DNS RR Anti-Patterns: Common Implementation Anti-Patterns Related to Domain Name System (DNS) Resource Record (RR) Processing

Independent Submission                                     S. Dashevskyi
Request for Comments: 9267                                 D. dos Santos
Category: Informational                                       J. Wetzels
ISSN: 2070-1721                                                  A. Amri
                                                  Forescout Technologies
                                                               July 2022


Common Implementation Anti-Patterns Related to Domain Name System (DNS)
                    Resource Record (RR) Processing

Abstract

   This memo describes common vulnerabilities related to Domain Name
   System (DNS) resource record (RR) processing as seen in several DNS
   client implementations.  These vulnerabilities may lead to successful
   Denial-of-Service and Remote Code Execution attacks against the
   affected software.  Where applicable, violations of RFC 1035 are
   mentioned.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates for any level of Internet Standard;
   see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9267.

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   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction
   2.  Compression Pointer and Offset Validation
   3.  Label and Name Length Validation
   4.  Null-Terminator Placement Validation
   5.  Response Data Length Validation
   6.  Record Count Validation
   7.  Security Considerations
   8.  IANA Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Major vulnerabilities in DNS implementations recently became evident
   and raised attention to this protocol as an important attack vector,
   as discussed in [SIGRED], [SADDNS], and [DNSPOOQ], the latter being a
   set of 7 critical issues affecting the DNS forwarder "dnsmasq".

   The authors of this memo have analyzed the DNS client implementations
   of several major TCP/IP protocol stacks and found a set of
   vulnerabilities that share common implementation flaws (anti-
   patterns).  These flaws are related to processing DNS resource
   records (RRs) (discussed in [RFC1035]) and may lead to critical
   security vulnerabilities.

   While implementation flaws may differ from one software project to
   another, these anti-patterns are highly likely to span multiple
   implementations.  In fact, one of the first "Common Vulnerabilities
   and Exposures" (CVE) documents related to one of the anti-patterns
   [CVE-2000-0333] dates back to the year 2000.  The observations are
   not limited to DNS client implementations.  Any software that
   processes DNS RRs may be affected, such as firewalls, intrusion
   detection systems, or general-purpose DNS packet dissectors (e.g.,
   the DNS dissector in Wireshark; see [CVE-2017-9345]).  Similar issues
   may also occur in DNS-over-HTTPS [RFC8484] and DNS-over-TLS [RFC7858]
   implementations.  However, any implementation that deals with the DNS
   wire format is subject to the considerations discussed in this
   document.

   [DNS-COMPRESSION] and [RFC5625] briefly mention some of these anti-
   patterns, but the main purpose of this memo is to provide technical
   details behind these anti-patterns, so that the common mistakes can
   be eradicated.

   We provide general recommendations on mitigating the anti-patterns.
   We also suggest that all implementations should drop malicious/
   malformed DNS replies and (optionally) log them.

2.  Compression Pointer and Offset Validation

   [RFC1035] defines the DNS message compression scheme that can be used
   to reduce the size of messages.  When it is used, an entire domain
   name or several name labels are replaced with a (compression) pointer
   to a prior occurrence of the same name.

   The compression pointer is a combination of two octets: the two most
   significant bits are set to 1, and the remaining 14 bits are the
   OFFSET field.  This field specifies the offset from the beginning of
   the DNS header, at which another domain name or label is located:

   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   | 1  1|                OFFSET                   |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

   The message compression scheme explicitly allows a domain name to be
   represented as one of the following: (1) a sequence of unpacked
   labels ending with a zero octet, (2) a pointer, or (3) a sequence of
   labels ending with a pointer.

   However, [RFC1035] does not explicitly state that blindly following
   compression pointers of any kind can be harmful [DNS-COMPRESSION], as
   we could not have had any assumptions about various implementations
   that would follow.

   Yet, any DNS packet parser that attempts to decompress domain names
   without validating the value of OFFSET is likely susceptible to
   memory corruption bugs and buffer overruns.  These bugs make it
   easier to perform Denial-of-Service attacks and may result in
   successful Remote Code Execution attacks.

   Pseudocode that illustrates a typical example of a broken domain name
   parsing implementation is shown below (Figure 1):

    1: decompress_domain_name(*name, *dns_payload) {
    2:
    3:   name_buffer[255];
    4:   copy_offset = 0;
    5:
    6:   label_len_octet = name;
    7:   dest_octet = name_buffer;
    8:
    9:   while (*label_len_octet != 0x00) {
   10:
   11:      if (is_compression_pointer(*label_len_octet)) {
   12:          ptr_offset = get_offset(label_len_octet,
                                           label_len_octet+1);
   13:          label_len_octet = dns_payload + ptr_offset + 1;
   14:      }
   15:
   16:      else {
   17:          length = *label_len_octet;
   18:          copy(dest_octet + copy_offset,
                           label_len_octet+1, *length);
   19:
   20:         copy_offset += length;
   21:          label_len_octet += length + 1;
   22:      }
   23:
   24:   }
   25: }

      Figure 1: A Broken Implementation of a Function That Is Used for
                Decompressing DNS Domain Names (Pseudocode)

   Such implementations typically have a dedicated function for
   decompressing domain names (for example, see [CVE-2020-24338] and
   [CVE-2020-27738]).  Among other parameters, these functions may
   accept a pointer to the beginning of the first name label within an
   RR ("name") and a pointer to the beginning of the DNS payload to be
   used as a starting point for the compression pointer ("dns_payload").
   The destination buffer for the domain name ("name_buffer") is
   typically limited to 255 bytes as per [RFC1035] and can be allocated
   either in the stack or in the heap memory region.

