diff --git a/.changeset/large-cats-reply.md b/.changeset/large-cats-reply.md
new file mode 100644
index 0000000000..0e5b3696e7
--- /dev/null
+++ b/.changeset/large-cats-reply.md
@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-auth-backend': patch
+---
+
+Limit the size of fetched client ID metadata documents to prevent oversized responses from being accepted.
diff --git a/plugins/auth-backend/src/service/CimdClient.test.ts b/plugins/auth-backend/src/service/CimdClient.test.ts
index 03d453e5e9..8c21ec3167 100644
--- a/plugins/auth-backend/src/service/CimdClient.test.ts
+++ b/plugins/auth-backend/src/service/CimdClient.test.ts
@@ -328,6 +328,29 @@ describe('CimdClient', () => {
         ).rejects.toThrow('Invalid client metadata document');
       });
 
+      it('should throw for oversized JSON without content-length', async () => {
+        const oversizedMetadata = {
+          client_id: 'https://example.com/oauth-metadata.json',
+          client_name: 'x'.repeat(64 * 1024),
+          redirect_uris: ['http://localhost:8080/callback'],
+        };
+        const fetchMock = jest.spyOn(global, 'fetch').mockResolvedValue(
+          new Response(JSON.stringify(oversizedMetadata), {
+            headers: { 'content-type': 'application/json' },
+          }),
+        );
+
+        try {
+          await expect(
+            fetchCimdMetadata({
+              clientId: 'https://example.com/oauth-metadata.json',
+            }),
+          ).rejects.toThrow('Client metadata document too large');
+        } finally {
+          fetchMock.mockRestore();
+        }
+      });
+
       it('should throw for client_id mismatch', async () => {
         const mismatchedMetadata = {
           client_id: 'https://different.com/metadata',
diff --git a/plugins/auth-backend/src/service/CimdClient.ts b/plugins/auth-backend/src/service/CimdClient.ts
index 7d8567f97d..eee9b9637f 100644
--- a/plugins/auth-backend/src/service/CimdClient.ts
+++ b/plugins/auth-backend/src/service/CimdClient.ts
@@ -187,6 +187,24 @@ function validateMetadata(
   }
 }
 
+async function readCappedResponseBody(response: Response): Promise<string> {
+  if (!response.body) {
+    return '';
+  }
+
+  const chunks: Buffer[] = [];
+  let received = 0;
+  for await (const chunk of response.body) {
+    received += chunk.byteLength;
+    if (received > MAX_RESPONSE_BYTES) {
+      throw new InputError('Client metadata document too large');
+    }
+    chunks.push(Buffer.from(chunk));
+  }
+
+  return Buffer.concat(chunks).toString('utf8');
+}
+
 /**
  * Fetches and validates a CIMD metadata document.
  * @throws InputError if fetching or validation fails
@@ -228,8 +246,12 @@ export async function fetchCimdMetadata(opts: {
 
   let metadata: CimdMetadata;
   try {
-    metadata = await response.json();
-  } catch {
+    const responseBody = await readCappedResponseBody(response);
+    metadata = JSON.parse(responseBody) as CimdMetadata;
+  } catch (error) {
+    if (isError(error) && error.name === 'InputError') {
+      throw error;
+    }
     throw new InputError('Invalid client metadata document');
   }