Agent Skills: Filesystem Security

Filesystem Security

File operations have race conditions, symlink attacks, and path traversal risks.

ikigai Application

Path traversal:

• Reject paths containing .. before canonicalization
• Use realpath() and verify result is under allowed directory
• Never concatenate user input directly into paths

TOCTOU (Time-of-Check to Time-of-Use):

// BAD: Race between check and use
if (access(path, R_OK) == 0) { fd = open(path, O_RDONLY); }

// GOOD: Open first, then check
fd = open(path, O_RDONLY);
if (fd >= 0) { /* use fd */ }

Symlink attacks:

• Use O_NOFOLLOW when opening files in shared directories
• lstat() to check for symlinks before operations
• Create temp files with mkstemp(), never mktemp()

Permissions:

• Config files: 0600 (owner read/write only)
• Directories: 0700
• Check permissions before reading sensitive files
• Set umask appropriately: umask(077)

Review red flags: access() before open(), path concatenation, missing O_NOFOLLOW, world-readable configs.