CVE-2026-39861: Claude Code: Sandbox Escape via Symlink Following Allows Arbitrary File Write Outside Workspace
Claude Code is an agentic coding tool. Prior to version 2.1.64, Claude Code's sandbox did not prevent sandboxed processes from creating symlinks pointing to locations outside the workspace. When Claude Code subsequently wrote to a path within such a symlink, its unsandboxed process followed the symlink and wrote to the target location outside the workspace without prompting the user for confirmation. This allowed a sandbox escape where neither the sandboxed command nor the unsandboxed app could independently write outside the workspace, but their combination could write to arbitrary locations, potentially leading to code execution outside the sandbox. Reliably exploiting this required the ability to add untrusted content into a Claude Code context window to trigger sandboxed code execution via prompt injection. Users on standard Claude Code auto-update have received this fix automatically. Users performing manual updates are advised to update to version 2.1.64 or later.
Other sources
Claude Code's sandbox did not prevent sandboxed processes from creating symlinks pointing to locations outside the workspace. When Claude Code subsequently wrote to a path within such a symlink, its unsandboxed process followed the symlink and wrote to the target location outside the workspace without prompting the user for confirmation. This allowed a sandbox escape where neither the sandboxed command nor the unsandboxed app could independently write outside the workspace, but their combination could write to arbitrary locations, potentially leading to code execution outside the sandbox. Reliably exploiting this required the ability to add untrusted content into a Claude Code context window to trigger sandboxed code execution via prompt injection.
Users on standard Claude Code auto-update have received this fix automatically. Users performing manual updates are advised to update to the latest version.
Claude Code thanks hackerone.com/philts for reporting this issue.
— GitHub
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2026-39861?
CVE-2026-39861 is classified as high severity due to its potential for arbitrary file writing outside the designated workspace.
How do I fix CVE-2026-39861?
To fix CVE-2026-39861, upgrade to Claude Code version 2.1.64 or later where the symlink following vulnerability has been addressed.
What impact does CVE-2026-39861 have?
CVE-2026-39861 allows an attacker to escape the sandbox and write files to arbitrary locations, compromising the integrity of the system.
Who is affected by CVE-2026-39861?
CVE-2026-39861 affects users of Claude Code versions prior to 2.1.64.
What is the nature of the attack vector for CVE-2026-39861?
The attack vector for CVE-2026-39861 exploits symlink following in the sandbox environment, enabling unauthorized file writes.