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Message-ID: <4E57697B.4010609@gmail.com> Date: Fri, 26 Aug 2011 12:38:03 +0300 From: Anestis Bechtsoudis <bechtsoudis.a@...il.com> To: full-disclosure@...ts.grok.org.uk Subject: Re: Advisory: Range header DoS vulnerability Apache HTTPD 1.3/2.x (CVE-2011-3192) On 08/24/2011 07:55 PM, Dirk-Willem van Gulik wrote: > Apache HTTPD Security ADVISORY > ============================== > UPDATE 1 > > Title: Range header DoS vulnerability Apache HTTPD 1.3/2.x > > CVE: CVE-2011-3192 > Last Change: 20110824 1800Z > Date: 20110824 1600Z > Product: Apache HTTPD Web Server > Versions: Apache 1.3 all versions, Apache 2 all versions > > Description: > ============ > > A denial of service vulnerability has been found in the way the multiple > overlapping ranges are handled by the Apache HTTPD server: > > http://seclists.org/fulldisclosure/2011/Aug/175 > > An attack tool is circulating in the wild. Active use of this tools has > been observed. > > The attack can be done remotely and with a modest number of requests can > cause very significant memory and CPU usage on the server. > > The default Apache HTTPD installation is vulnerable. > > There is currently no patch/new version of Apache HTTPD which fixes this > vulnerability. This advisory will be updated when a long term fix > is available. > > A full fix is expected in the next 48 hours. > > Mitigation: > ============ > > There are several immediate options to mitigate this issue until a full fix > is available: > > 1) Use SetEnvIf or mod_rewrite to detect a large number of ranges and then > either ignore the Range: header or reject the request. > > Option 1: (Apache 2.0 and 2.2) > > # Drop the Range header when more than 5 ranges. > # CVE-2011-3192 > SetEnvIf Range (,.*?){5,} bad-range=1 > RequestHeader unset Range env=bad-range > > # optional logging. > CustomLog logs/range-CVE-2011-3192.log common env=bad-range > > Option 2: (Also for Apache 1.3) > > # Reject request when more than 5 ranges in the Range: header. > # CVE-2011-3192 > # > RewriteEngine on > RewriteCond %{HTTP:range} !(^bytes=[^,]+(,[^,]+){0,4}$|^$) > RewriteRule .* - [F] > > The number 5 is arbitrary. Several 10's should not be an issue and may be > required for sites which for example serve PDFs to very high end eReaders > or use things such complex http based video streaming. > > 2) Limit the size of the request field to a few hundred bytes. Note that while > this keeps the offending Range header short - it may break other headers; > such as sizeable cookies or security fields. > > LimitRequestFieldSize 200 > > Note that as the attack evolves in the field you are likely to have > to further limit this and/or impose other LimitRequestFields limits. > > See: http://httpd.apache.org/docs/2.2/mod/core.html#limitrequestfieldsize > > 3) Use mod_headers to completely dis-allow the use of Range headers: > > RequestHeader unset Range > > Note that this may break certain clients - such as those used for > e-Readers and progressive/http-streaming video. > > 4) Deploy a Range header count module as a temporary stopgap measure: > > http://people.apache.org/~dirkx/mod_rangecnt.c > > Precompiled binaries for some platforms are available at: > > http://people.apache.org/~dirkx/BINARIES.txt > > 5) Apply any of the current patches under discussion - such as: > > http://mail-archives.apache.org/mod_mbox/httpd-dev/201108.mbox/%3cCAAPSnn2PO-d-C4nQt_TES2RRWiZr7urefhTKPWBC1b+K1Dqc7g@mail.gmail.com%3e > > OS and Vendor specific information > ================================== > > Red Hat: Option 1 cannot be used on Red Hat Enterprise Linux 4. > https://bugzilla.redhat.com/show_bug.cgi?id=732928 > > NetWare: Pre compiled binaries available. > > Actions: > ======== > > Apache HTTPD users who are concerned about a DoS attack against their server > should consider implementing any of the above mitigations immediately. > > When using a third party attack tool to verify vulnerability - know that most > of the versions in the wild currently check for the presence of mod_deflate; > and will (mis)report that your server is not vulnerable if this module is not > present. This vulnerability is not dependent on presence or absence of > that module. > > Planning: > ========= > > This advisory will be updated when new information, a patch or a new release > is available. A patch or new apache release for Apache 2.0 and 2.2 is expected > in the next 48 hours. Note that, while popular, Apache 1.3 is deprecated. > > > _______________________________________________ > Full-Disclosure - We believe in it. > Charter: http://lists.grok.org.uk/full-disclosure-charter.html > Hosted and sponsored by Secunia - http://secunia.com/ Everyone must be also aware of the "Request-Range" except the "Range" field in the header. >>From the byterange source (http://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x/modules/http/byterange_filter.c) if (!(range = apr_table_get(r->headers_in, "Range"))) { range = apr_table_get(r->headers_in, "Request-Range"); } Advisories must take into account this case too. Credits to Gappy. -- =============================================== * Anestis Bechtsoudis * * Undergraduate Student * * * * Network Operation Center (NOC Group) * * Dept. of Computer Engineering & Informatics * * University of Patras, Greece * * * * Email: bechtsoudis.a@...il.com * =============================================== _______________________________________________ Full-Disclosure - We believe in it. Charter: http://lists.grok.org.uk/full-disclosure-charter.html Hosted and sponsored by Secunia - http://secunia.com/
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