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HighVulnerabilityCVE-2026-76490updated 27 Oct 2026, 14:49 UTC

Cisco Expressway out-of-bounds write flaw, actively exploited

Attackers can send specially crafted data that makes Cisco Expressway devices write memory outside its allowed space, which can crash them or let attackers take control. Cisco confirms this is being exploited.

Exploited (Cisco PSIRT)Fix availableOfficial, 2 sources
SeverityHighCVSS 8.6 of 10
Being exploited?YesConfirmed by Cisco PSIRT · 27 Oct 2026
Fix available?YesCisco software update released in the October 2026 Expressway Series security hardening release (specific fixed version numbers: unknown in provided material). · since 27 Oct 2026
How sure is this?Official, 2 sourcesBacked by an official source

How the attack works simplified

  1. 1Attacker sends crafted inputMalicious data sent to the device
  2. 2Expressway processes itInput is not bounds-checked properly
  3. 3Memory written out of boundsWrites past the allowed buffer
  4. 4Device crashes or is compromisedMemory corruption breaks controls
  5. ResultAttacker may take control of the device

What happened

Out-of-bounds write vulnerabilities (CWE-787) in Cisco Expressway Series software (Expressway-C and Expressway-E), found in an internal review and fixed in a security hardening release; Cisco has grouped several related issues under this CVE.

Why it is dangerous: Successful exploitation can compromise Expressway devices, which handle video and voice traffic and sit at the network edge (Expressway-E), exposing internal communications and infrastructure to attackers.

Are you affected?

You are affected if you run Cisco Expressway Core (Expressway-C) or Cisco Expressway Edge (Expressway-E) devices on software versions before the October 2026 hardening release.

Cisco Expressway-C and Expressway-E devices running software versions prior to the October 2026 security hardening release (exact version list: unknown, see Cisco advisory cisco-sa-exwy-hardening-FhcqBVcs).

What to do

  1. Identify all Expressway-C and Expressway-E devices in your environment and their current software versions.
  2. Install the Cisco Expressway October 2026 security hardening release or later on every affected device.
  3. Check the Cisco advisory cisco-sa-exwy-hardening-FhcqBVcs for the exact fixed version for your device.
  4. Review device logs for unusual activity or crashes, as no workaround exists.

Timeline

  1. 21 Oct 2026, 17:19 UTC
    Publicly disclosed
  2. 21 Oct 2026, 17:19 UTC
    Reported by NVD · NVD
  3. 27 Oct 2026, 14:49 UTC
    Cisco PSIRT reported on it · Cisco PSIRT
  4. 27 Oct 2026, 14:49 UTC
    Confirmed by Cisco PSIRT

Sources 2 reports, oldest first

Official vendor, government CERT or advisory database

Technical details

ID
CVE-2026-76490
CVSS
8.6 / 10
EPSS
0.4% chance of exploitation within 30 days
Disclosed
21 Oct 2026, 17:19 UTC
On Zero Radar
before the radar started
Last change
27 Oct 2026, 14:49 UTC

The plain-language parts and the attack diagram were written by an AI model (anthropic/claude-haiku-5.5) from the sources above. Facts like versions, exploitation and dates come from the sources themselves. How Zero Radar works

Cisco Expressway out-of-bounds write flaw, actively exploited · Zero Radar