Overview
Apache HTTPD server contains a denial-of-service vulnerability in the way multiple overlapping ranges are handled. Both the 'Range' header and the 'Range-Request' header are vulnerable. An attack tool, commonly known as 'Apache Killer', has been released in the wild. The attack tool causes a significant increase in CPU and memory usage on the server.
Description
The Apache HTTPD Security Advisory Update 2 states: "Background and the 2007 report |
Impact
CPU and memory usage will spike causing a denial-of-service condition. |
Solution
Apply an Update |
Workarounds
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)
# 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+K1Dqc7g@mail.gmail.com%3e http://svn.apache.org/viewvc?view=revision&sortby=date&revision=1161534" |
Vendor Information
CVSS Metrics
| Group | Score | Vector |
|---|---|---|
| Base | ||
| Temporal | ||
| Environmental |
References
- http://blog.spiderlabs.com/2011/08/mitigation-of-apache-range-header-dos-attack.html
- http://mail-archives.apache.org/mod_mbox/httpd-announce/201108.mbox/%3C20110824161640.122D387DD@minotaur.apache.org%3E
- http://mail-archives.apache.org/mod_mbox/httpd-announce/201108.mbox/%3C20110826103531.998348F82@minotaur.apache.org%3E
- http://www.apache.org/dist/httpd/CHANGES_2.2.20
- http://www.apache.org/dist/httpd/Announcement2.2.html
Acknowledgements
This vulnerability was publicly disclosed.
This document was written by Jared Allar.
Other Information
| CVE IDs: | CVE-2011-3192 |
| Severity Metric: | 16.01 |
| Date Public: | 2011-08-24 |
| Date First Published: | 2011-08-26 |
| Date Last Updated: | 2011-09-19 12:41 UTC |
| Document Revision: | 22 |