diff --git a/src/internal.c b/src/internal.c
index 9759ca5f1..738518268 100644
--- a/src/internal.c
+++ b/src/internal.c
@@ -5566,9 +5566,9 @@ static int SendPacketFlush(WOLFSSH* ssh)
 }
 
 
-/* returns WS_SUCCESS on success. Transport failures record their code in
- * ssh->error, so a later write to that field on the same pass has to be
- * conditional on this having succeeded, or it hides the dead transport. */
+/* returns WS_SUCCESS on success. Flush failures record their code in
+ * ssh->error, so a later write there on the same pass must wait for this to
+ * succeed. HighwaterCheck() writes nothing there, but its callback may. */
 int wolfSSH_SendPacket(WOLFSSH* ssh)
 {
     int ret;
@@ -13894,11 +13894,16 @@ static int DoPacket(WOLFSSH* ssh, byte* bufferConsumed)
     if (padSz < MIN_PAD_LENGTH) {
         WLOG(WS_LOG_DEBUG, "Packet padding length %u below minimum %u",
              (word32)padSz, (word32)MIN_PAD_LENGTH);
+        /* Retries fail on this unconsumed packet, so no rekey may start. */
+        ssh->highwaterFlag = 1;
+        ssh->msgHighwaterFlag = 1;
         return WS_BUFFER_E;
     }
 
     /* check for underflow */
     if ((word32)(PAD_LENGTH_SZ + padSz + MSG_ID_SZ) > pktSz) {
+        ssh->highwaterFlag = 1;
+        ssh->msgHighwaterFlag = 1;
         return WS_OVERFLOW_E;
     }
 
@@ -14631,6 +14636,10 @@ int DoReceive(WOLFSSH* ssh)
                 if (ret != WS_SUCCESS) {
                     WLOG(WS_LOG_DEBUG, "PR: First decrypt fail");
                     ssh->error = ret;
+                    /* The input cannot be resynced, so no highwater rekey
+                     * may start. */
+                    ssh->highwaterFlag = 1;
+                    ssh->msgHighwaterFlag = 1;
                     return WS_FATAL_ERROR;
                 }
             }
@@ -14648,6 +14657,8 @@ int DoReceive(WOLFSSH* ssh)
                 WLOG(WS_LOG_DEBUG, "Packet length overflow: size = %u",
                         ssh->curSz);
                 ssh->error = WS_OVERFLOW_E;
+                ssh->highwaterFlag = 1;
+                ssh->msgHighwaterFlag = 1;
                 return WS_FATAL_ERROR;
             }
 
@@ -14663,6 +14674,8 @@ int DoReceive(WOLFSSH* ssh)
                         "block = %u, aead = %u",
                         alignSz, (word32)alignBlockSz, (word32)aeadMode);
                 ssh->error = WS_BUFFER_E;
+                ssh->highwaterFlag = 1;
+                ssh->msgHighwaterFlag = 1;
                 return WS_FATAL_ERROR;
             }
             ssh->processReplyState = PROCESS_PACKET_FINISH;
@@ -14702,11 +14715,15 @@ int DoReceive(WOLFSSH* ssh)
                     if (ret != WS_SUCCESS) {
                         WLOG(WS_LOG_DEBUG, "PR: Decrypt fail");
                         ssh->error = ret;
+                        ssh->highwaterFlag = 1;
+                        ssh->msgHighwaterFlag = 1;
                         return WS_FATAL_ERROR;
                     }
                     if (verifyResult != WS_SUCCESS) {
                         WLOG(WS_LOG_DEBUG, "PR: VerifyMac fail");
                         ssh->error = verifyResult;
+                        ssh->highwaterFlag = 1;
+                        ssh->msgHighwaterFlag = 1;
                         return WS_FATAL_ERROR;
                     }
                 }
@@ -14726,6 +14743,8 @@ int DoReceive(WOLFSSH* ssh)
                     if (ret != WS_SUCCESS) {
                         WLOG(WS_LOG_DEBUG, "PR: DecryptAead fail");
                         ssh->error = ret;
+                        ssh->highwaterFlag = 1;
+                        ssh->msgHighwaterFlag = 1;
                         return WS_FATAL_ERROR;
                     }
 #endif
diff --git a/tests/unit.c b/tests/unit.c
index 8dfa18669..50045dd91 100644
--- a/tests/unit.c
+++ b/tests/unit.c
@@ -1707,6 +1707,8 @@ static int test_DoReceive_VerifyMacFailure(void)
         ssh->curSz = 0;
         ssh->processReplyState = PROCESS_INIT;
         ssh->error = 0;
+        ssh->highwaterFlag = 0;
+        ssh->msgHighwaterFlag = 0;
 
