diff --git a/ironic/console/securityproxy/rfb.py b/ironic/console/securityproxy/rfb.py
index 566e1e682b..d6ea908691 100644
--- a/ironic/console/securityproxy/rfb.py
+++ b/ironic/console/securityproxy/rfb.py
@@ -134,7 +134,14 @@ def recv(sock, num):
             # Decode the reason why the request failed
             reason_len_raw = recv(host_sock, 4)
             reason_len = struct.unpack('!I', reason_len_raw)[0]
-            reason = recv(host_sock, reason_len)
+            if reason_len <= 256:
+                reason = recv(host_sock, reason_len)
+            else:
+                # NOTE(danms): If the reason is too long, we just assume a
+                # generic failure instead of reading up to 2^32 bytes to avoid
+                # a potential exhaustion attack.
+                reason = b'Unable to negotiate security with server'
+                reason_len_raw = struct.pack('!I', len(reason))
 
             tenant_sock.sendall(auth.AUTH_STATUS_FAIL
                                 + reason_len_raw + reason)
diff --git a/ironic/tests/unit/console/securityproxy/test_rfb.py b/ironic/tests/unit/console/securityproxy/test_rfb.py
index 5b6bd39754..828cf6fa4b 100644
--- a/ironic/tests/unit/console/securityproxy/test_rfb.py
+++ b/ironic/tests/unit/console/securityproxy/test_rfb.py
@@ -174,6 +174,30 @@ def test_fails_on_sec_type_cnt_zero(self):
 
         self._assert_expected_calls()
 
+    def test_fails_on_sec_type_cnt_zero_oversized_reason(self):
+        """Validate behavior if a server returns an oversized reason string.
+
+        If the server sends a reason length greater than 256 bytes, we should
+        not attempt to read the full payload (which could be up to 2^32 bytes)
+        and instead substitute a generic failure message.
+        """
+        import struct
+        self.proxy._fail = mock.Mock()
+        self._version_handshake()
+        self._expect_host_recv(1, "\x00")
+        # Send a reason length of 1000, which exceeds the 256-byte limit
+        self._expect_host_recv(4, struct.pack('!I', 1000))
+        generic_reason = b'Unable to negotiate security with server'
+        generic_reason_len = struct.pack('!I', len(generic_reason))
+        self._expect_tenant_send(
+            auth.AUTH_STATUS_FAIL + generic_reason_len + generic_reason)
+        ex = self.assertRaises(exception.SecurityProxyNegotiationFailed,
+                               self.proxy.connect,
+                               self.tenant_sock,
+                               self.host_sock)
+        self.assertIn('Unable to negotiate security', str(ex))
+        self._assert_expected_calls()
+
     @mock.patch.object(authnone.RFBAuthSchemeNone, "security_handshake",
                        autospec=True)
     def test_full_run(self, mock_handshake):
diff --git a/releasenotes/notes/rfb-reason-text-resource-exhaustion-5fc9485d9b0d6698.yaml b/releasenotes/notes/rfb-reason-text-resource-exhaustion-5fc9485d9b0d6698.yaml
new file mode 100644
index 0000000000..3b9189ca6d
--- /dev/null
+++ b/releasenotes/notes/rfb-reason-text-resource-exhaustion-5fc9485d9b0d6698.yaml
@@ -0,0 +1,19 @@
+---
+security:
+  - |
+    Fixes a vulnerability where security proxy could be crashed if attacker
+    controls host VNC server. The exploit requires that VNC server first
+    informs the client that there is 0 security types available and then sends
+    a reason with reason length for this. There is no checks for reason length,
+    so the attacker can make the security proxy to reserve up to 4 GB of memory
+    and causing it to crash. The security proxy is shared between all tenants,
+    so crashing the proxy denies console access for all tenants. More
+    information in the bug description
+    `bug 2155052 <https://bugs.launchpad.net/ironic/+bug/2155052>`_.
+fixes:
+  - |
+    Fixes a vulnerability where security proxy could be crashed if the VNC
+    server sends unreasonable large reason length. The security proxy is shared
+    between all tenants, so crashing the proxy denies console access for all
+    tenants. More information in the bug description
+    `bug 2155052 <https://bugs.launchpad.net/ironic/+bug/2155052>`_.