diff --git a/api/users/:uuid/inbox/index.ts b/api/users/:uuid/inbox/index.ts
index 6c2b520a..5fba0c54 100644
--- a/api/users/:uuid/inbox/index.ts
+++ b/api/users/:uuid/inbox/index.ts
@@ -25,7 +25,11 @@ export const schemas = {
     header: z.object({
         "x-signature": z.string().optional(),
         "x-nonce": z.string().optional(),
-        "x-signed-by": z.string().url().or(z.string().startsWith("instance ")),
+        "x-signed-by": z
+            .string()
+            .url()
+            .or(z.string().startsWith("instance "))
+            .optional(),
         authorization: z.string().optional(),
     }),
     body: z.any(),
@@ -111,6 +115,32 @@ export default apiRoute((app) =>
         const logger = getLogger(["federation", "inbox"]);
         const body: Entity = await context.req.valid("json");
 
+        if (authorization) {
+            const processor = new InboxProcessor(
+                context,
+                body,
+                null,
+                {
+                    signature,
+                    nonce,
+                    authorization,
+                },
+                logger,
+            );
+
+            return await processor.process();
+        }
+
+        // If not potentially from bridge, check for required headers
+        if (!(signature && nonce && signedBy)) {
+            return context.json(
+                {
+                    error: "Missing required headers: x-signature, x-nonce, or x-signed-by",
+                },
+                400,
+            );
+        }
+
         const sender = await User.resolve(signedBy);
 
         if (!(sender || signedBy.startsWith("instance "))) {
diff --git a/classes/inbox/processor.ts b/classes/inbox/processor.ts
index 1d9f6578..f33203b4 100644
--- a/classes/inbox/processor.ts
+++ b/classes/inbox/processor.ts
@@ -70,7 +70,7 @@ export class InboxProcessor {
      *
      * @param context Hono request context.
      * @param body Entity JSON body.
-     * @param senderInstance Sender of the request's instance (from X-Signed-By header).
+     * @param senderInstance Sender of the request's instance (from X-Signed-By header). Null if request is from a bridge.
      * @param headers Various request headers.
      * @param logger LogTape logger instance.
      * @param requestIp Request IP address. Grabs it from the Hono context if not provided.
@@ -78,7 +78,7 @@ export class InboxProcessor {
     public constructor(
         private context: Context,
         private body: Entity,
-        private senderInstance: Instance,
+        private senderInstance: Instance | null,
         private headers: {
             signature?: string;
             nonce?: string;
@@ -94,9 +94,9 @@ export class InboxProcessor {
      * @returns {Promise<boolean>} - Whether the signature is valid.
      */
     private async isSignatureValid(): Promise<boolean> {
-        if (!this.senderInstance.data.publicKey?.key) {
+        if (!this.senderInstance?.data.publicKey?.key) {
             throw new Error(
-                `Instance ${this.senderInstance.data.baseUrl} has no public key stored in database`,
+                `Instance ${this.senderInstance?.data.baseUrl} has no public key stored in database`,
             );
         }
 
@@ -196,7 +196,10 @@ export class InboxProcessor {
     public async process(): Promise<
         (Response & TypedResponse<{ error: string }, 500, "json">) | Response
     > {
-        if (isDefederated(this.senderInstance.data.baseUrl)) {
+        if (
+            this.senderInstance &&
+            isDefederated(this.senderInstance.data.baseUrl)
+        ) {
             // Return 201 to avoid
             // 1. Leaking defederated instance information
             // 2. Preventing the sender from thinking the message was not delivered and retrying