Source:
<code>public FormValidation doCheckName_(@Nonnull String value, @Nonnull AccessControlled subject, @Nonnull Permission permission) throws IOException, ServletException { final String v = value.substring(1,value.length()-1); String ev = Functions.escape(v); if(!subject.hasPermission(permission)) return FormValidation.ok(ev); SecurityRealm sr = Jenkins.getInstance().getSecurityRealm(); if(v.equals("authenticated")) return FormValidation.respond(Kind.OK, makeImg("user.png", ev, "Group", false)); try { try { sr.loadUserByUsername(v); <START> User u = User.get(v); <END> if (ev.equals(u.getFullName())) { return FormValidation.respond(Kind.OK, makeImg("person.png", ev, "User", false)); } return FormValidation.respond(Kind.OK, makeImg("person.png", u.getFullName(), "User " + ev, false)); } catch (UserMayOrMayNotExistException e) { return FormValidation.respond(Kind.OK, ev); } catch (UsernameNotFoundException e) { } catch (DataAccessException e) { } catch (AuthenticationException e) { return FormValidation.error(e,"Failed to test the validity of the user name "+v); } try { sr.loadGroupByGroupname(v); return FormValidation.respond(Kind.OK, makeImg("user.png", ev, "Group", false)); } catch (UserMayOrMayNotExistException e) { return FormValidation.respond(Kind.OK, ev); } catch (UsernameNotFoundException e) { } catch (DataAccessException e) { } catch (AuthenticationException e) { return FormValidation.error(e,"Failed to test the validity of the group name "+v); } return FormValidation.respond(Kind.ERROR, makeImg("user-disabled.png", formatNonexistentUser(ev), "User or group not found", true)); } catch (Exception e) { return FormValidation.error(e,ev); } }</code><technical_language>This call Jenkins create a user if a bogus security realm load/report properly. is a big deal, I recommend get user creation if (u == null || ev.equals(u.getFullName())) in next clause.</technical_language>

Target:
public FormValidation doCheckName_(@Nonnull String value, @Nonnull AccessControlled subject, @Nonnull Permission permission) throws IOException, ServletException { final String v = value.substring(1,value.length()-1); String ev = Functions.escape(v); if(!subject.hasPermission(permission)) return FormValidation.ok(ev); SecurityRealm sr = Jenkins.getInstance().getSecurityRealm(); if(v.equals("authenticated")) return FormValidation.respond(Kind.OK, makeImg("user.png", ev, "Group", false)); try { try { sr.loadUserByUsername(v); User u = User.get(v); if (ev.equals(u.getFullName())) { return FormValidation.respond(Kind.OK, makeImg("person.png", ev, "User", false)); } return FormValidation.respond(Kind.OK, makeImg("person.png", Util.escape(StringUtils.abbreviate(u.getFullName(), 50)), "User " + ev, false)); } catch (UserMayOrMayNotExistException e) { return FormValidation.respond(Kind.OK, ev); } catch (UsernameNotFoundException e) { } catch (DataAccessException e) { } catch (AuthenticationException e) { return FormValidation.error(e,"Failed to test the validity of the user name "+v); } try { sr.loadGroupByGroupname(v); return FormValidation.respond(Kind.OK, makeImg("user.png", ev, "Group", false)); } catch (UserMayOrMayNotExistException e) { return FormValidation.respond(Kind.OK, ev); } catch (UsernameNotFoundException e) { } catch (DataAccessException e) { } catch (AuthenticationException e) { return FormValidation.error(e,"Failed to test the validity of the group name "+v); } return FormValidation.respond(Kind.ERROR, makeImg("user-disabled.png", formatNonexistentUser(ev), "User or group not found", true)); } catch (Exception e) { return FormValidation.error(e,ev); } }