Security: Predictable temp file names in /tmp vulnerable to symlink attacks

Summary

Claude Code creates temporary files with predictable names in global temporary directories (e.g., /tmp). This is vulnerable to symlink attacks (TOCTOU race conditions), a well-known class of security vulnerabilities.

The Attack

1. Attacker predicts the filename Claude Code will create (e.g., /tmp/claude-export.csv)
2. Attacker pre-creates a symlink: /tmp/claude-export.csv → ~/.ssh/authorized_keys
3. Claude Code writes to what it believes is a temp file
4. The symlink target is overwritten instead, leading to privilege escalation or data corruption

This is especially concerning in multi-user systems or shared environments.

Recommended Mitigations

1. Use unpredictable filenames: Use mktemp or equivalent with random suffixes
2. Use private directories: Prefer $XDG_RUNTIME_DIR (user-private, tmpfs-backed), or fall back to $TMPDIR / ~/tmp
3. Use safe file creation flags: O_EXCL | O_NOFOLLOW to fail if file exists or is a symlink
4. Set restrictive permissions: 0600 for files, 0700 for directories

References

• CWE-377: Insecure Temporary File
• CWE-59: Improper Link Resolution Before File Access ('Link Following')
• OWASP: Insecure Temporary File

Current Workaround

Users can instruct Claude Code to use ~/tmp instead of /tmp via CLAUDE.md, but this relies on user awareness and doesn't address the predictable filename issue.