   The code of the function in Figure 1 reads the domain name label by
   label from an RR until it reaches the NUL octet ("0x00") that
   signifies the end of a domain name.  If the current label length
   octet ("label_len_octet") is a compression pointer, the code extracts
   the value of the compression offset and uses it to "jump" to another
   label length octet.  If the current label length octet is not a
   compression pointer, the label bytes will be copied into the name
   buffer, and the number of bytes copied will correspond to the value
   of the current label length octet.  After the copy operation, the
   code will move on to the next label length octet.

   The first issue with this implementation is due to unchecked
   compression offset values.  The second issue is due to the absence of
   checks that ensure that a pointer will eventually arrive at a
   decompressed domain label.  We describe these issues in more detail
   below.

   [RFC1035] states that a compression pointer is "a pointer to a prior
   occurance [sic] of the same name."  Also, according to [RFC1035], the
   maximum size of DNS packets that can be sent over UDP is limited to
   512 octets.

   The pseudocode in Figure 1 violates these constraints, as it will
   accept a compression pointer that forces the code to read outside the
   bounds of a DNS packet.  For instance, a compression pointer set to
   "0xffff" will produce an offset of 16383 octets, which is most
   definitely pointing to a label length octet somewhere past the bounds
   of the original DNS packet.  Supplying such offset values will most
   likely cause memory corruption issues and may lead to Denial-of-
   Service conditions (e.g., a Null pointer dereference after
   "label_len_octet" is set to an invalid address in memory).  For
   additional examples, see [CVE-2020-25767], [CVE-2020-24339], and
   [CVE-2020-24335].

   The pseudocode in Figure 1 allows jumping from a compression pointer
   to another compression pointer and does not restrict the number of
   such jumps.  That is, if a label length octet that is currently being
   parsed is a compression pointer, the code will perform a jump to
   another label, and if that other label is a compression pointer as
   well, the code will perform another jump, and so forth until it
   reaches a decompressed label.  This may lead to unforeseen side
   effects that result in security issues.

   Consider the DNS packet excerpt illustrated below:

           +----+----+----+----+----+----+----+----+----+----+----+----+
     +0x00 |    ID   |  FLAGS  | QDCOUNT | ANCOUNT | NSCOUNT | ARCOUNT |
           +----+----+----+----+----+----+----+----+----+----+----+----+
   ->+0x0c |0xc0|0x0c|   TYPE  |  CLASS  |0x04| t  | e  | s  | t  |0x03|
   |       +----+--|-+----+----+----+----+----+----+----+----+----+----+
   | +0x18 | c  | o| | m  |0x00|  TYPE   |  CLASS  | ................  |
   |       +----+--|-+----+----+----+----+----+----+----+----+----+----+
   |               |
   -----------------

   The packet begins with a DNS header at offset +0x00, and its DNS
   payload contains several RRs.  The first RR begins at an offset of 12
   octets (+0x0c); its first label length octet is set to the value
   "0xc0", which indicates that it is a compression pointer.  The
   compression pointer offset is computed from the two octets "0xc00c"
   and is equal to 12.  Since the broken implementation in Figure 1
   follows this offset value blindly, the pointer will jump back to the
   first octet of the first RR (+0x0c) over and over again.  The code in
   Figure 1 will enter an infinite-loop state, since it will never leave
   the "TRUE" branch of the "while" loop.

   Apart from achieving infinite loops, the implementation flaws in
   Figure 1 make it possible to achieve various pointer loops that have
   other undesirable effects.  For instance, consider the DNS packet
   excerpt shown below:

           +----+----+----+----+----+----+----+----+----+----+----+----+
     +0x00 |    ID   |  FLAGS  | QDCOUNT | ANCOUNT | NSCOUNT | ARCOUNT |
           +----+----+----+----+----+----+----+----+----+----+----+----+
   ->+0x0c |0x04| t  | e  | s  | t  |0xc0|0x0c| ...................... |
   |       +----+----+----+----+----+----+--|-+----+----+----+----+----+
   |                                        |
   ------------------------------------------

   With such a domain name, the implementation in Figure 1 will first
   copy the domain label at offset "0xc0" ("test"); it will then fetch
   the next label length octet, which happens to be a compression
   pointer ("0xc0").  The compression pointer offset is computed from
   the two octets "0xc00c" and is equal to 12 octets.  The code will
   jump back to offset "0xc0" where the first label "test" is located.
   The code will again copy the "test" label and then jump back to it,
   following the compression pointer, over and over again.

