Privilege Handling

Privilege Handling (CWE-271, CWE-426, CWE-377, CWE-61)

What this checks

Privileged programs (SUID/SGID, root daemons, container init) operate with a credentials gap. Classic mistakes: privileges dropped in the wrong order, environment variables trusted from an attacker-controlled shell, temp files where an attacker can symlink-swap before write. Local pattern, cross-trust-boundary consequence. LSMs provide defense in depth; this catches the source-level mistake.

Vulnerable patterns

• Privileged process drops the effective UID only, leaving the saved set-user-ID intact so the process can regain root later.
• Privileged exec resolves the program name via PATH, letting the attacker plant a lookalike earlier in the search path.
• Privileged code inherits LD_PRELOAD, LD_LIBRARY_PATH, or IFS from the calling environment without scrubbing them.
• Process runs with no umask call (or an explicit zero umask), leaving created files world-writable.
• Temp file created via a "generate name then open" pattern (non-atomic), allowing an attacker to symlink the name in between.
• File opened in a world-writable directory without the no-follow flag, letting the attacker pre-create the path as a symlink to a privileged target.
• Multi-call binary that picks behavior based on argv[0] without canonicalizing — the caller chooses the name via execve's second argument.

Fix immediately

Flag the vulnerable code and explain the risk. Then suggest a fix that establishes these properties. Translate each property into the audited file's language and operating-system API — use the platform's documented privilege, environment, and filesystem calls.

1. Privilege drops set all three credential IDs explicitly and check the return. Use the call that sets real, effective, and saved IDs in one operation; verify by re-reading afterwards. Any failure is fatal — abort rather than continue with ambiguous credentials.
2. Privileged execs pass an absolute path and a known environment. Do not inherit PATH, LD_*, or IFS from the caller. Never use exec variants that resolve via PATH in privileged code.
3. Temp files are created with an atomic open-and-create primitive that returns a file descriptor in one call — never "make a name, then open". Atomic creation closes the symlink-swap window.
4. Files opened in world-writable directories pass a no-follow flag so the open fails if the final path component is a symlink. Combine with exclusive-create when the file should not pre-exist.
5. The umask is set explicitly at process start to a restrictive value matching the intended file-mode policy. Do not rely on the inherited umask.

Verification

After rewriting, confirm:

• Every privilege drop sets real, effective, and saved IDs explicitly and checks the return values
• Every exec from privileged code passes an absolute path and a sanitized environment, never the inherited environment
• No path-searching exec variant (the family that resolves via PATH) is used in privileged code paths
• Every temp-file creation uses an atomic open-and-create primitive — never a name-then-open pattern
• Every file open in a world-writable directory passes a no-follow flag, plus exclusive-create when the file should not pre-exist
• The umask is set explicitly at process start to a restrictive value

References

• CWE-271 (Privilege Dropping / Lowering Errors)
• CWE-426 (Untrusted Search Path)
• CWE-377 (Insecure Temporary File)
• CWE-61 (UNIX Symbolic Link Following)