Advances in cybersecurity often focus on hackers and malware, yet one of the biggest vulnerabilities sits in the office chair. Studies show that the vast majority of breaches involve human error; one analysis found that 88% to 95% of cybersecurity incidents can be traced back to mistakes by people. Whether through inadvertent slip-ups or intentional misconduct, employees and insiders play a role in a huge portion of data breaches. No organization is immune: even with strong firewalls and encryption, a single click on a phishing email or an improperly secured account can undo all those defenses. It’s no wonder, then, that phishing and other social engineering tactics remain the top cause of corporate breaches, as attackers find it easier to trick a person than to hack a system. The financial stakes are enormous as well; the average cost of a data breach reached about $4.9 million in 2024, not to mention hard-to-quantify damage to a company’s reputation and customer trust.
Insider-related threats come in many forms: negligent errors, policy violations, or outright malicious acts. The examples below, drawn from real-world incidents, illustrate how employees have caused serious security breaches across different industries. These cases, ranging from misplaced trust and oversight failures to deliberate sabotage, underscore the need for organizations to build a culture of security awareness. From HR departments to IT teams, everyone has a part to play in protecting sensitive data. Let’s examine ten notable breaches caused by employees or insiders, what happened in each, and the lessons that enterprises can learn from them.
In February 2016, Snapchat learned the hard way that even tech-savvy companies are not immune to old-fashioned deception. A payroll department employee was tricked by an email impersonating the CEO (Evan Spiegel) and unwittingly sent confidential payroll information to a cybercriminal. The attacker’s spoofed email looked convincing enough that the employee believed the request was legitimate. As a result, sensitive data for about 700 current and former Snapchat employees, including names, Social Security numbers, salaries, and more, was handed over to the fraudster. Fortunately, the breach was limited to employee data (user accounts were not affected), but it was a serious embarrassment for a company that trades on privacy. This incident highlights how social engineering exploits human trust: no matter how many security tools are in place, an untrained or over-trusting employee can be duped into opening the vault. Snapchat responded by apologizing and offering credit monitoring to those affected, and importantly, vowed to ramp up employee security training to prevent similar scams in the future.
Even one of the world’s largest social media platforms fell victim to an insider-targeted attack. In July 2020, hackers gained access to Twitter’s internal administration tools by socially engineering a handful of employees over the phone. Posing as IT support in a “spear phishing” phone call, the attackers tricked employees into revealing credentials and used those to penetrate deeper into Twitter’s network. With control of privileged internal accounts, the attackers hijacked 130 celebrity and corporate Twitter accounts, including those of Barack Obama, Elon Musk, and Apple, and tweeted out a cryptocurrency scam from 45 of them. While the immediate haul was only about $100,000 in Bitcoin, the real damage was to Twitter’s credibility and security image, as it became clear that insider access was too easily obtained by impostors. Twitter later acknowledged that the attackers “misled certain employees and exploited human vulnerabilities” to gain access to critical support tools. The lesson is stark: even at highly tech-focused firms, robust employee verification and access controls are essential. Organizations must train staff to spot unusual requests (especially via phone or email) and implement checks so that one compromised login can’t unlock the keys to the kingdom.
Not all breaches are high-tech; sometimes, a simple mistake can cause a major data leak. In 2016, an employee of the City of Calgary (Canada) accidentally emailed a file containing sensitive personal information of 3,716 other city employees to an external recipient. The spreadsheet, which was meant for internal use, included confidential details such as employee names, home addresses, dates of birth, social insurance numbers, medical records, and salary information. This one errant email, sent to a colleague in another municipality, amounted to a serious privacy breach for thousands of public sector workers. The fallout was significant: the affected employees launched a class-action lawsuit seeking damages for the exposure of their data. For the City of Calgary, it was a wake-up call on the risks of human error. This case shows how “fat-finger” mistakes or lapses in attention can defeat technical safeguards. Organizations should mitigate these risks by implementing data loss prevention tools (e.g., warnings for external email recipients or limits on mass data exports) and by reinforcing policies around the handling of sensitive files. In addition, double-checking recipients before hitting send is a habit every employee should learn; it could prevent a costly breach.
Insider risk can spike during employee turnover. A notable example occurred at the U.S. Federal Deposit Insurance Corporation (FDIC) in February 2016, when a departing employee inadvertently took a trove of data on a portable storage device. The FDIC staffer copied files containing personal information of approximately 44,000 banking customers onto a USB flash drive before leaving the agency. According to an internal investigation, the individual had legitimate access to the data as part of their job and apparently copied it “without malicious intent”, essentially, they claimed it was an accident. Still, the incident was classified as a major breach. It took the FDIC several days to detect that the data had been downloaded, after which they contacted the ex-employee and recovered the USB drive. An affidavit from the former employee indicated the data was not misused, averting harm, but the potential consequences were alarming. This case underlines the need for strict off-boarding procedures and technological controls. Agencies and companies should ensure that when an employee leaves, their access is immediately revoked and any downloading of bulk data is flagged or blocked. In fact, following this breach, the FDIC banned most employees from using portable storage devices. The broader point is clear: data exposure doesn’t require ill intent; simple lack of oversight can lead to serious breaches.
