diff --git a/esphome/components/api/proto.cpp b/esphome/components/api/proto.cpp
index eac26997cf..945a192b92 100644
--- a/esphome/components/api/proto.cpp
+++ b/esphome/components/api/proto.cpp
@@ -48,14 +48,16 @@ uint32_t ProtoDecodableMessage::count_repeated_field(const uint8_t *buffer, size
         }
         uint32_t field_length = res->as_uint32();
         ptr += consumed;
-        if (ptr + field_length > end) {
+        // Use subtraction to avoid integer overflow on 32-bit systems
+        if (field_length > static_cast<size_t>(end - ptr)) {
           return count;  // Out of bounds
         }
         ptr += field_length;
         break;
       }
       case WIRE_TYPE_FIXED32: {  // 32-bit - skip 4 bytes
-        if (ptr + 4 > end) {
+        // Use subtraction to avoid integer overflow on 32-bit systems
+        if (static_cast<size_t>(end - ptr) < 4) {
           return count;
         }
         ptr += 4;
@@ -110,7 +112,8 @@ void ProtoDecodableMessage::decode(const uint8_t *buffer, size_t length) {
         }
         uint32_t field_length = res->as_uint32();
         ptr += consumed;
-        if (ptr + field_length > end) {
+        // Use subtraction to avoid integer overflow on 32-bit systems
+        if (field_length > static_cast<size_t>(end - ptr)) {
           ESP_LOGV(TAG, "Out-of-bounds Length Delimited at offset %ld", (long) (ptr - buffer));
           return;
         }
@@ -121,7 +124,8 @@ void ProtoDecodableMessage::decode(const uint8_t *buffer, size_t length) {
         break;
       }
       case WIRE_TYPE_FIXED32: {  // 32-bit
-        if (ptr + 4 > end) {
+        // Use subtraction to avoid integer overflow on 32-bit systems
+        if (static_cast<size_t>(end - ptr) < 4) {
           ESP_LOGV(TAG, "Out-of-bounds Fixed32-bit at offset %ld", (long) (ptr - buffer));
           return;
         }
diff --git a/tests/integration/fixtures/proto_bounds_check_overflow_noise.yaml b/tests/integration/fixtures/proto_bounds_check_overflow_noise.yaml
new file mode 100644
index 0000000000..4c04d74d0d
--- /dev/null
+++ b/tests/integration/fixtures/proto_bounds_check_overflow_noise.yaml
@@ -0,0 +1,11 @@
+esphome:
+  name: proto-overflow-noise
+
+host:
+
+api:
+  encryption:
+    key: "N4Yle5YirwZhPiHHsdZLdOA73ndj/84veVaLhTvxCuU="
+
+logger:
+  level: VERY_VERBOSE
diff --git a/tests/integration/fixtures/proto_bounds_check_overflow_plaintext.yaml b/tests/integration/fixtures/proto_bounds_check_overflow_plaintext.yaml
new file mode 100644
index 0000000000..feb4bb5725
--- /dev/null
+++ b/tests/integration/fixtures/proto_bounds_check_overflow_plaintext.yaml
@@ -0,0 +1,9 @@
+esphome:
+  name: proto-overflow-plaintext
+
+host:
+
+api:
+
+logger:
+  level: VERY_VERBOSE
diff --git a/tests/integration/fixtures/proto_fixed32_bounds_check_plaintext.yaml b/tests/integration/fixtures/proto_fixed32_bounds_check_plaintext.yaml
new file mode 100644
index 0000000000..feb4bb5725
--- /dev/null
+++ b/tests/integration/fixtures/proto_fixed32_bounds_check_plaintext.yaml
@@ -0,0 +1,9 @@
+esphome:
+  name: proto-overflow-plaintext
+
+host:
+
+api:
+
+logger:
+  level: VERY_VERBOSE
diff --git a/tests/integration/test_proto_bounds_check_overflow.py b/tests/integration/test_proto_bounds_check_overflow.py
new file mode 100644
index 0000000000..1160696561
--- /dev/null
+++ b/tests/integration/test_proto_bounds_check_overflow.py
@@ -0,0 +1,278 @@
+"""Integration tests for protobuf bounds check integer overflow fix (GHSA-4h3h-63v6-88qx).
+
+This tests the fix for CVE where an integer overflow in the comparison
+`ptr + field_length > end` could be bypassed by sending a large field_length value,
+causing the device to crash by reading out-of-bounds memory.
+
+The fix changes the comparison to `field_length > static_cast<size_t>(end - ptr)`
+which avoids the overflow by comparing against the remaining buffer size directly.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import contextlib
+import socket
+
+import pytest
+
+from .const import LOCALHOST
+from .types import APIClientConnectedWithDisconnectFactory, RunCompiledFunction
+
+
+def _encode_varint(value: int) -> bytes:
+    """Encode an integer as a protobuf varint."""
