Learn how to identify and stop the most common email security threats before they lead to breaches.
• Phishing and BEC attacks cause the most financial damage, but they’re just the entry point. The real cost comes when stolen credentials get reused across your systems.
• Email account compromise is worse than phishing because attackers send from a trusted account. Your spam filters won’t catch it.
• SPF, DKIM, and DMARC stop spoofing, but they can’t stop an attacker who’s already logged into a real account. You need credential monitoring too.
• Infostealer malware on a personal device can compromise your corporate email without ever touching your network. Stolen session tokens even bypass MFA.
Business Email Compromise attacks cost companies $2.77 billion in 2024 (FBI IC3 Report). That’s just one type of email threat.
Email is still the easiest way into your network. Attackers know this. That’s why phishing and credential theft start in your inbox.
Most email security focuses on filtering spam and blocking attachments. But the real danger is what happens after an email succeeds. One phishing click can give attackers a foothold inside your network.
This guide covers the most common email security threats, what makes each one dangerous, and how to protect against them.
Your email is the front door to your organization. It’s the top initial access vector for breaches, and attackers exploit it daily.
Email wasn’t designed with security in mind. The SMTP protocol doesn’t encrypt messages by default and makes it trivial to spoof sender addresses. Authentication protocols like SPF, DKIM, and DMARC help, but they can’t stop every attack.
The real problem isn’t the email itself. It’s what happens after. Stolen credentials end up on dark web marketplaces where anyone can buy them. Compromised accounts get used to launch more attacks from inside your organization, bypassing all your perimeter defenses.
Here are the email threats your security team needs to watch for, ranked by how much damage they cause.
Phishing is the most common email attack. Attackers send emails that look like they’re from a trusted source, like your bank or your IT team. The goal is to get you to click a link and enter your credentials on a fake login page.
Mass phishing campaigns hit thousands of inboxes at once. They’re not targeted, but they don’t need to be. Even a 1% success rate gives attackers dozens of valid credentials.
Spear phishing is more dangerous. Attackers research specific employees and craft personalized emails. They reference real projects and real deadlines. These are much harder to spot.
BEC attacks cost more than any other email threat. Attackers impersonate executives or vendors and request wire transfers or invoice payments. No malware, no links. Just a convincing email from someone who appears to be the CEO.
The FBI reports BEC caused $2.77 billion in losses in 2024 alone. These attacks work because they exploit trust, not technology. Your spam filter won’t catch an email that looks legitimate.
For more on preventing these attacks, see our guide on business email compromise and data theft.
This is what happens after a successful phishing attack. Attackers log into a real employee’s email account using stolen credentials. Now they’re operating from inside a trusted account.
From there, they can read sensitive emails and set up forwarding rules to intercept communications. They can also send attacks to other employees from a legitimate address. Your email filters won’t flag messages from a real internal account.
The most common way in? Password reuse. This technique has a name, and it’s worth knowing.
Credential stuffing is an automated attack where criminals take usernames and passwords leaked from one breach and try them against other login pages. It works because people reuse passwords. One leaked password from a shopping site can unlock your corporate email.
Credentials leaked from a third-party breach get tried against your corporate email. If an employee used the same password, attackers walk right in. Compromised credential monitoring catches these leaked passwords before attackers use them.
Attackers use email to deliver malware through attachments or links. Common payloads include ransomware and infostealers.
Infostealers are especially dangerous for email security. Malware like RedLine and Vidar grabs saved passwords from browsers, including email credentials and session tokens. Those stolen credentials get posted to infostealer channels within hours, available for anyone to buy.
Even if your email gateway blocks the initial malware attachment, infostealer infections on personal devices can still compromise corporate email accounts.
Spoofing means forging the sender address so an email appears to come from someone it didn’t. It’s how phishing emails look like they’re from your CEO or your bank.
SPF, DKIM, and DMARC protocols were designed to stop this. If your domain has these configured correctly, spoofed emails from your domain get blocked or flagged. But many companies still haven’t implemented DMARC enforcement. And attackers can still spoof lookalike domains that bypass these checks.
Not every email attack uses links or attachments. Some just ask for information. Attackers pose as IT support asking for credentials. They pretend to be HR requesting employee details. They impersonate vendors asking for payment information.
