kovidgoyal/kitty · critical

Incorrect permissions on SHM file

Error message

Incorrect permissions on SHM file

What it means

Companion check to the owner verification: the /dev/shm bootstrap file must have exactly 0600 permissions. Looser modes would let other local users read the transmitted data (which can include sensitive payloads), so the read is aborted.

Solutions

1. Fix your umask before launching kitty (e.g. umask 022 or 077)
2. Delete and let the kitten recreate the file: rm /dev/shm/<name> and rerun
3. Verify no other tool/user is pre-creating the shm file: ls -l /dev/shm/
4. If it recurs, audit what writes to /dev/shm with wide modes (could be tampering)

Example fix

# before
umask 000
kitty +kitten ssh user@host
# after
umask 077
kitty +kitten ssh user@host

if info, err := os.Stat(shmPath); err == nil && info.Mode().Perm() != 0o600 {
    os.Chmod(shmPath, 0o600) // or remove to force recreation
}

if err != nil && strings.Contains(err.Error(), "Incorrect permissions") {
    os.Remove(shmPath) // recreate with correct mode
}

When it happens

Trigger: read_data_from_shared_memory seeing a shm file whose mode bits are not 0600 — e.g. created under a permissive umask (0002/0666 gone wide), or chmod'ed afterwards, or a foreign file with the same name.

Common situations: Users with umask 000 in shell profiles; containers or CI images with odd umask defaults; multi-user systems; files copied into /dev/shm manually for debugging.

Related errors

• Incorrect owner on SHM file
• Incorrect owner on pwfile: uid=
• Incorrect permissions on pwfile: 0o
• Attempted path traversal in tar file
• Cannot access the mmap of a closed shared memory object

AI-assisted analysis of kovidgoyal/kitty@6d5d0c4406 (2026-08-27). Data as JSON: /api/errors/e015331e53723841. Report an issue: GitHub.

Appendix: source

Thrown at kittens/ssh/main.go:82

	} else if strings.Contains(hostname, "@") && hostname[0] != '@' {
		username, hostname_for_match, _ = strings.Cut(hostname, "@")
		parsed = true
	}
	if !parsed && strings.Contains(hostname, "@") && hostname[0] != '@' {
		_, hostname_for_match, _ = strings.Cut(hostname, "@")
	}
	return
}

func read_data_from_shared_memory(shm_name string) ([]byte, error) {
	data, err := shm.ReadWithSizeAndUnlink(shm_name, func(s fs.FileInfo) error {
		if stat, ok := s.Sys().(syscall.Stat_t); ok {
			if os.Getuid() != int(stat.Uid) || os.Getgid() != int(stat.Gid) {
				return fmt.Errorf("Incorrect owner on SHM file")
			}
		}
		if s.Mode().Perm() != 0o600 {
			return fmt.Errorf("Incorrect permissions on SHM file")
		}
		return nil
	})
	return data, err
}

func add_cloned_env(val string) (ans map[string]string, err error) {
	data, err := read_data_from_shared_memory(val)
	if err != nil {
		return nil, err
	}
	err = json.Unmarshal(data, &ans)
	return ans, err
}

func parse_kitten_args(found_extra_args []string, username, hostname_for_match string) (overrides []string, literal_env map[string]string, ferr error) {
	literal_env = make(map[string]string)
	overrides = make([]string, 0, 4)

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