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Simple Machines Forum Username Faking / Clickjacking

Simple Machines Forum Username Faking / Clickjacking
Posted Dec 14, 2013
Authored by Jakob Lell | Site jakoblell.com

Simple Machines Forum suffers from username impersonation and clickjacking issues. These issues are are present in SMF1 up to version 1.1.18 and SMF2 up to version 2.0.5.

tags | advisory
SHA-256 | ec054b0bcc023ef1325986cda6d0998e1dc4e6a4098ffcf06f2400521afdec66

Simple Machines Forum Username Faking / Clickjacking

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Advisory location:

I. Introduction

Simple Machines Forum (abbreviated as SMF) is a free Internet forum
(BBS) software written in PHP.

II. Username faking via Unicode homoglyphs or duplicate spaces allows
user impersonation

The forum registration process allows registering UTF8 usernames. Since
Unicode contains a lot of additional symbols and some of them look very
similar (or even identical) to standard ASCII characters, this allows
registering a user with a name which is visually indistinguishable from
an existing forum user. As an example, someone may register a user named
"admiÕ¸" with the "n" replaced by the Unicode letter u+0578 (ARMENIAN
SMALL LETTER VO), which looks more or less exactly like the ASCII
character "n" depending on the font. This may be used in order to
impersonate users e.g. in forum messages. Additionally to choosing a
name which looks more or less exactly as the victim, an attacker can
also steal the avatar of the victim in order to further improve the

The following page simplifies finding matching homoglyph characters for
a given string:


If the original username contains a space, user impersonation is also
possible by registering the same username with two or more consecutive
spaces. These spaces will be passed to all HTML pages containing the
username and since web browsers ignore multiple consecutive spaces in
HTML, there is no visible difference between the original and the faked

III. Clickjacking in SMF forum allows user-assisted remote arbitrary
code execution

The forum software SMF contains no protection against clickjacking. This
allows tricking a currently logged in user to do various unintended
actions in the forum when the user visits a malicious website. I have a
working POC exploit which requires no more than 2 clicks to a
predictable location to achieve full remote code execution when
exploited against a forum administrator (although I will not disclose
the exact attack vector in this public advisory). A cleverly designed
attack site may trick the user do these two clicks without much
thinking. The first click can be achieved by displaying one of the
annoying overlays which requests the user to fill out a survey, like the
site on facebook or subscribe to a newsletter. Most users are
conditioned to directly click on the small x on the top right of the
overlay to close it. For the second click, the attack site may just not
react to the first click hoping that the victim tries again.
Alternatively, the site could also pretend to be a video site waiting
for the user to click on the play button.

IV. Affected versions

All three vulnerabilities are present in SMF1 up to version 1.1.18 and
SMF2 up to version 2.0.5. The SMF team has released updates (version
1.1.19 and 2.0.6) which fix the clickjacking problem (via an
X-Frame-Options header) and the username faking possibility via multiple
consecutive spaces. However, the Unicode homoglyph attack has not yet
been fixed since it is not trivial to filter out all confusable
characters while still allowing legitimate Unicode characters in
usernames (especially if you can't use the Spoofchecker class because
you have to support PHP versions below 5.4.0).

V. Credits

Jakob Lell

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