         flatSeq[0] = (byte)(ssh->peerSeq >> 24);
         flatSeq[1] = (byte)(ssh->peerSeq >> 16);
@@ -1757,6 +1759,10 @@ static int test_DoReceive_VerifyMacFailure(void)
             result = -207;
             goto done;
         }
+        if (!ssh->highwaterFlag || !ssh->msgHighwaterFlag) {
+            result = -209;
+            goto done;
+        }
     }
 
 done:
@@ -1862,6 +1868,10 @@ static int test_DoReceive_AeadTagFailure(void)
         result = -229;
         goto done;
     }
+    if (!ssh->highwaterFlag || !ssh->msgHighwaterFlag) {
+        result = -230;
+        goto done;
+    }
 
 done:
     if (aesInited)
@@ -2106,6 +2116,10 @@ static int test_DoReceive_RejectsShortPadding(void)
         result = -764;
         goto done2;
     }
+    if (!ssh->highwaterFlag || !ssh->msgHighwaterFlag) {
+        result = -765;
+        goto done2;
+    }
 
 done2:
     wolfSSH_free(ssh);
@@ -2114,6 +2128,58 @@ static int test_DoReceive_RejectsShortPadding(void)
 }
 
 
+/* Verify DoReceive rejects a cleartext packet whose padding_length leaves no
+ * room for the message id, returning WS_OVERFLOW_E, and stops the highwater
+ * callback. Layout: packet_length=4, padding_length=4 => 8 bytes total. */
+static int test_DoReceive_RejectsPaddingUnderflow(void)
+{
+    WOLFSSH_CTX* ctx = NULL;
+    WOLFSSH* ssh = NULL;
+    int ret;
+    int result = 0;
+    byte pkt[8];
+    word32 totalLen = (word32)sizeof(pkt);
+
+    ctx = wolfSSH_CTX_new(WOLFSSH_ENDPOINT_CLIENT, NULL);
+    if (ctx == NULL)
+        return -766;
+    ssh = wolfSSH_new(ctx);
+    if (ssh == NULL) {
+        wolfSSH_CTX_free(ctx);
+        return -767;
+    }
+
+    WMEMSET(pkt, 0, sizeof(pkt));
+    pkt[3] = 4;             /* packet_length */
+    pkt[4] = MIN_PAD_LENGTH;
+
+    ShrinkBuffer(&ssh->inputBuffer, 1);
+    ret = GrowBuffer(&ssh->inputBuffer, totalLen);
+    if (ret != WS_SUCCESS) {
+        result = -768;
+        goto done;
+    }
+    WMEMCPY(ssh->inputBuffer.buffer, pkt, totalLen);
+    ssh->inputBuffer.length = totalLen;
+    ssh->inputBuffer.idx = 0;
+
+    ret = wolfSSH_TestDoReceive(ssh);
+    if (ret != WS_FATAL_ERROR || ssh->error != WS_OVERFLOW_E) {
+        result = -769;
+        goto done;
+    }
+    if (!ssh->highwaterFlag || !ssh->msgHighwaterFlag) {
+        result = -793;
+        goto done;
+    }
+
+done:
+    wolfSSH_free(ssh);
+    wolfSSH_CTX_free(ctx);
+    return result;
+}
+
+
 /* Verify DoReceive rejects a cleartext binary packet whose length field is
  * not block aligned. The packet is valid in every other respect, so the
  * RFC 4253 section 6 alignment check is the only thing that can reject it. */
@@ -2173,6 +2239,10 @@ static int test_DoReceive_RejectsMisalignedPacket(void)
         result = -774;
         goto done3;
     }
+    if (!ssh->highwaterFlag || !ssh->msgHighwaterFlag) {
+        result = -794;
+        goto done3;
+    }
 
 done3:
     wolfSSH_free(ssh);
@@ -6576,6 +6646,117 @@ static int test_WorkerReportsExtDataChannelKeying(void)
     return result;
 }
 
