An attacker, opening a HTTP/2 connection with an initial window size of 0, was able to block handling of that connection indefinitely in Apache HTTP Server. This could be used to exhaust worker resources in the server, similar to the well known "slow loris" attack pattern. This has been fixed in version 2.4.58, so that such connection are terminated properly after the configured connection timeout. This issue affects Apache HTTP Server: from 2.4.55 through 2.4.57. Users are recommended to upgrade to version 2.4.58, which fixes the issue. A flaw was found in the mod_http2 module of httpd. This flaw allows an attacker opening an HTTP/2 connection with an initial window size of 0 to block handling of that connection indefinitely. This vulnerability can exhaust worker resources in the server, similar to the well-known "slow loris" attack pattern.
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