These attacks work because they exploit human behavior, not technical vulnerabilities. Training helps, but it’s not enough on its own. You need data breach monitoring to catch compromised accounts quickly when social engineering succeeds.
BEC deserves its own definition because it’s often confused with regular phishing. The difference matters for how you defend against it.
Business Email Compromise (BEC) is a targeted email attack where attackers impersonate executives or vendors to trick employees into transferring money or sharing sensitive data. Unlike phishing, BEC emails rarely contain malware or malicious links, making them harder for security tools to detect.
Email threats don’t exist in isolation. They’re the first step in a bigger attack chain.
An employee clicks a phishing link and enters their credentials. Those credentials get harvested by the attacker. If the employee reused that password for corporate email or VPN, the attacker now has access to your systems.
From a compromised email account, attackers can access shared drives and customer data. They can also send phishing emails to other employees from a trusted account, expanding their access.
The credentials from these attacks often end up on dark web markets. Other attackers buy them and try them against your systems months later. The Verizon 2025 DBIR found that 88% of basic web app breaches involved stolen credentials. That’s why catching leaked passwords early matters so much.
Here’s what actually works to prevent email threats, listed by impact.
Configure SPF, DKIM, and DMARC for all your domains. These protocols verify sender identity and block spoofed emails. DMARC enforcement (p=reject) is the goal. Without it, attackers can send emails that appear to come from your domain.
Enable MFA on all email accounts. It blocks most credential-based attacks. But remember that MFA isn’t perfect. Session token theft from infostealer malware can bypass it. Still, MFA stops the vast majority of email account compromise attempts.
Monitor for your employees’ email credentials on dark web markets and in stealer logs. When passwords leak, force immediate resets. This closes the window between a credential being stolen and an attacker using it.
This is where Breachsense fits in. We monitor dark web marketplaces and infostealer channels for your company’s leaked credentials. When employee email addresses and passwords appear, you get alerted so you can reset them before attackers log in.
Deploy a secure email gateway with anti-malware scanning and URL filtering. Modern gateways use sandboxing to detonate suspicious attachments safely. But don’t rely on filtering alone. Determined attackers will eventually get through.
Train employees to recognize phishing and BEC attempts. Focus on the attacks that bypass technical controls, like BEC emails with no malicious links. Test with simulated phishing campaigns and track improvement over time.
Use DLP rules to prevent sensitive data from leaving via email. Flag emails containing credit card numbers or SSNs. DLP catches accidental data leaks and makes intentional exfiltration harder.
Catching a compromised account fast limits the damage. Watch for these signs:
Automate detection by integrating your email security with your SIEM. Set alerts for impossible travel (logging in from two countries within minutes) and after-hours access from new devices.
The fastest detection method? Dark web monitoring. You’ll often find your credentials for sale on criminal markets before the attacker even uses them. That gives you time to reset passwords and lock down accounts before any damage happens.
Email security threats aren’t getting simpler. Phishing and BEC attacks continue to evolve. But the core defense hasn’t changed: stop attackers from getting valid credentials, and catch it fast when they do.
Technical controls like DMARC and MFA handle prevention. Detection closes the gap. You need both.
Check your dark web exposure to see if your company’s email credentials are already on criminal markets.
Phishing is the most common email threat. Attackers send emails that impersonate trusted sources to steal credentials or deliver malware. But BEC attacks cause the most financial damage because they target people with access to money and sensitive data.
The most common method is credential stuffing. Attackers take passwords leaked from other breaches and try them against your email login. If employees reuse passwords, they’re in. Infostealer malware is the other big source. It grabs saved passwords directly from browsers.
MFA blocks most credential-based attacks but not all. Infostealer malware can steal session tokens that bypass MFA entirely. Attackers also use real-time phishing proxies that capture MFA codes as you enter them. MFA is essential, but it’s not bulletproof.
Use compromised credential monitoring to check if employee email addresses and passwords appear in breach data or stealer logs. You can also run a dark web scan for a quick check.
Regular phishing casts a wide net with generic emails sent to thousands of people. Spear phishing targets specific individuals with personalized messages. Attackers research their targets on LinkedIn and company websites to make the emails convincing.
Immediately. Reset the password and revoke all active sessions. Then check for forwarding rules attackers may have set up and scan for any emails sent from the compromised account. Every hour you wait gives attackers more time to move laterally.
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