Topic:	Apache (mod_deflate) Denial of Service Vulnerability
Credit:	François Guerraz
Date:	2009.07.12
CWE:	CWE-399 (Show similar)
CVE:	CVE-2009-1891 (Show details) Use CVE to see details like: - CVSS2, - Affected Software, - References

Hi,

we have received a bug report [1] that a DoS is possible with
mod_deflate since it does not stop to compress large files even after
the network connection has been closed. This allows to use large
amounts of CPU if there is a largish (>10 MB) file available that has
mod_deflate enabled.

An exploit is as easy as

for a in $(seq 1 100) ; do
curl -H "Accept-Encoding: gzip" -is http://url.to/large/file.txt |
head -1 ; done

François Guerraz, the bug submitter, also suggested a patch:

--- mod_deflate.c 2008-01-04 15:23:50.000000000 +0100
+++ mod_deflate.c.new 2009-06-26 16:50:36.000000000 +0200
@@ -691,6 +691,10 @@
continue;
}

+ if (r->connection->aborted) {
+ return APR_ECONNABORTED;
+ }
+
/* read */
apr_bucket_read(e, &data, &len, APR_BLOCK_READ);

This greatly reduces the problem. But even with it, quite a bit of
data is compressed (maybe determined by APR_MMAP_LIMIT == 4MB?).

Cheers,
Stefan

[1] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=534712

References: