[Owasp-modsecurity-core-rule-set] [mod-security-users] Advanced Topic of the Week: Mitigating Slow HTTP DoS Attacks

christian.folini at post.ch christian.folini at post.ch
Wed Nov 24 07:31:13 EST 2010

Hi Breno,

Could explain the term "bad connection" a bit? Ryan's blog post implies a client IP is considered bad
when it has too many connections in read state. Your entry in the CHANGES document reads,
"Add SecReadStateLimit to limit the number of BUSY connections".

I can't see why a proxy can't have a lot of legitimate connections
in read state. AFAIK Request Body reading is also considered "read".
So uploads can remain in READ for a certain time - depending on service.

I do not want to pester you too much, but I just want to make sure I
get this correctly - and people are aware that telling good from bad
connections is very tricky. Especially when it comes to request delaying and
you want to make sure you are not locking legitimate users.

Best Regs,


Von: Breno Silva [mailto:breno.silva at gmail.com]
Gesendet: Mittwoch, 24. November 2010 13:20
An: Folini Christian, IT222 extern
Cc: RBarnett at trustwave.com; mod-security-users at lists.sourceforge.net; owasp-modsecurity-core-rule-set at lists.owasp.org
Betreff: Re: [mod-security-users] [Owasp-modsecurity-core-rule-set] Advanced Topic of the Week: Mitigating Slow HTTP DoS Attacks

Hi Christian,

The SecReadStateLimit is not only a threshold for ip address. It is looking for an "anomaly" in connection process. So if you are behind a proxy or a NAT only the bad connections will be dropped. The good ones will pass normally. So legit connections behind the proxy will works fine.


On Wed, Nov 24, 2010 at 1:17 AM, <christian.folini at post.ch<mailto:christian.folini at post.ch>> wrote:
Hi Ryan,

Nice post. Thanks. Especially the combination of mod_reqtimeout and ModS
is very elegant in my eyes.

I am not so happy with SecReadStateLimit looking only at the IP address.
How do protect proxies from your countermeasures? A proxy might share multiple
hundred legitimate connections with your server for multiple hundred legitimate
clients, all appearing to come from the same IP address.



-----Ursprüngliche Nachricht-----
Von: owasp-modsecurity-core-rule-set-bounces at lists.owasp.org<mailto:owasp-modsecurity-core-rule-set-bounces at lists.owasp.org> [mailto:owasp-modsecurity-core-rule-set-bounces at lists.owasp.org<mailto:owasp-modsecurity-core-rule-set-bounces at lists.owasp.org>] Im Auftrag von Ryan Barnett
Gesendet: Mittwoch, 24. November 2010 02:45
An: mod-security-users at lists.sourceforge.net<mailto:mod-security-users at lists.sourceforge.net>; owasp-modsecurity-core-rule-set at lists.owasp.org<mailto:owasp-modsecurity-core-rule-set at lists.owasp.org>
Betreff: [Owasp-modsecurity-core-rule-set] Advanced Topic of the Week: Mitigating Slow HTTP DoS Attacks

This week's blog post -


Ryan Barnett
Senior Security Researcher
Trustwave - SpiderLabs

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