diff --git a/src/internal.c b/src/internal.c
index b49708b4c..7de51bb02 100644
--- a/src/internal.c
+++ b/src/internal.c
@@ -15420,6 +15420,17 @@ int ValidateProtoId(const char* protoIdStr, word32 len)
         }
     }
 
+    /* softwareversion runs to the first space and must not contain a '-'
+     * (RFC 4253 section 4.2); the comments may. */
+    for (i = SSH_PROTO_SZ;
+            i < len - SSH_PROTO_EOL_SZ && protoIdStr[i] != ' '; i++) {
+        if (protoIdStr[i] == '-') {
+            WLOG(WS_LOG_ERROR, "Proto Id was invalid: softwareversion "
+                    "must not contain '-'");
+            return WS_BAD_ARGUMENT;
+        }
+    }
+
     return WS_SUCCESS;
 }
 
diff --git a/tests/unit.c b/tests/unit.c
index e94996edf..9bb2c02e4 100644
--- a/tests/unit.c
+++ b/tests/unit.c
@@ -663,6 +663,9 @@ static int test_DoProtoId(void)
              * 0x7e must both be accepted. */
             { "body w/ SP comments",   "SSH-2.0-app comment\r\n",     1 },
             { "body w/ tilde",         "SSH-2.0-app~1\r\n",           1 },
+            /* RFC 4253 section 4.2 bars '-' only from softwareversion. */
+            { "comments w/ minus",     "SSH-2.0-app my-comment\r\n",  1 },
+            { "comments are minus",    "SSH-2.0-app -\r\n",           1 },
             /* Failing Tests */
             { "wrong version prefix",  "SSH-2-this_is_my_app\r\n",    0 },
             { "bad casing prefix",     "sSH-2.0-this_is_my_app\r\n",  0 },
@@ -675,7 +678,10 @@ static int test_DoProtoId(void)
             { "Body End in CR",        "SSH-2.0-s\r\r\n",             0 },
             { "Body End in TAB",       "SSH-2.0-s\t\r\n",             0 },
             { "body starts w/ space",  "SSH-2.0-\x20-a-b\r\n",        0 },
-            { "body has embedded TAB", "SSH-2.0-\x7e-a\t\r\n",        0 },
+            { "body has embedded TAB", "SSH-2.0-\x7e" "a\t\r\n",      0 },
+            { "swversion w/ minus",    "SSH-2.0-my-app\r\n",          0 },
+            { "swversion is minus",    "SSH-2.0--\r\n",               0 },
+            { "minus before comments", "SSH-2.0-app- comment\r\n",    0 },
             { "too long id",           tooLongProtoId,                0 },
             { "null pointer",          NULL,                          0 },
         };
diff --git a/wolfssh/ssh.h b/wolfssh/ssh.h
index a91b43734..54a82c0bd 100644
--- a/wolfssh/ssh.h
+++ b/wolfssh/ssh.h
@@ -821,6 +821,8 @@ WOLFSSH_API int wolfSSH_CTX_SetBanner(WOLFSSH_CTX* ctx, const char* newBanner);
  *     body as softwareversion [SP comments], so a leading space would
  *     make softwareversion empty. A space later in the body is accepted
  *     and starts the optional comments field.
+ * MUST NOT carry a '-' in softwareversion, the body up to the first
+ *     space (RFC 4253 section 4.2). The comments may carry one.
  * If these are not adhered to the function will return WS_BAD_ARGUMENT
  * and not load the ProtoId into the WOLFSSH_CTX struct.
  * ProtoIdStr is stored by reference and is not copied, so it must remain