+    result = []
+    while value > 127:
+        result.append((value & 0x7F) | 0x80)
+        value >>= 7
+    result.append(value & 0x7F)
+    return bytes(result)
+
+
+def _create_malicious_hello_request(field_length: int) -> bytes:
+    """Create a malicious HelloRequest packet with overflow-inducing field_length.
+
+    The packet structure is:
+    - 0x00: Plaintext protocol indicator
+    - VarInt: Total message size
+    - 0x01: Message type (HelloRequest)
+    - 0x02: Field tag (field_id=0, wire_type=2 LENGTH_DELIMITED)
+    - VarInt: field_length (the malicious value)
+
+    When field_length is large (e.g., 0xe0000000), on 32-bit systems the comparison
+    `ptr + field_length > end` would overflow, bypassing the bounds check.
+    """
+    field_length_varint = _encode_varint(field_length)
+    # Message content: field tag (0x02) + field_length varint
+    message_content = bytes([0x02]) + field_length_varint
+    # Full message: message type (0x01) + content
+    full_message = bytes([0x01]) + message_content
+    # Size varint
+    size_varint = _encode_varint(len(full_message))
+    # Complete packet: indicator (0x00) + size + message
+    return bytes([0x00]) + size_varint + full_message
+
+
+def _send_malicious_packets_raw(host: str, port: int, packets: list[bytes]) -> None:
+    """Send malicious packets using a raw socket connection.
+
+    This bypasses the aioesphomeapi client to send raw malformed data directly
+    to the ESPHome API server.
+    """
+    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+    sock.settimeout(5.0)
+    try:
+        sock.connect((host, port))
+        for packet in packets:
+            sock.sendall(packet)
+    except (TimeoutError, ConnectionResetError, BrokenPipeError):
+        # Expected - server may close connection after malformed packet
+        pass
+    finally:
+        sock.close()
+
+
+@pytest.mark.asyncio
+async def test_proto_bounds_check_overflow_plaintext(
+    yaml_config: str,
+    run_compiled: RunCompiledFunction,
+    api_client_connected_with_disconnect: APIClientConnectedWithDisconnectFactory,
+    unused_tcp_port: int,
+) -> None:
+    """Test that protobuf bounds check overflow doesn't crash the device (plaintext).
+
+    This tests the fix for GHSA-4h3h-63v6-88qx where sending a HelloRequest
+    with a large field_length could cause an integer overflow in the bounds check,
+    leading to out-of-bounds memory access and device crash.
+
+    The attack works by sending a packet where field_length is large enough that
+    `ptr + field_length` wraps around to a smaller value, bypassing the > end check.
+    """
+    process_crashed = False
+    invalid_length_logged = False
+
+    def check_logs(line: str) -> None:
+        nonlocal process_crashed, invalid_length_logged
+        # Check for signs that the process crashed
+        if "Segmentation fault" in line or "core dumped" in line:
+            process_crashed = True
+        # Check if the bounds check caught the malicious packet
+        if "Out-of-bounds Length Delimited" in line:
+            invalid_length_logged = True
+
+    async with run_compiled(yaml_config, line_callback=check_logs):
+        # First verify the API is working normally
+        async with api_client_connected_with_disconnect() as (client, _):
+            device_info = await client.device_info()
+            assert device_info is not None
+            assert device_info.name == "proto-overflow-plaintext"
+
+        # Now send malicious packets using raw socket
+        # Test with multiple field_length values that would cause overflow on 32-bit
+        # These values are chosen to cause ptr + field_length to wrap around
+        overflow_values = [
+            0xE0000000,  # Causes crash on ESP32 and RPi Pico W
+            0xD0000000,  # Crashes ESP32
+            0xF0000000,  # May not crash but reads unrelated memory
+            0xFFFFFFFF,  # Maximum uint32 value
+        ]
+
+        malicious_packets = [
+            _create_malicious_hello_request(val) for val in overflow_values
+        ]
+
+        # Send malicious packets in executor to not block event loop
+        loop = asyncio.get_running_loop()
+        await loop.run_in_executor(
+            None,
+            _send_malicious_packets_raw,
+            LOCALHOST,
+            unused_tcp_port,
+            malicious_packets,
+        )
+
+        # Small delay to let ESPHome process the packets
+        await asyncio.sleep(0.5)
+
+        # After the malicious packets, verify the process didn't crash
+        assert not process_crashed, (
+            "ESPHome process crashed! The bounds check overflow fix is not working."
+        )
+
+        # Most importantly: verify we can reconnect, proving the process is still running
+        async with api_client_connected_with_disconnect() as (client2, _):
+            device_info = await client2.device_info()
+            assert device_info is not None
+            assert device_info.name == "proto-overflow-plaintext"
+
+
+@pytest.mark.asyncio
+async def test_proto_bounds_check_overflow_noise(
+    yaml_config: str,
+    run_compiled: RunCompiledFunction,
+    api_client_connected_with_disconnect: APIClientConnectedWithDisconnectFactory,
+) -> None:
+    """Test that protobuf bounds check overflow doesn't crash the device (noise encryption).
