How Claude Code’s 256KB blind spot put developers’ secrets in the preview pane
Noma says Claude Code's HTML preview could run malicious JavaScript and steal developer secrets; Anthropic patched it.
Noma Security disclosed SneakPeek, a high-severity flaw in Anthropic's Claude Code Desktop preview pane, scored CVSS 8.7 with no CVE published yet. HTML files larger than 256KB were loaded as file:// URLs, skipping the strict content-security policy applied to smaller files, so malicious JavaScript could read local files and exfiltrate AWS credentials, GitHub secrets, and SSH keys. An attacker could plant the file in a public repository and wait for a developer to ask Claude Code to inspect it. Anthropic's first patch was bypassed through a Web Worker; Noma says a deeper fix has fully resolved the issue.
- HTML files over 256KB rendered via file URLs and bypassed the preview CSP.
- Embedded JavaScript could read local secrets, including cloud, GitHub, and SSH keys.
- Anthropic assigned CVSS 8.7; a CVE was requested but not yet public.
- A first patch missed Web Worker reads; a later architectural fix closed it.
- Vendors
- Anthropic
- Products
- Claude CodeClaude Code Desktop
- Organizations
- Noma SecurityAnthropicBlack DuckSysdig
- AI models
- Claude
From the original article
4 min read at csoonline.com
Simply asking Anthropic’s Claude Code to review an HTML file could hand attackers access to a developer’s machine, cloud credentials and software development environment, according to a new Noma Security finding.
csoonline.com
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