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Date: Tue, 9 Nov 2021 11:22:58 +0100
From: "Philipp Jeitner (SIT)" <>
To: <>
Subject: [CVE-2021-43523] Incorrect handling of special characters in domain
 names in uclibc and uclibc-ng

Security Issue: Incorrect handling of special characters in domain names 
in uclibc and uclibc-ng

#### Description of the vulnerability

Incorrect handling of special characters in domain names returned by 
Domain Name Servers in uclibc and uclibc-ng below 1.0.39 via the 
gethostbyname(), getaddrinfo(), gethostbyaddr() and getnameinfo() calls 
can lead to output of wrong hostnames (leading to Domain Hijacking) and 
injection vulnerabilities in applications using the library (leading to 
Remote Code Execution, XSS, Applications crashes, etc.).

Examples of incorrect behavior:\ is returned as
     www\ is returned as
     <script>alert('xss')</script> is not filtered even 
though this is not a valid hostname.

#### CVE ID

This vulnerability has been assigned CVE-2021-43523.

#### Patch

This vulnerability was patched with uclibc-ng version 1.0.39:

#### Attack vector(s):

Lookup of attacker controlled domain name or other cases where an 
attacker can control the DNS response returned by used DNS servers.

#### Attack type


#### Impact

Impact depends on the application using the libc stub resolver. In case 
a cache is implemented, misinterpretation of \000 or \. can lead to 
cache poisoning. In case unfiltered input is processed, this can lead to 
XSS, SQL-injection, etc. We are aware of applications which are 
vulnerable due to such stub-resolver behavior, such as CVE-2021-33425.

#### Affected Components:

DNS resolver library functions `gethostbyname()`, `getaddrinfo()`, 
`gethostbyaddr()` and `getnameinfo()`.

#### Discoverer(s)/Credits

Philipp Jeitner and Haya Shulman, Fraunhofer SIT

#### Reference(s)

  - Injection Attacks Reloaded: Tunnelling Malicious Payloads over DNS

#### Additional information

The POSIX Standard for Information Technology [1] defines interfaces for 
DNS lookups in systems standard C libraries. This Standard includes 
functions for forward lookups (gethostbyname, getaddrinfo) as well as 
backward-lookups (gethostbyaddr, getnameinfo). These functions cannot 
only return IP addresses but can also contain hostnames of aliases 
(CNAME) of the requested host name in case of forward-lookups, or the 
primary host name of that ip address in the case of backward-lookups 
(PTR). The POSIX Standard defines the data format of these host names as 
a null-terminated C-String containing a "hostname" or "nodename", which 
are typically expected by developers and defined by RFC952 [2] and 
RFC1123 [3] to only contain alphanumeric characters (a-z,A-Z,0-9), 
hyphens ("-") and periods (".") to split labels. This creates a mismatch 
of allowed characters between "hostnames" and "domain names" as defined 
by the DNS standard [4] which defines "domain names" as a series of 
"text labels" which are textually represented by concatenating all "text 
labels" and joining them together with period signs. However, "text 
labels" can contain any octet value, even zero-bytes ("\x00") and period 
signs (".") and recursive DNS resolvers are required by the DNS standard 
to support any of these characters in DNS records, thus not implementing 
any sanitiy checks on domain names.

When DNS responses are parsed by the stub DNS resolver implemented by 
stub resolver library as part of the `gethostbyname()`, `getaddrinfo()`, 
`gethostbyaddr()` and `getnameinfo()` functions, these functions must 
therefore ensure that the returned, null-terminated C-Strings must be 
valid domain names as defined by the POSIX standard, else applications 
which use these values might include that information in contexts where 
malicious data can included inside the domain name and used for command 
injection attacks like Cross-Site-Scripting, SQL-injections, etc. 
Furthermore, if domain names contain text labels with periods (`"\."`) 
or zero-bytes (`"\000"`) and the stub resolver library does naively 
decode these domain names into strings, attackers can create malicious 
domain names which are misinterpreted by the naive decoding logic to 
look like different domain names than they actually are. When these 
misinterpreted domain names are than cached by applications using the 
stub resolver, this allows for domain hijacking by poisoning of the 
applications DNS cache which uses the vulnerable stub resolver library.

For an example on how stub resolvers should sanitize domain names 
returned by those functions, we refer to the `ns_name_ntop` [5] and 
`res_hnok` [6] functions in glibc.

[1]: Standard for Information Technology - Portable Operating System 
Interface (POSIX(R)) Base Specifications, Issue 7 -


[3]: RFC1123 - Requirements for Internet Hosts -- Application and 
Support -

[4]: RFC6895 - Domain Name System (DNS) IANA Considerations -

[5]: Github:

[6]: Github:

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