diff --git a/.changeset/giant-bananas-sit.md b/.changeset/giant-bananas-sit.md
new file mode 100644
index 000000000000..f310f9982fc5
--- /dev/null
+++ b/.changeset/giant-bananas-sit.md
@@ -0,0 +1,6 @@
+---
+'astro': patch
+'@astrojs/node': patch
+---
+
+Add a default body size limit for server actions to prevent oversized requests from exhausting memory.
diff --git a/packages/astro/src/actions/runtime/server.ts b/packages/astro/src/actions/runtime/server.ts
index b7c2daeffca2..7cca3aeac17a 100644
--- a/packages/astro/src/actions/runtime/server.ts
+++ b/packages/astro/src/actions/runtime/server.ts
@@ -327,6 +327,9 @@ export function getActionContext(context: APIContext): AstroActionContext {
 				try {
 					input = await parseRequestBody(context.request);
 				} catch (e) {
+					if (e instanceof ActionError) {
+						return { data: undefined, error: e };
+					}
 					if (e instanceof TypeError) {
 						return { data: undefined, error: new ActionError({ code: 'UNSUPPORTED_MEDIA_TYPE' }) };
 					}
@@ -378,16 +381,75 @@ function getCallerInfo(ctx: APIContext) {
 	return undefined;
 }
 
+const DEFAULT_ACTION_BODY_SIZE_LIMIT = 1024 * 1024;
+
 async function parseRequestBody(request: Request) {
 	const contentType = request.headers.get('content-type');
-	const contentLength = request.headers.get('Content-Length');
+	const contentLengthHeader = request.headers.get('content-length');
+	const contentLength = contentLengthHeader ? Number.parseInt(contentLengthHeader, 10) : undefined;
+	const hasContentLength = typeof contentLength === 'number' && Number.isFinite(contentLength);
 
 	if (!contentType) return undefined;
+	if (hasContentLength && contentLength > DEFAULT_ACTION_BODY_SIZE_LIMIT) {
+		throw new ActionError({
+			code: 'CONTENT_TOO_LARGE',
+			message: `Request body exceeds ${DEFAULT_ACTION_BODY_SIZE_LIMIT} bytes`,
+		});
+	}
 	if (hasContentType(contentType, formContentTypes)) {
+		if (!hasContentLength) {
+			const body = await readRequestBodyWithLimit(request.clone(), DEFAULT_ACTION_BODY_SIZE_LIMIT);
+			const formRequest = new Request(request.url, {
+				method: request.method,
+				headers: request.headers,
+				body: toArrayBuffer(body),
+			});
+			return await formRequest.formData();
+		}
 		return await request.clone().formData();
 	}
 	if (hasContentType(contentType, ['application/json'])) {
-		return contentLength === '0' ? undefined : await request.clone().json();
+		if (contentLength === 0) return undefined;
+		if (!hasContentLength) {
+			const body = await readRequestBodyWithLimit(request.clone(), DEFAULT_ACTION_BODY_SIZE_LIMIT);
+			if (body.byteLength === 0) return undefined;
+			return JSON.parse(new TextDecoder().decode(body));
+		}
+		return await request.clone().json();
 	}
 	throw new TypeError('Unsupported content type');
 }
+
+async function readRequestBodyWithLimit(request: Request, limit: number): Promise<Uint8Array> {
+	if (!request.body) return new Uint8Array();
+	const reader = request.body.getReader();
+	const chunks: Uint8Array[] = [];
+	let received = 0;
+	while (true) {
+		const { done, value } = await reader.read();
+		if (done) break;
+		if (value) {
+			received += value.byteLength;
+			if (received > limit) {
+				throw new ActionError({
+					code: 'CONTENT_TOO_LARGE',
+					message: `Request body exceeds ${limit} bytes`,
+				});
+			}
+			chunks.push(value);
+		}
+	}
+	const buffer = new Uint8Array(received);
+	let offset = 0;
+	for (const chunk of chunks) {
+		buffer.set(chunk, offset);
+		offset += chunk.byteLength;
+	}
+	return buffer;
+}
+
+function toArrayBuffer(buffer: Uint8Array): ArrayBuffer {
+	const copy = new Uint8Array(buffer.byteLength);
+	copy.set(buffer);
+	return copy.buffer;
+}
diff --git a/packages/astro/test/actions.test.js b/packages/astro/test/actions.test.js
index 7471ebcd0961..60f95bcaa212 100644
--- a/packages/astro/test/actions.test.js
+++ b/packages/astro/test/actions.test.js
@@ -66,6 +66,26 @@ describe('Astro Actions', () => {
 			assert.equal(data.subscribeButtonState, 'smashed');
 		});
 
+		it('Rejects oversized JSON action body', async () => {
+			const largeActionPayload = JSON.stringify({
+				channel: 'a'.repeat(2 * 1024 * 1024),
+			});
+			const res = await fixture.fetch('/_actions/subscribe', {
+				method: 'POST',
+				body: largeActionPayload,
+				headers: {
+					'Content-Type': 'application/json',
+				},
+			});
+
+			assert.equal(res.ok, false);
+			assert.equal(res.status, 413);
+			assert.equal(res.headers.get('Content-Type'), 'application/json');
+
+			const data = await res.json();
+			assert.equal(data.code, 'CONTENT_TOO_LARGE');
+		});
+
 		it('Exposes comment action', async () => {
 			const formData = new FormData();
 			formData.append('channel', 'bholmesdev');
@@ -179,6 +199,27 @@ describe('Astro Actions', () => {
 			assert.equal(data.subscribeButtonState, 'smashed');
 		});
 
+		it('Rejects oversized JSON action body', async () => {
+			const largeActionPayload = JSON.stringify({
+				channel: 'a'.repeat(2 * 1024 * 1024),
+			});
+			const req = new Request('http://example.com/_actions/subscribe', {
+				method: 'POST',
+				headers: {
+					'Content-Type': 'application/json',
+				},
+				body: largeActionPayload,
+			});
+			const res = await app.render(req);
+
+			assert.equal(res.ok, false);
+			assert.equal(res.status, 413);
+			assert.equal(res.headers.get('Content-Type'), 'application/json');
+
+			const data = await res.json();
+			assert.equal(data.code, 'CONTENT_TOO_LARGE');
+		});
+
 		it('Exposes comment action', async () => {
 			const formData = new FormData();
 			formData.append('channel', 'bholmesdev');