-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Apache HTTPD Security ADVISORY ============================== UPDATE 2
Title: Range header DoS vulnerability Apache HTTPD 1.3/2.x CVE: CVE-2011-3192 Last Change: 20110826 1030Z Date: 20110824 1600Z Product: Apache HTTPD Web Server Versions: Apache 1.3 all versions, Apache 2 all versions Changes since last update ========================= In addition to the 'Range' header - the 'Range-Request' header is equally affected. Furthermore various vendor updates, improved regexes (speed and accommodating a different and new attack pattern). 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 tool 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 24 hours. Background and the 2007 report ============================== There are two aspects to this vulnerability. One is new, is Apache specific; and resolved with this server side fix. The other issue is fundamentally a protocol design issue dating back to 2007: http://seclists.org/bugtraq/2007/Jan/83 The contemporary interpretation of the HTTP protocol (currently) requires a server to return multiple (overlapping) ranges; in the order requested. This means that one can request a very large range (e.g. from byte 0- to the end) 100's of times in a single request. Being able to do so is an issue for (probably all) webservers and currently subject of an IETF discussion to change the protocol: http://trac.tools.ietf.org/wg/httpbis/trac/ticket/311 This advisory details a problem with how Apache httpd and its so called internal 'bucket brigades' deal with serving such "valid" request. The problem is that currently such requests internally explode into 100's of large fetches, all of which are kept in memory in an inefficient way. This is being addressed in two ways. By making things more efficient. And by weeding out or simplifying requests deemed too unwieldy. Mitigation: =========== There are several immediate options to mitigate this issue until a full fix is available. Below examples handle both the 'Range' and the legacy 'Request-Range' with various levels of care. Note that 'Request-Range' is a legacy name dating back to Netscape Navigator 2-3 and MSIE 3. Depending on your user community - it is likely that you can use option '3' safely for this older 'Request-Range'. 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 2.2) # Drop the Range header when more than 5 ranges. # CVE-2011-3192 SetEnvIf Range (?:,.*?){5,5} bad-range=1 RequestHeader unset Range env=bad-range # We always drop Request-Range; as this is a legacy # dating back to MSIE3 and Netscape 2 and 3. RequestHeader unset Request-Range # optional logging. CustomLog logs/range-CVE-2011-3192.log common env=bad-range CustomLog logs/range-CVE-2011-3192.log common env=bad-req-range Above may not work for all configurations. In particular situations mod_cache and (language) modules may act before the 'unset' is executed upon during the 'fixup' phase. Option 2: (Pre 2.2 and 1.3) # Reject request when more than 5 ranges in the Range: header. # CVE-2011-3192 # RewriteEngine on RewriteCond %{HTTP:range} !(bytes=[^,]+(,[^,]+){0,4}$|^$) # RewriteCond %{HTTP:request-range} !(bytes=[^,]+(?:,[^,]+){0,4}$|^$) RewriteRule .* - [F] # We always drop Request-Range; as this is a legacy # dating back to MSIE3 and Netscape 2 and 3. RequestHeader unset Request-Range 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. Furthermore to ignore the Netscape Navigator 2-3 and MSIE 3 specific legacy header - add: RequestHeader unset Request-Range Unlike the commonly used 'Range' header - dropping the 'Request-Range' is not likely to affect many clients. 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+k1dq...@mail.gmail.com%3e http://svn.apache.org/viewvc?view=revision&sortby=date&revision=1161534 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. mod_security: Has updated their rule set; see http://blog.spiderlabs.com/2011/08/mitigation-of-apache-range-header-dos-attack.html 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 - note 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 24 hours. Note that, while popular, Apache 1.3 is deprecated. - -- end of advisory - update 2 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (Darwin) iEYEARECAAYFAk5Xdu8ACgkQ/W+IxiHQpxvN8ACgwsUJ6oYMq3SyoPHCR7rqsbP6 DFkAoKhZcF87F96h40tQdM1SZsiVX9N5 =07sc -----END PGP SIGNATURE-----