server can end up processing parts of the browser’s HTTP request as its own commands, sometimes reflecting attacker-controlled content straight back to the browser.

Consequences: For users, this can mean stolen session cookies or malicious JavaScript executing in the browser through cross-site scripting, which, for enterprises, translates to data breaches and compliance violations. 

Legacy SSL/TLS Attacks

Other threats depend on protocols and cipher suites that browsers and servers phased out years ago, so they aren’t as current as those listed above, although they still appear in compliance checklists. They are still worth knowing because they tend to resurface in older infrastructure or legacy systems.

7. Heartbleed Vulnerability

The Heartbleed vulnerability was a security issue in OpenSSL’s implementation of the heartbeat extension, a feature that keeps connections alive. An attacker could send a heartbeat request with a false payload size, tricking the server into replying with up to 64 kilobytes of its own memory, and repeat this as many times as needed to harvest passwords, session cookies, and even private keys.

Heartbleed (CVE-2014-0160) was patched in OpenSSL 1.0.1g over a decade ago. Any server still running an unpatched version has far bigger problems than Heartbleed, but the vulnerability remains one of the most cited in cybersecurity history.

8. Padding Oracle On Downgraded Legacy Encryption (POODLE)

POODLE (CVE-2014-3566) requires SSL 3.0 support, which was dropped by major browsers in 2015. Compliance scans still flag the threat because PCI DSS checks for insecure cryptography issues, and POODLE is one such issue. However, exploiting it today requires finding a server or appliance that deliberately kept SSL 3.0 alive, which is rare.

The attack forces a connection down to that outdated protocol, then exploits the way SSL 3.0 handles padding in cipher block chaining (CBC) mode. Attackers repeatedly modify encrypted data and analyze how the server responds so they can recover a byte of plaintext — often a session cookie — one byte at a time.

9. Logjam

Logjam (CVE-2015-4000) exploits export-grade Diffie-Hellman key exchange, a form of encryption that dates back to 1990s-era U.S. restrictions on exporting strong cryptography. Many servers kept support for these weak parameters around for backward compatibility long after the restrictions were lifted.

The attack goes as follows: an attacker intercepts the TLS handshake and tricks the client and server into using weak, export-grade Diffie-Hellman encryption instead of stronger settings. Because many servers reused the same weak parameters, attackers could do much of the difficult cracking work ahead of time. They could then break individual connections more quickly, recover the session keys, and use them to read or alter traffic between the two parties.

10. Decrypting RSA with Obsolete and Weakened Encryption (DROWN)

DROWN (CVE-2016-0800) targets servers that still support the old SSLv2 protocol somewhere in their infrastructure, even if the attacked server runs only modern TLS. The vulnerability relies on a padding oracle weakness built into SSLv2 itself.

The attack works because organizations often reuse the same private key and certificate across multiple services. If a single SSLv2-enabled service, commonly an old mail server, shares a key with a properly hardened web server, an attacker can query that weaker SSLv2 server and use it as an oracle. By sending specially crafted requests and analyzing its responses, the attacker can recover enough information to decrypt captured TLS sessions from the hardened server — even though that server never accepted SSLv2 connections.

11. Return of Bleichenbacher’s Oracle Threat (ROBOT)

ROBOT is a revival of a padding oracle vulnerability against RSA-based key exchange first identified by cryptographer Daniel Bleichenbacher in 1998. When a TLS connection uses RSA to encrypt the session key during the handshake, the server has to decrypt that value and verify that it’s formatted correctly.

Older implementations respond differently depending on whether that decrypted value has valid padding, through a distinct error message, a connection timeout, or a reset. An attacker can send many slightly altered versions of a captured ciphertext and use those differing responses as an oracle, gradually narrowing down the correct session key, or even forging a valid signature, without ever needing the server’s actual private key.

Other SSL Threats to Watch For

There are other ways SSL/TLS protection can fail, aside from SSL certificate expiry and the threats listed above. Some of these issues include:

• Incorrect hostname validation, where a client accepts a certificate for the wrong domain
• Users ignoring certificate warnings instead of backing away, often after being conditioned to click through them
• Compromised certificate authorities issuing SSL certificates to attackers
• Malicious root CAs installed on endpoints, through phishing or malware, defeating browser trust checks
• Stolen server private keys, which let an attacker impersonate a site without any protocol-level attack at all
• Weak random-number generators, which can make supposedly random cryptographic keys predictable
• Poor session-ticket key management, which can let an attacker decrypt past sessions if a ticket key is leaked
• TLS interception products, such as some corporate proxies and antivirus tools, that don’t properly validate the upstream certificate before re-encrypting traffic to the user

Quantum Computing as a Threat to TLS  

Quantum computing threatens the mathematical foundation of SSL/TLS. Today’s SSL/TLS traffic relies on encryption algorithms like RSA and elliptic curve cryptography for two separate jobs during the handshake: 

1. Key exchange, where client and server agree on a shared session key. 
2. Authentication, where a certificate authority’s signature proves a certificate is genuine.

