diff --git a/server/token.c b/server/token.c
index cba6782..f1fba3b 100644
--- a/server/token.c
+++ b/server/token.c
@@ -161,6 +161,7 @@
     for (i = 0; i < acl->AceCount; i++)
     {
         const SID *sid;
+        size_t sid_size;
 
         if (size < sizeof(ACE_HEADER))
             return FALSE;
@@ -171,21 +172,25 @@
         {
         case ACCESS_DENIED_ACE_TYPE:
             sid = (const SID *)&((const ACCESS_DENIED_ACE *)ace)->SidStart;
+            sid_size = ace->AceSize - FIELD_OFFSET(ACCESS_DENIED_ACE, SidStart);
             break;
         case ACCESS_ALLOWED_ACE_TYPE:
             sid = (const SID *)&((const ACCESS_ALLOWED_ACE *)ace)->SidStart;
+            sid_size = ace->AceSize - FIELD_OFFSET(ACCESS_ALLOWED_ACE, SidStart);
             break;
         case SYSTEM_AUDIT_ACE_TYPE:
             sid = (const SID *)&((const SYSTEM_AUDIT_ACE *)ace)->SidStart;
+            sid_size = ace->AceSize - FIELD_OFFSET(SYSTEM_AUDIT_ACE, SidStart);
             break;
         case SYSTEM_ALARM_ACE_TYPE:
             sid = (const SID *)&((const SYSTEM_ALARM_ACE *)ace)->SidStart;
+            sid_size = ace->AceSize - FIELD_OFFSET(SYSTEM_ALARM_ACE, SidStart);
             break;
         default:
             return FALSE;
         }
-        if (size < sizeof(SID) ||
-            size < FIELD_OFFSET(SID, SubAuthority[sid->SubAuthorityCount]))
+        if (sid_size < FIELD_OFFSET(SID, SubAuthority[0]) ||
+            sid_size < FIELD_OFFSET(SID, SubAuthority[sid->SubAuthorityCount]))
             return FALSE;
         ace = ace_next( ace );
     }
diff --git a/server/trace.c b/server/trace.c
index de386e3..04b2e57 100644
--- a/server/trace.c
+++ b/server/trace.c
@@ -429,9 +429,12 @@
     DWORD i;
 
     /* security check */
-    if ((size < sizeof(SID)) ||
-      (FIELD_OFFSET(SID, SubAuthority[sid->SubAuthorityCount]) > size))
+    if ((FIELD_OFFSET(SID, SubAuthority[0]) > size) ||
+        (FIELD_OFFSET(SID, SubAuthority[sid->SubAuthorityCount]) > size))
+    {
+        fprintf( stderr, "<invalid sid>" );
         return;
+    }
 
     fputc( '{', stderr );
     fprintf( stderr, "S-%u-%lu", sid->Revision, MAKELONG(
@@ -453,12 +456,16 @@
     if (size)
     {
         if (size < sizeof(ACL))
+        {
+            fprintf( stderr, "<invalid acl>}\n" );
             return;
+        }
         size -= sizeof(ACL);
         ace = (const ACE_HEADER *)(acl + 1);
         for (i = 0; i < acl->AceCount; i++)
         {
             const SID *sid = NULL;
+            size_t sid_size = 0;
 
             if (size < sizeof(ACE_HEADER))
                 return;
@@ -471,21 +478,25 @@
             {
             case ACCESS_DENIED_ACE_TYPE:
                 sid = (const SID *)&((const ACCESS_DENIED_ACE *)ace)->SidStart;
+                sid_size = ace->AceSize - FIELD_OFFSET(ACCESS_DENIED_ACE, SidStart);
                 fprintf( stderr, "ACCESS_DENIED_ACE_TYPE,Mask=%lx",
                          ((const ACCESS_DENIED_ACE *)ace)->Mask );
                 break;
             case ACCESS_ALLOWED_ACE_TYPE:
                 sid = (const SID *)&((const ACCESS_ALLOWED_ACE *)ace)->SidStart;
+                sid_size = ace->AceSize - FIELD_OFFSET(ACCESS_ALLOWED_ACE, SidStart);
                 fprintf( stderr, "ACCESS_ALLOWED_ACE_TYPE,Mask=%lx",
                          ((const ACCESS_ALLOWED_ACE *)ace)->Mask );
                 break;
             case SYSTEM_AUDIT_ACE_TYPE:
                 sid = (const SID *)&((const SYSTEM_AUDIT_ACE *)ace)->SidStart;
+                sid_size = ace->AceSize - FIELD_OFFSET(SYSTEM_AUDIT_ACE, SidStart);
                 fprintf( stderr, "SYSTEM_AUDIT_ACE_TYPE,Mask=%lx",
                          ((const SYSTEM_AUDIT_ACE *)ace)->Mask );
                 break;
             case SYSTEM_ALARM_ACE_TYPE:
                 sid = (const SID *)&((const SYSTEM_ALARM_ACE *)ace)->SidStart;
+                sid_size = ace->AceSize - FIELD_OFFSET(SYSTEM_ALARM_ACE, SidStart);
                 fprintf( stderr, "SYSTEM_ALARM_ACE_TYPE,Mask=%lx",
                          ((const SYSTEM_ALARM_ACE *)ace)->Mask );
                 break;
@@ -495,7 +506,7 @@
             }
             fprintf( stderr, ",AceFlags=%x,Sid=", ace->AceFlags );
             if (sid)
-                dump_inline_sid( sid, size );
+                dump_inline_sid( sid, sid_size );
             ace = (const ACE_HEADER *)((const char *)ace + ace->AceSize);
             fputc( '}', stderr );
         }