[News](/news) Hackers spy on Mimecast customers [Blog](/blog)[Breaches](/breaches)[Resources](/resources)[News](/news) # Hackers spy on Mimecast customers ##### [Edward Kost](/team/edward-kost) January 19, 2021 Email security firm Mimecast has had its digital certificate compromised giving threat actors access to private customer communications. In an [official statement](https://www.mimecast.com/blog/important-update-from-mimecast/) Mimecast announced that their digital certificate used to protect connections between its products and Microsoft’s cloud services was compromised. Microsoft informed them of the incident. “Approximately 10 percent of our customers use this connection. Of those that do, there are indications that a low single digit number of our customers’ M365 tenants were targeted. We have already contacted these customers to remediate the issue.” Mimecast said in their statement. By compromising a certificate used to encrypt data, threat actors are able to decrypt, read and even modify the data.  This type of cyberattack is exceedingly difficult to execute since it requires private encryption keys stored in highly secure internal servers. Without hacking acuity, such a breach is only possible through insider access.  Impacted Mimecast customers are advised to delete their current Microsoft 365 connection and to reconnect with a replacement certificate. The small subset of impact customers is a reason for concern. If threat actors were sophisticated enough to access highly guarded encryption keys, why was the impact so prosaic?\ \ The chilling possibility is that this was a highly targeted attack intercepting the communications of specific Mimecast customers. An echo, and possible connection, to the surgical hacking methods of the [SolarWinds breach](https://www.upguard.com/news/u-s-government-data-breach). ## How secure is Mimecast? Mimecast provides email security and human risk management solutions that protect organizations from phishing, ransomware, and other cyber threats. The company offers AI-powered threat detection, data loss prevention, compliance archiving, and security awareness training through a unified platform. Its services integrate with collaboration tools to reduce security risks associated with employee behavior and unsanctioned technology use. * View our free preliminary report on Mimecast’s security posture * 13 risk factors, including email security, SSL, DNS health, open ports and common vulnerabilities [View Mimecast's score](/security-report/mimecast) [View score](/security-report/mimecast) [https://www.mimecast.com](/security-report/mimecast) ### Sign up for our newsletter UpGuard's monthly newsletter cuts through the noise and brings you what matters most: our breaking research, in-depth analysis of emerging threats, and actionable strategic insights. ## Latest news Stay up-to-date with the latest news in cybersecurity. [](/news/hugging-face-data-breach-2026-07-20) #### [Hugging Face data breach: key facts and what we know so far](/news/hugging-face-data-breach-2026-07-20) A data breach involving Hugging Face was reported in July 2026. See incident details, impact on customers, and recommended security measures. ##### [UpGuard Team](/team/upguard) July 20, 2026 [](/news/ey-data-breach-2026-07-19) #### [EY data breach: what happened and what's at risk](/news/ey-data-breach-2026-07-19) A data breach involving EY was reported in July 2026. See incident details, impact on customers, and recommended security measures. ##### [UpGuard Team](/team/upguard) July 15, 2026 [](/news/south-florida-injury-convenient-care-data-breach-2026-07-16) #### [South Florida Injury & Convenient Care data breach exposes names and Social Security numbers](/news/south-florida-injury-convenient-care-data-breach-2026-07-16) A data breach involving South Florida Injury & Convenient Care was reported in July 2026. See incident details, impact on customers, and security measures. ##### [UpGuard Team](/team/upguard) July 15, 2026 [](/news/park-west-psychology-data-breach-2026-07-16) #### [Park West Psychology data breach: what happened and what's at risk](/news/park-west-psychology-data-breach-2026-07-16) A data breach involving Park West Psychology was reported in July 2026. See incident details, impact on customers, and recommended security measures. ##### [UpGuard Team](/team/upguard) July 15, 2026 [](/news/fall-river-municipal-credit-union-data-breach-2026-07-16) #### [Fall River Municipal Credit Union data breach: what happened and what's at risk](/news/fall-river-municipal-credit-union-data-breach-2026-07-16) A data breach involving Fall River Municipal Credit Union was reported in July 2026. See incident details, impact on customers, and recommended security measures. ##### [](#) July 14, 2026 [](/news/npcil-data-breach-2026-07-16) #### [NPCIL data breach: World Leaks claims exposure of Kudankulam Nuclear Power Plant files](/news/npcil-data-breach-2026-07-16) A data breach involving NPCIL was reported in July 2026. See incident details, impact on customers, and recommended security measures. ##### [UpGuard Team](/team/upguard) July 14, 2026 [View all news](/news) ## Protect your organization Get in touch or book a free demo. [Contact sales](/demo) [Free demo](/demo) ##### Free instant security score ## How secure is your organization? Request a free cybersecurity report to discover key risks on your website, email, network, and brand. * Instant insights you can act on immediately * Hundreds of risk factors including email security, SSL, DNS health, open ports and common vulnerabilities [Free score](/instant-security-score)