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https://issues.apache.org/jira/browse/ZOOKEEPER-2693?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15876313#comment-15876313
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Rakesh R commented on ZOOKEEPER-2693:
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bq. There are some 4lw commands which ZooKeeper is using by itself
Good catch, [~arshad.mohammad].

bq. even though we are not doing much work in these commands but still the 
connections will be created for every call.
IIUC, these are two problems -> {{case-1)}} restrict 4lw cmd execution as few 
cmds taking too much time for execution. {{case-2)}} protection against overuse 
because it creates many connections.
 
 I think, {{case-1}} is matching with this jira reported issue and we could 
provide {{whitelist}} config in both branches 3.4 and 3.5+ to solve this 
problem. 

In branch-3.4, we could give a simple fix by exposing {{whitelist}} 
configuration and include {{srvr, isro}} cmds in default list. Since we have 
plans to deprecate 4lws in branch-3.5, we could find alternative ways instead 
of using {{srvr, isro}} cmd internally. Probably, we could raise separate jira 
task and handle this case. 
[~hanm], could you create a PR for branch-3.4, if no objection from anyone 
about {{whitelist}} idea and that would help to unblock 3.4.10 releasing.
 
{{case-2}}, I like [~phunt]'s idea of introducing configuration to limit the 
number of 4lw that would be allowed to run in parallel. For example, allows 
only 1-n number of 4lw cmds to run in parallel. How about raising another jira 
to implement this instead of clubbing with this issue?

> DOS attack on wchp/wchc four letter words (4lw)
> -----------------------------------------------
>
>                 Key: ZOOKEEPER-2693
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2693
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: security, server
>    Affects Versions: 3.4.0, 3.5.1, 3.5.2
>            Reporter: Patrick Hunt
>            Assignee: Michael Han
>            Priority: Blocker
>             Fix For: 3.4.10, 3.5.3
>
>         Attachments: ZOOKEEPER-2693-01.patch
>
>
> The wchp/wchc four letter words can be exploited in a DOS attack on the ZK 
> client port - typically 2181. The following POC attack was recently published 
> on the web:
> https://webcache.googleusercontent.com/search?q=cache:_CNGIz10PRYJ:https://www.exploit-db.com/exploits/41277/+&cd=14&hl=en&ct=clnk&gl=us
> The most straightforward way to block this attack is to not allow access to 
> the client port to non-trusted clients - i.e. firewall the ZooKeeper service 
> and only allow access to trusted applications using it for coordination.



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