One of the most infamous data breaches in history, the Equifax incident of 2017, demonstrates how an internal oversight can open the door to attackers. Equifax, a major credit bureau, had a known security patch available for a web application framework (Apache Struts) in March 2017. However, Equifax’s IT staff failed to apply the critical patch to its systems. This lapse proved devastating: attackers took advantage of the unpatched vulnerability and for over two months, enjoyed unfettered access, ultimately stealing personal data (including Social Security numbers, birth dates, and credit card details) of approximately 147 million people. The breach was not discovered until late July 2017, by which time the damage was done. Equifax’s CEO later testified that a specific employee’s failure to implement the software update allowed the breach to occur. The consequences were enormous. Equifax agreed to a settlement of up to $700 million with regulators and states, and its reputation was in tatters. The Equifax case is a textbook example of breach by negligence. It highlights the importance of robust patch management, clear accountability, and oversight within organizations. In an era when vendors issue fixes for known flaws, failing to patch is akin to leaving the door unlocked. Companies must foster a culture where security procedures (like timely updates) are followed diligently and double-checked to avoid such easily preventable calamities.
Not all threats come from outside hackers; sometimes the call is coming from inside the house. In 2014, U.K. supermarket chain Morrisons experienced a major internal breach when a disgruntled employee with IT access leaked the payroll data of about 100,000 staff members. The employee, an internal auditor named Andrew Skelton, had legitimate access to payroll records. Motivated by a personal grudge against the company (he had been disciplined at work), Skelton maliciously uploaded the data, including names, addresses, bank account details, and salaries, to a public file-sharing website and even sent it to newspapers. This deliberate insider attack caused panic among Morrisons employees, who feared identity theft and fraud, and led to years of legal battles. (The case even reached the UK Supreme Court, which eventually ruled that Morrisons was not vicariously liable for the rogue employee’s criminal actions.) The Morrisons leak underscores the damage a single rogue insider can inflict. It reinforces the need for companies to implement strict access controls (limit who can download such sensitive data), monitor employee actions (especially those with elevated privileges), and have response plans for insider incidents. It’s a stark reminder that employees angry at their employer can turn into threats, hence the importance of fostering a positive workplace culture and watching for warning signs. Trust is vital, but as Morrisons learned, trust must be paired with verification and safeguards.
Insider breaches aren’t limited to data theft; they can also take the form of destructive attacks. A dramatic example comes from energy company EnerVest: in 2012, a network engineer learned he was about to be terminated and decided to exact revenge by sabotaging the company’s IT systems. Shortly after hearing of his impending firing, the employee remotely accessed EnerVest’s network and reset dozens of critical servers to their factory settings, essentially wiping all the data and configurations. He didn’t stop there, the disgruntled engineer also snuck into the office after hours to disconnect equipment and disable cooling systems in the server room. The impact was catastrophic: EnerVest’s operations were knocked offline for about 30 days, disrupting business across its eastern US operations. The company spent hundreds of thousands of dollars attempting to recover data (some of which was permanently lost) and restore its network. Ultimately, the rogue employee was caught and sentenced to four years in federal prison for the attack. This case highlights the extreme damage a malicious insider with IT administrator access can cause. It reinforces the importance of rigorous off-boarding (cut access immediately when an employee is to be let go) and possibly monitoring for abnormal actions by employees under stress or facing termination. Technical safeguards like privileged access management, server snapshots, and backup systems can also limit the destruction an angry insider can inflict. EnerVest’s nightmare scenario is a cautionary tale: insider sabotage can be as crippling as any external cyberattack, and preparedness is key.
One of the largest insider-caused breaches on record took place at Desjardins Group, a Canadian financial cooperative, between 2017 and 2019. Over about two years, a malicious employee in Desjardins’ marketing department surreptitiously exfiltrated data on nearly 9.7 million customers. The stolen information included personal details like names, addresses, social insurance numbers, and transaction histories, basically a treasure trove of customer data. What makes this breach chilling is the slow, stealthy nature of the insider’s actions: the employee had authorized access to customer databases (which were too broadly accessible within the department) and regularly siphoned data onto portable media without detection. Desjardins only discovered the breach in 2019 when alerted by the police, by which point the damage was done. The fallout was massive. Millions of members had to be notified, credit monitoring was offered, and Desjardins eventually agreed to a settlement of over CAD 200 million for those affected. An investigation by Canada’s Privacy Commissioner found Desjardins had multiple gaps in safeguards and oversight, such as inadequate access controls and a lack of employee training on data protection. The Desjardins breach underscores how critical it is to enforce the principle of least privilege (employees should only access data truly needed for their job) and to monitor for unusual data access patterns. It also shows that insiders can operate as stealth data thieves for long periods if proper audits and controls are not in place. In short, organizations must treat insider threat detection with the same seriousness as external threats; sometimes the call is coming from inside, and it can go unnoticed for years.
