Opera discloses MCP Hub flaw in Opera Neon letting a malicious extension control the browser

Opera published a security advisory on November 12, 2026, disclosing CVE-2026-123139, a medium-severity flaw in the MCP Hub extension that ships with its Opera Neon browser. The details are laid out in Opera's security advisory.

What changed

• CVE-2026-123139 carries a CVSS score of 5.8, rated Medium severity.
• The flaw is classified as CWE-940, improper verification of source of a communication channel.
• It affects the Opera Neon MCP Hub extension version 0.0.1 running on Opera Neon 1.0.0.
• Both macOS and Windows installations are affected.
• A malicious browser extension can send unauthorized commands through MCP Hub to Opera Neon's built in Browser Operator extension, which does not verify that commands come from a trusted source.
• The CVE was reserved on November 5, 2026 and published on November 12, 2026 by Opera Norway AS, the CVE Numbering Authority.

What it means for people building with AI

The flaw requires local access through another installed browser extension rather than remote exploitation, so an attacker first needs a malicious extension present in the same browser. This is notable as the first disclosed MCP-specific vulnerability in a mainstream consumer AI browser's extension and tool-hub architecture, relevant as AI browsers increasingly wire MCP into extension APIs that carry real system and browser-control privileges.

Sources

• Improper verification of source of a communication channel - Opera security advisories (Opera, November 12, 2026, official)

Written with AI assistance, strictly from the sources listed above. Before it went live, automated checks verified its dates, sources and every language edition.