fix: default to restricted OCI capability set (#786)

Changes the default Linux capability set for container processes from
`.allCapabilities` to `.defaultOCICapabilities`, making the library
secure-by-default. Callers that genuinely need elevated capabilities
must now opt in explicitly.

Signed-off-by: michael_crosby <michael_crosby@apple.com>

Sources/Containerization/LinuxProcessConfiguration.swift

@@ -376,8 +376,12 @@ public struct LinuxProcessConfiguration: Sendable {
     /// process and its children cannot gain additional privileges via setuid/setgid binaries
     /// or file capabilities.
     public var noNewPrivileges: Bool = false
-    /// The Linux capabilities for the container process.
-    public var capabilities: LinuxCapabilities = .allCapabilities
+    /// The Linux capabilities for the container process. Defaults to
+    /// ``LinuxCapabilities/defaultOCICapabilities`` — the restricted baseline used by
+    /// runc/containerd, which excludes privileged capabilities such as `CAP_SYS_ADMIN`.
+    /// Callers that require additional capabilities (for example, privileged containers)
+    /// must opt in explicitly, e.g. by setting ``LinuxCapabilities/allCapabilities``.
+    public var capabilities: LinuxCapabilities = .defaultOCICapabilities
     /// Whether to allocate a pseudo terminal for the process. If you'd like interactive
     /// behavior and are planning to use a terminal for stdin/out/err on the client side,
     /// this should likely be set to true.
@@ -398,7 +402,7 @@ public struct LinuxProcessConfiguration: Sendable {
         user: ContainerizationOCI.User = .init(),
         rlimits: [LinuxRLimit] = [],
         noNewPrivileges: Bool = false,
-        capabilities: LinuxCapabilities = .allCapabilities,
+        capabilities: LinuxCapabilities = .defaultOCICapabilities,
         terminal: Bool = false,
         stdin: ReaderStream? = nil,
         stdout: Writer? = nil,

Sources/cctl/RunCommand.swift

@@ -114,7 +114,6 @@ extension Application {
                 config.process.setTerminalIO(terminal: current)
                 config.process.arguments = arguments
                 config.process.workingDirectory = cwd
-                config.process.capabilities = .allCapabilities
 
                 for mount in self.mounts {
                     let paths = mount.split(separator: ":")

Tests/ContainerizationTests/LinuxContainerTests.swift

@@ -15,6 +15,7 @@
 //===----------------------------------------------------------------------===//
 
 import ContainerizationOCI
+import ContainerizationOS
 import Foundation
 import Testing
 
@@ -66,4 +67,29 @@ struct LinuxContainerTests {
 
         #expect(process.arguments == ["/bin/sh", "-c", "echo 'hello'", "&&", "sleep 10"])
     }
+
+    @Test func defaultCapabilitiesAreRestrictedOCISet() {
+        // Regression guard against shipping `.allCapabilities` as the default.
+        // A default container must not receive CAP_SYS_ADMIN, which would let it
+        // write /proc/sys/kernel/core_pattern and escape to guest-root. Cover both
+        // construction paths: the no-argument init (property default) and the full
+        // memberwise init (parameter default).
+        let viaProperty = LinuxProcessConfiguration()
+        let viaInit = LinuxProcessConfiguration(arguments: ["/bin/sh"])
+
+        for caps in [viaProperty.capabilities, viaInit.capabilities] {
+            for set in [caps.bounding, caps.effective, caps.permitted, caps.inheritable, caps.ambient] {
+                #expect(!set.contains(.sysAdmin), "default capabilities must not include CAP_SYS_ADMIN")
+            }
+        }
+
+        // The default must be exactly the documented OCI baseline.
+        let expected = LinuxCapabilities.defaultOCICapabilities
+        #expect(viaProperty.capabilities.bounding == expected.bounding)
+        #expect(viaProperty.capabilities.effective == expected.effective)
+        #expect(viaProperty.capabilities.permitted == expected.permitted)
+        #expect(viaProperty.capabilities.inheritable == expected.inheritable)
+        #expect(viaProperty.capabilities.ambient == expected.ambient)
+        #expect(viaInit.capabilities.bounding == expected.bounding)
+    }
 }