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Home : Advisories : Possible security issues with Apache in some configurations

Title: Possible security issues with Apache in some configurations
Released by: APACHE
Date: 20th January 1998
Printable version: Click here


Release Date: Tuesday, January 6 1998

Topic: Possible security issues with Apache in some configurations

Summary of Issues


This advisory is to inform all Apache users of several possible

security issues that have been discovered during an internal security

review of the Apache source code.

DO NOT BE ALARMED BY THIS ADVISORY.  This is a pro-active step

designed to be certain that users of Apache are advised of the

issues and can take appropriate action to minimize their risk.

None of these holes allow for a root compromise (they only impact

the user Apache runs as, as set with the "User" directive; if you

have this user set to root, then fix your configuration now because

you probably have a gaping security hole) and they generally

require that a user already have access to the system before they

can exploit them, meaning that on a large number of systems they

are of little practical concern.  Some of the issues that have been

addressed might not be exploitable in real-world conditions.

In some security environments, however, they may be of more concern.

The administrator of the system running Apache is the only one who

can make the judgment call as to how significant the below issues

are in their environment.

Resolution of Problems


We very strongly recommend that anyone using versions of Apache

previous to 1.2 or earlier 1.2 versions upgrade to the newly released

1.2.5.  It is now available at


There are no plans for an immediate 1.3b4 release to correct these

problems in the 1.3 beta development tree, however we will make

patches for 1.3b3 to correct these issues available at


in the near future.

Technical Description of Issues


Below is a step by step technical description of the potential

problems discovered.  Read the below only if you wish to understand

the details of the problems to better judge how they impact your

server and if you have a solid grounding in how Apache works.  If

in doubt, you are advised to simply upgrade to 1.2.5 as soon as


I.   Buffer overflow in cfg_getline()

        RISK: medium

        cfg_getline() is a function that the Apache core and several

        Apache modules use to read certain types of files from disk.

        Some examples of the type of files that read with this are

        htaccess, htpasswd and mod_imap files.

        It is possible to create a sequence of data such that a

        buffer overflow occurs while cfg_getline is reading from

        a file.  If someone has access to create any of these types

        of files on the server, this hole is generally exploitable

        to gain full access to the user Apache runs as.

        On most systems, this is of little consequence since users

        already have such access via methods such as the creation of

        their own CGI scripts.  If, however, the server is secured

        so that the user has no access to the server other than to

        create and modify files (eg. a "ftp only" account with no

        ability to create CGI scripts) this could allow increased

        access to the server.

II.  Several coding errors in mod_include

        RISK: medium

        There are several coding problems in mod_include which can

        result in a buffer overflow or in the child process going

        into an infinite loop.

        The same comments about the nature of the risk apply here as

        do for the cfg_getline() overflow.  Generally, a user already

        needs to have access to the server to exploit this.  Note that

        it is possible to setup a document which deliberately allows this

        to be remotely exploited, however such a document would be very

        rare in practice.

        If you do not allow users to use mod_include, then they

        can not exploit these holes.

III. Inefficient removal of duplicate '/'s ("beck" exploit)

        RISK: medium

        The code in the no2slash() function used to collapse multiple

        '/'s in a request for access checking purposes is very

        inefficient.  It is O(n^2) in the number of '/'s in the

        input.  What this means is that as the input size grows,

        it very quickly requires vastly increased CPU time to

        process the request.  By sending many requests with a large

        number of '/'s in to a server, it is possible to cause a

        large amount of CPU time to be used in processing these

        requests.  Making multiple simultaneous requests of this

        nature could result in a high load average, high CPU usage,

        and possibly starving other processes for CPU resulting in

        a denial of service attack.  This does not allow for any

        compromise of the server.

        The fixed version of the no2slash() function is O(n) and

        does not allow for this attack.

        Thanks to Michal Zalewski  for

        discovering this bug and reporting it on the BUGTRAQ

        mailing list along with the "beck" script that can be

        used to exploit it.

IV.  Possible buffer overflow in "logresolve" program.

        RISK: low

        The logresolve program is used for non-realtime processing of

        logfiles to convert numeric IP addresses into host names.

        In some cases, it may be possible for a remote user who has

        control of a DNS server to return a hostname specifically

        designed to exploit a coding hole in logresolve.

        This can only happen on a system where either the MAXDNAME

        define does not exist and the resolver can return names

        longer than 256 characters or where the MAXDNAME define

        does exist but is less than the maximum length of hostname

        that the resolver can return.  Even on such (arguably

        broken) systems, this would be very difficult to exploit.

        The number of systems which are impacted by this is very


        This problem is a potential concern only if you use the

        logresolve program.

V.   Insufficient data validation in mod_proxy

        RISK: low

        The ftp proxy part of mod_proxy accepts directory listings from

        remote ftp servers and converts them to HTML to send to the

        client.  It is possible to deliberately create a listing that

        will cause Apache to dump core.

        This hole does not compromise the server; the only risk

        is that it would be possible to use this to create a

        denial of service attack which would render the server

        effectively inoperative.

        If you do not use mod_proxy, you are not vulnerable to this.

        If you restrict the use of mod_proxy, then only those users

        who are permitted to use it can attempt to exploit this


VI.  Possible buffer overflow reading from the proxy cache

        RISK: low

        When caching is enabled in mod_proxy, Apache writes cached

        files to disk as the user that the server runs as.  If an

        attacker can gain access to this user id (eg. by running

        a CGI script from a pre-existing account on the machine)

        then they can modify the filenames on disk resulting in a

        buffer overflow.

        Because the data is limited to what can be stored in a

        filename (not the file, just the filename), and they already

        need to have access to the user ID the server runs as to

        exploit this, the risk is minimal.

        The main instance where this may be a cause for concern is if

        there is privileged information stored in memory by the

        web server, such as an unencrypted SSL key.  This same

        caution, however, applies to the other buffer overflows


        If you do not use mod_proxy, this problem can not be


VII. Unreadable htaccess files were ignored

        RISK: low

        Previously, if a htaccess file was unreadable Apache ignored

        it.  This is, from a security standpoint, a poor idea

        because it goes against the principle of "if in doubt, deny

        access".  This had already been corrected in the 1.3

        development tree, but we had refrained from making the

        change in 1.2 because it could cause unexpected behavior

        on existing sites.  We have since reconsidered, and as of

        1.2.5, Apache will now reject requests if there is a htaccess

        file present in the relevant directory tree that is unreadable

        for any reason.

        It is also possible, in very rare conditions, for this to

        to be used to bypass htaccess files restricting access to

        a directory or file.  The only case where this can happen

        is if the attacker can form a request that results in the

        full path to the htaccess file being too long (on most

        systems, meaning over 1024 characters) yet the request for

        the protected file in the same directory is not too long.

        The only normal case where such an attack could be possible

        is if there is a symbolic link such as "somedir -> ."

        created in the document tree.

Contact Information


Full information about Apache and the 1.2.5 release which fixes

these issues is available at http://www.apache.org/

Normal bugs can be reported via http://www.apache.org/bug_report.html

If you believe you have discovered a security hole in Apache, please

be sure to contact us at security@apache.org so that we can verify

and resolve the problem.  Support questions to this address will

not get a response.  We fully support the concept of full disclosure,

however it is always preferable to try to work with the vendor

first before publicizing information about security holes.

- --

     Marc Slemko     | Apache team member

     marcs@znep.com  | marc@apache.org

(C) 1999-2000 All rights reserved.