Both jobs depend on math problems, such as factoring large numbers or solving discrete logarithms, that are extremely hard for regular computers. However, they could become solvable by a quantum computer running Shor’s algorithm.

Without a transition plan, anything encrypted with TLS today (and not protected by post-quantum cryptography) could be decrypted retroactively once quantum computers mature. An adversary can capture encrypted traffic today, store it, and wait for quantum computers to become capable of recovering the session key. At that point, everything in that captured traffic becomes readable retroactively. That makes it a serious concern right now for any data that needs to stay confidential for years, such as health records, government communications, or trade secrets, even though no quantum computer can break the encryption yet.

The risk is different for CAs, since SSL certificates are verified in real time during each handshake rather than recorded and cracked later; they can’t be attacked retroactively. However, a future quantum computer could still forge certificates and enable new man-in-the-middle attacks.

Some experts estimate that quantum computers capable of breaking RSA encryption could be available by 2030. In response, NIST finalized its first three post-quantum cryptography standards in 2024, and some major cloud providers and CDNs, such as Cloudflare and AWS, have started implementing hybrid key exchange, which runs the regular encryption algorithm and a post-quantum algorithm side by side in the same handshake so a connection stays secure even if one of the two is broken.

Consequences of SSL Issues

Whether the root cause is an expired SSL certificate or a downgraded connection, Chrome may warn the user that the connection is insecure, although the exact warning depends on the problem and browser settings

However, a large-scale study of more than 25 million real-life browser warnings found that Chrome users clicked through 70.2% of SSL certificate warnings. This happens partly because organizations’ struggles with certificate management have taught the users that expired certificates happen and this is a normal situation rather than a security issue. That essentially renders the warnings useless. 

Here’s what’s at stake when that warning gets ignored:  

• Intercepted communications: Without an encrypted connection, anyone positioned between a user and the site, on a compromised network, public WiFi, or through a man-in-the-middle attack, can read whatever passes between browser and server, including passwords, credit card numbers, and other personal details.
• Convincing phishing sites: A missing or broken SSL connection makes it easier for attackers to create a fake version of a legitimate site. Anything typed into that fake page goes straight to the attacker instead of the real business.
• Identity theft: An unencrypted connection gives attackers a way to gather personal details over time, harvested from intercepted forms, session data, or account information, until there’s enough to impersonate someone.
• Exposed browsing activity: Even browsing that doesn’t involve typing anything sensitive still has value. Without encryption, that activity is visible to anyone watching the connection, not just attackers but any third party positioned to intercept it.
• Malware injection: An insecure connection gives an attacker a way to slip malicious code into a page before it reaches the browser. That code can steal data, damage the device, or monitor the user going forward.

For organizations, these individual risks add up to something bigger, such as a data breach, regulatory exposure under laws like GDPR, and the reputational damage that follows once customers learn their information passed through an attacker rather than a secure connection.

How to Reduce SSL Security Risk

Most of the threats above can be avoided with the help of the following best practices: 

• Monitor certificates across all domains and subdomains using SSL Certificates API to keep track of expiration dates and unexpected changes in SSL certificate chains. 
• Automate certificate renewal through ACME-based tools.
• Enforce HTTP Strict Transport Security (HSTS) so browsers can’t be forced to downgrade connections to HTTP.
• Disable legacy protocols and weak cipher suites.
• Monitor CT logs to detect any certificate for your domain that some CA issued to someone else. 
• Use short-lived certificates to shrink the window an attacker has if a key is ever compromised.
• Restrict root CA installations on managed endpoints so employees and malware can’t add a trusted, attacker-controlled CA.

Conclusion

SSL security threats range from the mundane, such as an expired certificate, to highly technical attacks such as SSL stripping and TLS 1.3 0-RTT replay. Depending on the weakness, attackers may compromise confidentiality or authentication, replay sensitive requests, or simply make services unavailable — leading to data exposure, downtime, reputational damage, and potential compliance violations.