Docker Compose check · critical
Docker starts containers with a reduced set of Linux capabilities. cap_add: ALL gives back all of them, and SYS_ADMIN alone covers a very wide range of administrative operations, including mounting filesystems. Both remove much of the isolation privileged mode would remove, so the explainer marks them critical. Add only the specific capability the service needs.
$ explain docker-compose.yml ! app adds capability SYS_ADMIN line 4 fix: see below $
What the free Docker Compose explainer reports on the risky example below.
Capabilities split root's power into pieces so a container can have only some of them. Docker's default set leaves out the dangerous ones, including SYS_ADMIN, SYS_MODULE and SYS_PTRACE.
SYS_ADMIN is the broadest single capability in Linux. It allows mount operations and many other administrative calls, and it is a frequent ingredient in published container escape techniques. `ALL` includes it and every other capability.
Like privileged mode, these are often added to make one feature work (a FUSE mount, a VPN client) and then never narrowed.
services:
app:
image: example/app:1.4.2
cap_add:
- SYS_ADMIN
restart: unless-stopped
mem_limit: 512m
services:
app:
image: example/app:1.4.2
# Drop everything, then add back the one capability needed
cap_drop:
- ALL
cap_add:
- NET_BIND_SERVICE
restart: unless-stopped
mem_limit: 512m
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1 service defined: app.
Runs the image example/app:1.4.2.
Adds Linux capabilities: SYS_ADMIN.
These capabilities are close to root on the host and are a common step in container escapes.
Safer: Remove it and add back only the narrow capabilities the service actually needs.
Restarts if it stops or after a daemon restart (unless-stopped), but stays down if you stopped it manually.
Limits memory to 512m. The container is killed if it exceeds this.
It covers a very wide set of privileged operations, including mounting filesystems, which is a common step in container escapes. Docker leaves it out of the default set for that reason.
Not exactly. privileged also gives access to all host devices and turns off seccomp and AppArmor. But cap_add: ALL already removes most of the capability-based limits, so both are treated as critical.
Yes. It accepts both SYS_ADMIN and CAP_SYS_ADMIN, in any case, and flags ALL the same way.