+#if defined(WOLFSSH_TEST_INTERNAL) && !defined(WOLFSSH_NO_HMAC_SHA2_256)
+/* An inbound packet that fails its checks starts no highwater rekey, neither
+ * from the worker's flush on that pass nor from a later send. */
+static int test_HighwaterQuietAfterBadPacket(void)
+{
+    WOLFSSH_CTX* ctx = NULL;
+    WOLFSSH*     ssh = NULL;
+    HwTestCtx    hc;
+    Hmac         hmac;
+    int          result = 0;
+    int          ret;
+    word32       prefixLen;
+    byte         seq[LENGTH_SZ];
+    byte         macKey[WC_SHA256_DIGEST_SIZE];
+    byte         digest[WC_SHA256_DIGEST_SIZE];
+    byte         pkt[UINT32_SZ + 12 + WC_SHA256_DIGEST_SIZE];
+
+    WMEMSET(&hc, 0, sizeof(hc));
+    s_extSendCount = 0;
+
+    ctx = wolfSSH_CTX_new(WOLFSSH_ENDPOINT_SERVER, NULL);
+    if (ctx == NULL)
+        return -1950;
+    wolfSSH_SetIOSend(ctx, CountIoSend);
+    wolfSSH_SetIORecv(ctx, PacketIoRecv);
+    wolfSSH_SetHighwaterCb(ctx, 1, HwTestCb);
+
+    ssh = wolfSSH_new(ctx);
+    if (ssh == NULL) { result = -1951; goto done; }
+    hc.ssh = ssh;
+    wolfSSH_SetHighwaterCtx(ssh, &hc);
+    wolfSSH_SetMsgHighwater(ssh, 1);
+    ssh->txCount = 1;
+    ssh->txMsgCount = 1;
+
+    /* Control: the flush after an idle receive fires the callback. */
+    ssh->outputBuffer.length = 1;
+    ssh->outputBuffer.idx = 0;
+    ssh->outputBuffer.buffer[0] = 0;
+    ret = wolfSSH_worker(ssh, NULL);
+    if (ret != WS_FATAL_ERROR || wolfSSH_get_error(ssh) != WS_WANT_READ) {
+        result = -1952;
+        goto done;
+    }
+    if (hc.count != 1) { result = -1953; goto done; }
+
+    /* A new key epoch clears both flags. */
+    ssh->highwaterFlag = 0;
+    ssh->msgHighwaterFlag = 0;
+    hc.count = 0;
+
+    /* An IGNORE packet under HMAC-SHA2-256 at peerSeq 0, one MAC bit off. */
+    prefixLen = BuildMacTestPacketPrefix(MSGID_IGNORE, 10, pkt, sizeof(pkt));
+    if (prefixLen == 0) { result = -1954; goto done; }
+    WMEMSET(macKey, 0xA5, sizeof(macKey));
+    WMEMSET(seq, 0, sizeof(seq));
+    ret = wc_HmacInit(&hmac, ssh->ctx->heap, INVALID_DEVID);
+    if (ret == 0) {
+        ret = wc_HmacSetKey(&hmac, WC_SHA256, macKey, sizeof(macKey));
+        if (ret == 0)
+            ret = wc_HmacUpdate(&hmac, seq, sizeof(seq));
+        if (ret == 0)
+            ret = wc_HmacUpdate(&hmac, pkt, prefixLen);
+        if (ret == 0)
+            ret = wc_HmacFinal(&hmac, digest);
+        wc_HmacFree(&hmac);
+    }
+    if (ret != 0) { result = -1955; goto done; }
+    WMEMCPY(pkt + prefixLen, digest, sizeof(digest));
+    pkt[prefixLen] ^= 0x01;
+
+    ssh->peerMacId = ID_HMAC_SHA2_256;
+    ssh->peerMacSz = WC_SHA256_DIGEST_SIZE;
+    WMEMCPY(ssh->peerKeys.macKey, macKey, sizeof(macKey));
+    ssh->peerKeys.macKeySz = sizeof(macKey);
+    s_recvPkt = pkt;
+    s_recvPktSz = prefixLen + WC_SHA256_DIGEST_SIZE;
+    s_recvPktOff = 0;
+
+    /* The worker still flushes what was queued, but fires nothing. */
+    ssh->outputBuffer.length = 1;
+    ssh->outputBuffer.idx = 0;
+    ssh->outputBuffer.buffer[0] = 0;
+    s_extSendCount = 0;
+    ret = wolfSSH_worker(ssh, NULL);
+    if (ret != WS_FATAL_ERROR || wolfSSH_get_error(ssh) != WS_VERIFY_MAC_E) {
+        result = -1957;
+        goto done;
+    }
+    if (wolfSSH_OutputPending(ssh) || s_extSendCount == 0) {
+        result = -1958;
+        goto done;
+    }
+    if (hc.count != 0) { result = -1959; goto done; }
+
+    /* Nor does the next send, as a teardown would make. */
+    ret = wolfSSH_SendIgnore(ssh, NULL, 0);
+    if (ret != WS_SUCCESS) { result = -1960; goto done; }
+    if (hc.count != 0) { result = -1961; goto done; }
+
+done:
+    s_recvPkt = NULL;
+    s_recvPktSz = 0;
+    s_recvPktOff = 0;
+    s_extSendCount = 0;
+    wolfSSH_free(ssh);
+    wolfSSH_CTX_free(ctx);
+    return result;
+}
+#endif /* WOLFSSH_TEST_INTERNAL && !WOLFSSH_NO_HMAC_SHA2_256 */
+
 /* channelId=0, type=1 (stderr), dataSz=10, payload all 0x44. */
 static const byte s_workerExtBlob[] = {
     0x00, 0x00, 0x00, 0x00,
@@ -6781,6 +6962,10 @@ static int test_WorkerHardRecvErrorOutranksFlush(void)
     ret = wolfSSH_worker(ssh, NULL);
     if (ret != WS_FATAL_ERROR) { result = -1735; goto done; }
     if (wolfSSH_get_error(ssh) != WS_OVERFLOW_E) { result = -1736; goto done; }
+    if (!ssh->highwaterFlag || !ssh->msgHighwaterFlag) {
+        result = -1784;
+        goto done;
+    }
 