Insider threats can emerge even at the very start of employment. In early 2021, Tesla experienced a breach of its intellectual property when it hired a software engineer who, within his first week on the job, stole around 26,000 confidential files from Tesla’s network. According to a lawsuit Tesla filed, the new employee (hired in December 2020) secretly uploaded thousands of sensitive files, including proprietary source code for Tesla’s manufacturing operating system, to his cloud storage (Dropbox). Tesla’s security team detected the unusual data transfers on January 6, 2021, just about three days into the engineer’s tenure, and confronted him. The engineer claimed it was an accident and that he had merely backed up some personal work documents, but Tesla alleged he attempted to delete evidence when caught. The stolen scripts represented “years of Tesla’s engineering work” and could be extremely valuable to competitors, according to the complaint. Tesla fired the employee and took legal action for trade secret theft. This case illustrates the risk of entrusting new or unvetted hires with broad access. It underscores why companies should implement probationary period safeguards, for example, closely monitor new employees’ access activities, impose tighter permissions until trust is earned, and use technical controls to flag large data exports. Tesla’s experience also shows insider threats aren’t limited to personal data; corporate intellectual property is a prime target as well. The key lesson is to maintain a zero-trust mindset: verify everyone’s actions, especially those who haven’t yet proven their loyalty.
Sometimes, the insider causing a breach isn’t even with the company anymore, due to oversight, their access persists post-termination. This was the case in the 2022 Cash App Investing breach, which occurred after a former employee left the company. In this incident, a departed employee was able to access and download internal reports containing sensitive customer information because their access to systems hadn’t been properly revoked. The reports, accessed in December 2021, included data such as customers’ full names, brokerage account numbers, portfolio holdings and values, and other stock trading details. It wasn’t until April 2022 that Cash App’s parent company (Block, formerly Square) discovered the breach and disclosed it. By then, about 8.2 million U.S. customers had been affected. Thankfully, crucial personal details like Social Security numbers and passwords were not exposed, but the incident was serious enough to trigger regulatory notifications and customer outreach. The breach’s cause was an oversight in access management; the ex-employee still had credentials able to reach critical reports. Cash App’s case drives home a straightforward lesson: when employees leave, whether voluntarily or not, organizations must promptly disable their accounts and access privileges. “Lingering access” is a well-known security hole that can lead to breaches either accidentally or intentionally. In addition, companies should consider technical measures like expiring credentials and routine audits of account permissions. This episode also highlights the value of monitoring and logging access to sensitive data; had the company noticed unusual report downloads sooner, it could have mitigated the impact. In sum, rigorous off-boarding and continuous access audits are essential to plug the insider threat, even after insiders depart.
These examples span a range of industries and breach types, but they all point to a common truth: employees are often the weakest link in security, whether through mistakes or malice. Human factors, be it curiosity, carelessness, or criminal intent, can defeat the best technical defenses. Organizations must therefore take a proactive, multi-layered approach to manage insider risk:
In the end, truly effective cybersecurity blends technology with savvy governance of people and processes. Firewalls and encryption won’t stop an authorized user who decides to abuse their access or an employee who unknowingly clicks a bad link. That’s why leading organizations invest heavily in security awareness, insider threat programs, and rigorous internal controls. The breaches caused by employees that we’ve explored are sobering, but they also serve as valuable lessons. By learning from these real incidents, companies can implement safeguards to avoid becoming the next headline. Remember that trust in the workplace is important, but when it comes to security, trust must be verified. With the right mindset and measures in place, businesses can significantly lower the risk that their own people will become the source of the next big breach.
Studies show that 88% to 95% of cybersecurity incidents involve human error, ranging from accidental mistakes to malicious insider actions.
Hackers used social engineering over the phone to trick Twitter employees into revealing credentials, gaining access to admin tools and hijacking high-profile accounts.
Failing to revoke access after termination, as seen in the FDIC and Cash App cases, can allow former employees to access and misuse sensitive data.
Negligence. Equifax failed to apply a critical software patch, allowing hackers to exploit the vulnerability and steal data on 147 million people.
Organizations should implement employee training, enforce the principle of least privilege, monitor data access, and follow strict off-boarding protocols.