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Date:   Mon, 13 May 2019 20:06:37 +0800
From:   Weilong Chen <>
To:     Michal Kubecek <>, <>
CC:     <>, <>,
Subject: Re: [PATCH net-next] ipv4: Add support to disable icmp timestamp

On 2019/5/13 19:49, Michal Kubecek wrote:
> On Mon, May 13, 2019 at 07:38:37PM +0800, Weilong Chen wrote:
>> On 2019/5/13 15:49, Michal Kubecek wrote:
>>> On Mon, May 13, 2019 at 09:33:13AM +0800, Weilong Chen wrote:
>>>> The remote host answers to an ICMP timestamp request.
>>>> This allows an attacker to know the time and date on your host.
>>> Why is that a problem? If it is, does it also mean that it is a security
>>> problem to have your time in sync (because then the attacker doesn't
>>> even need ICMP timestamps to know the time and date on your host)?
>> It's a low risk vulnerability(CVE-1999-0524). TCP has
>> net.ipv4.tcp_timestamps = 0 to disable it.
> That does not really answer my question. Even if "CVE" meant much more
> back in 1999 than it does these days, none of the CVE-1999-0524
> descriptions I found cares to explain why it's considered a problem that
> an attacker knows time on your machine. They just claim it is. If we
> assume it is a security problem, then we would have to consider having
> correct time a security problem which is something I certainly don't
> agree with.
> One idea is that there may be applications using current time as a seed
> for random number generator - but then such application is the real
> problem, not having correct time.
Yes, the target computer responded to an ICMP timestamp request. By 
accurately determining the target's clock state, an attacker can more 
effectively attack certain time-based pseudorandom number generators 
(PRNGs) and the authentication systems that rely on them.

So, the 'time' may become sensitive information. The OS should not leak 
it out.

> Michal Kubecek
> .

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