 done:
     s_recvPkt = NULL;
@@ -22776,6 +22961,11 @@ int wolfSSH_UnitTest(int argc, char** argv)
     printf("DoReceiveRejectsShortPadding: %s\n",
             (unitResult == 0 ? "SUCCESS" : "FAILED"));
     testResult = testResult || unitResult;
+
+    unitResult = test_DoReceive_RejectsPaddingUnderflow();
+    printf("DoReceiveRejectsPaddingUnderflow: %s\n",
+            (unitResult == 0 ? "SUCCESS" : "FAILED"));
+    testResult = testResult || unitResult;
 #endif
 
 #ifdef WOLFSSH_TEST_INTERNAL
@@ -23004,6 +23194,13 @@ int wolfSSH_UnitTest(int argc, char** argv)
            (unitResult == 0 ? "SUCCESS" : "FAILED"));
     testResult = testResult || unitResult;
 
+#if defined(WOLFSSH_TEST_INTERNAL) && !defined(WOLFSSH_NO_HMAC_SHA2_256)
+    unitResult = test_HighwaterQuietAfterBadPacket();
+    printf("HighwaterQuietAfterBadPacket: %s\n",
+           (unitResult == 0 ? "SUCCESS" : "FAILED"));
+    testResult = testResult || unitResult;
+#endif
+
     unitResult = test_WorkerFlushesOnIdleReceive();
     printf("WorkerFlushesOnIdleReceive: %s\n",
            (unitResult == 0 ? "SUCCESS" : "FAILED"));
diff --git a/wolfssh/internal.h b/wolfssh/internal.h
index 7044912cd..ba4a8cf9f 100644
--- a/wolfssh/internal.h
+++ b/wolfssh/internal.h
@@ -1192,8 +1192,10 @@ struct WOLFSSH {
     word32 txFlushCount;   /* Output buffer drained, whatever came after */
     word32 highwaterMark;
     word32 msgHighwaterMark; /* Per-key packet limit (RFC 4344 Sec 3.1) */
-    byte highwaterFlag;    /* Set when highwater CB called */
-    byte msgHighwaterFlag; /* Set when msg-count highwater CB called */
+    byte highwaterFlag;    /* Set when highwater CB called, or when an
+                            * inbound packet fails a framing or integrity
+                            * check */
+    byte msgHighwaterFlag; /* Same, for the msg-count highwater CB */
     void* highwaterCtx;    /* Highwater CB context */
     void* globalReqCtx;    /* Global Request CB context */
     void* reqSuccessCtx;   /* Global Request Success CB context */