diff --git a/backend/src/config.ts b/backend/src/config.ts
index 616d8349..a52df4b3 100644
--- a/backend/src/config.ts
+++ b/backend/src/config.ts
@@ -41,6 +41,33 @@ function absoluteRegistryPath(value: string, label: string): string {
   return pathValue;
 }
 
+function refSafeGitBranch(value: string): string {
+  const branch = requiredRegistryValue(value, "branch");
+  if (
+    branch === "@"
+    || branch.startsWith("-")
+    || branch.startsWith("/")
+    || branch.endsWith("/")
+    || branch.endsWith(".")
+    || branch.includes("..")
+    || branch.includes("@{")
+    || branch.includes("//")
+    || branch.split("/").some((component) => component.startsWith(".") || component.endsWith(".lock"))
+    || /[\p{Cc} ~^:?*\[\\]/u.test(branch)
+  ) {
+    throw new Error("workspace registry branch configuration is invalid");
+  }
+  return branch;
+}
+
+function safeInstallationId(value: string): string {
+  const installationId = requiredRegistryValue(value, "installation ID");
+  if (/\p{Cc}/u.test(installationId)) {
+    throw new Error("workspace registry installation ID configuration is invalid");
+  }
+  return installationId;
+}
+
 function positiveImportLimit(value: string | undefined, fallback: number): number {
   const limit = Number(value ?? fallback);
   if (!Number.isSafeInteger(limit) || limit <= 0) {
@@ -119,10 +146,9 @@ export function loadConfig(env: Record<string, string | undefined>): AppConfig {
     env.THT_WORKSPACE_REGISTRY_ROOT ?? "/data/workspace-registry",
     "root",
   );
-  const registryBranch = requiredRegistryValue(env.THT_WORKSPACE_GIT_BRANCH ?? "main", "branch");
-  const installationId = requiredRegistryValue(
+  const registryBranch = refSafeGitBranch(env.THT_WORKSPACE_GIT_BRANCH ?? "main");
+  const installationId = safeInstallationId(
     env.THT_WORKSPACE_INSTALLATION_ID ?? "local",
-    "installation ID",
   );
   const remoteUrl = env.THT_WORKSPACE_GIT_REMOTE === undefined
     ? undefined
diff --git a/backend/test/workspaces-config.test.ts b/backend/test/workspaces-config.test.ts
index c624b6e6..10c14853 100644
--- a/backend/test/workspaces-config.test.ts
+++ b/backend/test/workspaces-config.test.ts
@@ -46,3 +46,19 @@ test("rejects unsafe registry branch, installation ID, and secret roots", () =>
   expect(() => loadConfig({ THT_WORKSPACE_SECRET_ROOTS: "/run/secrets,relative" }))
     .toThrow(/secret/i);
 });
+
+test("rejects ref-unsafe Git branches", () => {
+  for (const branch of ["topic..bad", "--upload-pack=/tmp/x", "release/.hidden", "release.lock"]) {
+    expect(() => loadConfig({ THT_WORKSPACE_GIT_BRANCH: branch })).toThrow(/branch/i);
+  }
+});
+
+test("rejects control characters in installation IDs", () => {
+  for (const codePoint of [...Array(0x20).keys(), ...Array(0x21).keys()].map((code, index) => (
+    index < 0x20 ? code : code + 0x7f
+ ))) {
+    expect(() => loadConfig({
+      THT_WORKSPACE_INSTALLATION_ID: `server-psd-1${String.fromCodePoint(codePoint)}`,
+    })).toThrow(/installation/i);
+  }
+});