Cisco shares fell on Thursday, October 15, after the company disclosed that hackers were actively exploiting a critical vulnerability in its Secure Email Gateway. The flaw, identified as CVE-2026-76519, allows an unauthenticated remote attacker to gain complete control of an affected device.
The vulnerability has a severity score of 9.8 out of 10. Cisco’s security team learned of the exploitation in October 2026 and issued a security advisory on October 14.
It is the second critical, actively exploited flaw in the same product line in a month. The U.S. Cybersecurity and Infrastructure Security Agency (CISA) added the vulnerability to its Known Exploited Vulnerabilities catalog.
A broader technology-sector sell-off also weighed on Cisco shares, but the security news was the primary company-specific catalyst. Cisco has released software updates to address the flaw and is contacting affected customers.