security: sanitize env vars from agent Bash subprocesses (#171)

Use a PreToolUse SDK hook to prepend `unset ANTHROPIC_API_KEY
CLAUDE_CODE_OAUTH_TOKEN` to every Bash command Kit runs, preventing
secret leakage via env/printenv/echo/$PROC. Secrets are now passed
via stdin JSON instead of mounted env files, closing all known
exfiltration vectors.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

container/agent-runner/src/index.ts

@@ -16,7 +16,7 @@
 
 import fs from 'fs';
 import path from 'path';
-import { query, HookCallback, PreCompactHookInput } from '@anthropic-ai/claude-agent-sdk';
+import { query, HookCallback, PreCompactHookInput, PreToolUseHookInput } from '@anthropic-ai/claude-agent-sdk';
 import { fileURLToPath } from 'url';
 
 interface ContainerInput {
@@ -26,6 +26,7 @@ interface ContainerInput {
   chatJid: string;
   isMain: boolean;
   isScheduledTask?: boolean;
+  secrets?: Record<string, string>;
 }
 
 interface ContainerOutput {
@@ -183,6 +184,30 @@ function createPreCompactHook(): HookCallback {
   };
 }
 
+// Secrets to strip from Bash tool subprocess environments.
+// These are needed by claude-code for API auth but should never
+// be visible to commands Kit runs.
+const SECRET_ENV_VARS = ['ANTHROPIC_API_KEY', 'CLAUDE_CODE_OAUTH_TOKEN'];
+
+function createSanitizeBashHook(): HookCallback {
+  return async (input, _toolUseId, _context) => {
+    const preInput = input as PreToolUseHookInput;
+    const command = (preInput.tool_input as { command?: string })?.command;
+    if (!command) return {};
+
+    const unsetPrefix = `unset ${SECRET_ENV_VARS.join(' ')} 2>/dev/null; `;
+    return {
+      hookSpecificOutput: {
+        hookEventName: 'PreToolUse',
+        updatedInput: {
+          ...(preInput.tool_input as Record<string, unknown>),
+          command: unsetPrefix + command,
+        },
+      },
+    };
+  };
+}
+
 function sanitizeFilename(summary: string): string {
   return summary
     .toLowerCase()
@@ -422,7 +447,8 @@ async function runQuery(
         },
       },
       hooks: {
-        PreCompact: [{ hooks: [createPreCompactHook()] }]
+        PreCompact: [{ hooks: [createPreCompactHook()] }],
+        PreToolUse: [{ matcher: 'Bash', hooks: [createSanitizeBashHook()] }],
       },
     }
   })) {
@@ -477,6 +503,12 @@ async function main(): Promise<void> {
     process.exit(1);
   }
 
+  // Set secrets as env vars for the SDK (needed for API auth).
+  // The PreToolUse hook strips these from every Bash subprocess.
+  for (const [key, value] of Object.entries(containerInput.secrets || {})) {
+    process.env[key] = value;
+  }
+
   const __dirname = path.dirname(fileURLToPath(import.meta.url));
   const mcpServerPath = path.join(__dirname, 'ipc-mcp-stdio.js');