+
+    With noise encryption, the attack requires knowledge of the encryption key.
+    This test verifies that even with a valid encryption session, malicious
+    protobuf content doesn't crash the device.
+    """
+    noise_key = "N4Yle5YirwZhPiHHsdZLdOA73ndj/84veVaLhTvxCuU="
+    process_crashed = False
+
+    def check_logs(line: str) -> None:
+        nonlocal process_crashed
+        if "Segmentation fault" in line or "core dumped" in line:
+            process_crashed = True
+
+    async with run_compiled(yaml_config, line_callback=check_logs):
+        async with api_client_connected_with_disconnect(noise_psk=noise_key) as (
+            client,
+            disconnect_event,
+        ):
+            # Verify basic connection works first
+            device_info = await client.device_info()
+            assert device_info is not None
+            assert device_info.name == "proto-overflow-noise"
+
+            # With noise encryption, we need to send through the frame helper
+            # which will encrypt the data. We'll send a message with a malformed
+            # protobuf body that has a large length-delimited field.
+            frame_helper = client._connection._frame_helper
+
+            # Create a malformed protobuf body with overflow-inducing field length
+            # This is the content after encryption/decryption
+            # Tag 0x02 (field_id=0, wire_type=2) followed by large length
+            malformed_bodies = [
+                bytes([0x02]) + _encode_varint(0xE0000000),  # Overflow value
+                bytes([0x02]) + _encode_varint(0xFFFFFFFF),  # Max uint32
+            ]
+
+            for body in malformed_bodies:
+                # Send as HelloRequest (type 1)
+                try:
+                    frame_helper.write_packets([(1, body)], True)
+                except (ConnectionResetError, BrokenPipeError, OSError):
+                    # Connection may be closed after malformed packet
+                    break
+                await asyncio.sleep(0.1)
+
+            # Wait briefly for any disconnect
+            with contextlib.suppress(TimeoutError):
+                await asyncio.wait_for(disconnect_event.wait(), timeout=1.0)
+
+        # Verify process didn't crash
+        assert not process_crashed, (
+            "ESPHome process crashed! The bounds check overflow fix is not working."
+        )
+
+        # Verify we can reconnect
+        async with api_client_connected_with_disconnect(noise_psk=noise_key) as (
+            client2,
+            _,
+        ):
+            device_info = await client2.device_info()
+            assert device_info is not None
+            assert device_info.name == "proto-overflow-noise"
+
+
+@pytest.mark.asyncio
+async def test_proto_fixed32_bounds_check_plaintext(
+    yaml_config: str,
+    run_compiled: RunCompiledFunction,
+    api_client_connected_with_disconnect: APIClientConnectedWithDisconnectFactory,
+    unused_tcp_port: int,
+) -> None:
+    """Test that fixed32 bounds check works correctly.
+
+    This tests the simpler case where we check if there are 4 bytes remaining.
+    While less likely to overflow, the fix ensures consistent bounds checking.
+    """
+    process_crashed = False
+
+    def check_logs(line: str) -> None:
+        nonlocal process_crashed
+        if "Segmentation fault" in line or "core dumped" in line:
+            process_crashed = True
+
+    async with run_compiled(yaml_config, line_callback=check_logs):
+        # First verify the API is working normally
+        async with api_client_connected_with_disconnect() as (client, _):
+            device_info = await client.device_info()
+            assert device_info is not None
+            assert device_info.name == "proto-overflow-plaintext"
+
+        # Create a packet with a fixed32 field (wire type 5) but truncated data
+        # Tag: field_id=1, wire_type=5 (fixed32) = (1 << 3) | 5 = 0x0D
+        # This should be caught by the bounds check
+        truncated_fixed32 = bytes(
+            [
+                0x00,  # Plaintext indicator
+                0x03,  # Size (3 bytes of message)
+                0x01,  # Message type (HelloRequest)
+                0x0D,  # Field tag (field_id=1, wire_type=5 fixed32)
+                0x42,  # Only 1 byte of data instead of 4
+            ]
+        )
+
+        # Send using raw socket
+        loop = asyncio.get_running_loop()
+        await loop.run_in_executor(
+            None,
+            _send_malicious_packets_raw,
+            LOCALHOST,
+            unused_tcp_port,
+            [truncated_fixed32],
+        )
+
+        await asyncio.sleep(0.5)
+
+        assert not process_crashed, "ESPHome process crashed on truncated fixed32!"
+
+        # Verify we can still reconnect
+        async with api_client_connected_with_disconnect() as (client2, _):
+            device_info = await client2.device_info()
+            assert device_info is not None
+            assert device_info.name == "proto-overflow-plaintext"