Fix processing of response body when gzip compression is enabled#107
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…_t buffers. The documentation [http://nginx.org/en/docs/dev/development_guide.html#buffer] clearly states that .pos, .last must be used to reference actual data contained by the buffer. Whereas .start, .end denote the boundaries of the memory block allocated for the buffer (in case of dynamically allocated data) or just NULL (when .pos, .last reference a static memory location - one can see that kind of usage in ngx_http_gzip_filter_module.c:ngx_http_gzip_filter_gzheader()). To back up my words I invite to examine ngx_http_charset_filter_module.c:ngx_http_charset_recode() as an example of iteration over data contained in data buffer. Without this fix ngx_http_modsecurity_body_filter feeds random bytes from memory pointed by .start, .end range to msc_append_response_body. In my case is was 8KB of data instead of 10 bytes when referenced by (.pos, .last). That is this vulnerability may disclose sensitive data like passwords or whatever from nginx heap. The fix for ngx_http_modsecurity_pre_access_handler is to use .pos not .start to reference data as they may differ in general case.
… can be called by Nginx several times during request processing. And each time with it own unique set of chained buf pointers. For example, suppose a complete response consists of this chain of data: A->B->C->D->E Ngix may (and actually does, as verified by me in gdb) call body filter two times like this: handler(r, in = A->B->C) handler(r, in = D->E), E has last_buf set Current implementation delays feeding chain->buf to msc_append_response_body until it comes upon a chain with buf->last_buf set. So we loose chain containing A->B->C sequence. We must process body bufs as soon as we see them in body handler otherwise we will not see them again. N.B. You have PR owasp-modsecurity#84 pending. It goes further and fixes the problem when a blocking decision is made after headers were sent. I intentionally retained current (buggy) behavior to make my patch less intrusive and easier to review. Besides owasp-modsecurity#84 impose an excessive memory usage due to a complete copy of all bufs passed through body filter (we have sometimes 500K and more replies in our applications) - I will elaborate on this in code review for owasp-modsecurity#84.
…of modules to come after ngx_http_gzip_filter_module so that we could read a response body before it gets compressed. Prior to this fix ngx_http_modsecurity_body_filter was called after gzip filter so that msc_append_response_body was fed with compressed body bytes thus effectively making all further response body processing meaningless.
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zimmerle commented May 15, 2018
Merged! Thanks! |
pracj3am pushed a commit to cdn77/ModSecurity-nginx that referenced this pull request Nov 4, 2022
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Changed placement of ngx_http_modsecurity_module in nginx load order of modules
to come after ngx_http_gzip_filter_module so that we could read a response body
before it gets compressed.
Prior to this fix ngx_http_modsecurity_body_filter was called after gzip filter
so that msc_append_response_body was fed with compressed body bytes thus
effectively making all further response body processing meaningless.