diff --git a/srtcore/core.cpp b/srtcore/core.cpp
index 3d31ba331..8da10938d 100644
--- a/srtcore/core.cpp
+++ b/srtcore/core.cpp
@@ -8690,6 +8690,23 @@ void srt::CUDT::updateSndLossListOnACK(int32_t ackdata_seqno)
 
 void srt::CUDT::processCtrlAck(const CPacket &ctrlpkt, const steady_clock::time_point& currtime)
 {
+    // Valid ACK payloads are either LITE (just an ack seqno) or at least SMALL
+    // (RCVLASTACK + RTT + RTTVAR + BUFFERLEFT = 16 B). Anything else would OOB-read.
+    const size_t pktlen = ctrlpkt.getLength();
+    if (pktlen < (size_t)SEND_LITE_ACK)
+    {
+        LOGC(inlog.Warn, log << CONID() << "ACK: payload " << pktlen
+                             << " bytes is shorter than LITE (" << SEND_LITE_ACK << ") - rejecting");
+        return;
+    }
+    if (pktlen != (size_t)SEND_LITE_ACK && pktlen < ACKD_TOTAL_SIZE_SMALL * ACKD_FIELD_SIZE)
+    {
+        LOGC(inlog.Warn, log << CONID() << "ACK: non-lite payload " << pktlen
+                             << " bytes is shorter than SMALL (" << (ACKD_TOTAL_SIZE_SMALL * ACKD_FIELD_SIZE)
+                             << ") - rejecting");
+        return;
+    }
+
     const int32_t* ackdata       = (const int32_t*)ctrlpkt.m_pcData;
     const int32_t  ackdata_seqno = ackdata[ACKD_RCVLASTACK];
 
diff --git a/test/test_fec_rebuilding.cpp b/test/test_fec_rebuilding.cpp
index 3910c959b..96281d9c5 100644
--- a/test/test_fec_rebuilding.cpp
+++ b/test/test_fec_rebuilding.cpp
@@ -119,6 +119,11 @@ namespace srt {
         {
             return core->checkApplyFilterConfig(s);
         }
+
+        // Exposed for unit tests that need to drive private CUDT entry points.
+        void processCtrlAck(const CPacket& pkt, const sync::steady_clock::time_point& t) { core->processCtrlAck(pkt, t); }
+        int flowWindowSize() const { return core->m_iFlowWindowSize; }
+        void setFlowWindowSize(int v) { core->m_iFlowWindowSize = v; }
     };
 }
 
@@ -946,3 +951,43 @@ TEST_F(TestFECRebuilding, Rebuild)
 
     EXPECT_EQ(memcmp(skipped.data(), rebuilt.data(), rebuilt.size()), 0);
 }
+
+// processCtrlAck has two OOB-read sites for intermediate payload sizes:
+//  - ackdata[ACKD_RCVLASTACK] (index 0) is read up front, OOB for 0-3 byte payloads;
+//  - ackdata[ACKD_BUFFERLEFT] (index 3) is read in the slow path, OOB for 5-15 byte
+//    payloads (the lite-ACK fast path matches exactly 4 bytes).
+// Valid payloads are LITE (4 B) or SMALL+ (>=16 B). The guard at the top of the
+// handler rejects everything else.
+TEST(TestCUDT, AckRejectsIntermediatePayload)
+{
+    srt::TestInit srtinit;
+
+    CUDTSocket* s1 = NULL;
+    SRTSOCKET sid1 = CUDT::uglobal().newSocket(&s1);
+
+    TestMockCUDT m1;
+    m1.core = &s1->core();
+
+    const int sentinel = 0x5A5A5A5A;
+    m1.setFlowWindowSize(sentinel);
+
+    CPacket pkt;
+    pkt.allocate(1500);
+
+    // Fill the payload with bytes that would be plausible ack-seqnos if interpreted
+    // as int32 (non-negative), so the ackdata_seqno < 0 early return doesn't mask
+    // the bug for the 0-3 byte cases.
+    std::memset(pkt.m_pcData, 0x01, 1500);
+
+    const size_t bad_lens[] = { 0, 1, 3, 5, 8, 12, 15 };
+    const sync::steady_clock::time_point now = sync::steady_clock::now();
+    for (size_t i = 0; i < sizeof(bad_lens) / sizeof(bad_lens[0]); ++i)
+    {
+        pkt.setLength(bad_lens[i]);
+        m1.processCtrlAck(pkt, now);
+        EXPECT_EQ(m1.flowWindowSize(), sentinel)
+            << "ACK with payload " << bad_lens[i] << " bytes must not corrupt m_iFlowWindowSize";
+    }
+
+    srt_close(sid1);
+}