   Figure 1 does not contain any logic that restricts multiple jumps
   from the same compression pointer and does not ensure that no more
   than 255 octets are copied into the name buffer ("name_buffer").  In
   fact,

   *  the code will continue to write the label "test" into it,
      overwriting the name buffer and the stack of the heap metadata.

   *  attackers would have a significant degree of freedom in
      constructing shell code, since they can create arbitrary copy
      chains with various combinations of labels and compression
      pointers.

   Therefore, blindly following compression pointers may lead not only
   to Denial-of-Service conditions, as pointed out by [DNS-COMPRESSION],
   but also to successful Remote Code Execution attacks, as there may be
   other implementation issues present within the corresponding code.

   Some implementations may not follow [RFC1035], which states:

   |  The first two bits are ones.  This allows a pointer to be
   |  distinguished from a label, since the label must begin with two
   |  zero bits because labels are restricted to 63 octets or less.
   |  (The 10 and 01 combinations are reserved for future use.)

   Figures 2 and 3 show pseudocode that implements two functions that
   check whether a given octet is a compression pointer; Figure 2 shows
   a correct implementation, and Figure 3 shows an incorrect (broken)
   implementation.

   1: unsigned char is_compression_pointer(*octet) {
   2:     if ((*octet & 0xc0) == 0xc0)
   3:         return true;
   4:     } else {
   5:         return false;
   6:     }
   7: }

                Figure 2: Correct Compression Pointer Check

   1: unsigned char is_compression_pointer(*octet) {
   2:     if (*octet & 0xc0) {
   3:         return true;
   4:     } else {
   5:         return false;
   6:     }
   7: }

                 Figure 3: Broken Compression Pointer Check

   The correct implementation (Figure 2) ensures that the two most
   significant bits of an octet are both set, while the broken
   implementation (Figure 3) would consider an octet with only one of
   the two bits set to be a compression pointer.  This is likely an
   implementation mistake rather than an intended violation of
   [RFC1035], because there are no benefits in supporting such
   compression pointer values.  The implementations related to
   [CVE-2020-24338] and [CVE-2020-24335] had a broken compression
   pointer check, similar to the code shown in Figure 3.

   While incorrect implementations alone do not lead to vulnerabilities,
   they may have unforeseen side effects when combined with other
   vulnerabilities.  For instance, the first octet of the value "0x4130"
   may be incorrectly interpreted as a label length by a broken
   implementation.  Such a label length (65) is invalid and is larger
   than 63 (as per [RFC1035]); a packet that has this value should be
   discarded.  However, the function shown in Figure 3 will consider
   "0x41" to be a valid compression pointer, and the packet may pass the
   validation steps.

   This might give attackers additional leverage for constructing
   payloads and circumventing the existing DNS packet validation
   mechanisms.

   The first occurrence of a compression pointer in an RR (an octet with
   the two highest bits set to 1) must resolve to an octet within a DNS
   record with a value that is greater than 0 (i.e., it must not be a
   Null-terminator) and less than 64.  The offset at which this octet is
   located must be smaller than the offset at which the compression
   pointer is located; once an implementation makes sure of that,
   compression pointer loops can never occur.

   In small DNS implementations (e.g., embedded TCP/IP stacks), support
   for nested compression pointers (pointers that point to a compressed
   name) should be discouraged: there is very little to be gained in
   terms of performance versus the high probability of introducing
   errors such as those discussed above.

   The code that implements domain name parsing should check the offset
   with respect to not only the bounds of a packet but also its position
   with respect to the compression pointer in question.  A compression
   pointer must not be "followed" more than once.  We have seen several
   implementations using a check that ensures that a compression pointer
   is not followed more than several times.  A better alternative may be
   to ensure that the target of a compression pointer is always located
   before the location of the pointer in the packet.

3.  Label and Name Length Validation

   [RFC1035] restricts the length of name labels to 63 octets and
   lengths of domain names to 255 octets (i.e., label octets and label
   length octets).  Some implementations do not explicitly enforce these
   restrictions.

   Consider the function "copy_domain_name()" shown in Figure 4 below.
   The function is a variant of the "decompress_domain_name()" function
   (Figure 1), with the difference that it does not support compressed
   labels and only copies decompressed labels into the name buffer.

    1: copy_domain_name(*name, *dns_payload) {
    2:
    3:   name_buffer[255];
    4:   copy_offset = 0;
    5:
    6:   label_len_octet = name;
    7:   dest_octet = name_buffer;
    8:
    9:   while (*label_len_octet != 0x00) {
   10:
   11:      if (is_compression_pointer(*label_len_octet)) {
   12:          length = 2;
   13:          label_len_octet += length + 1;
   14:      }
   15:
   16:      else {
   17:          length = *label_len_octet;
   18:          copy(dest_octet + copy_offset,
                                label_len_octet+1, *length);
   19:
   20:         copy_offset += length;
   21:          label_len_octet += length + 1;
   22:      }
   23:
   24:  }
   25: }

      Figure 4: A Broken Implementation of a Function That Is Used for
                    Copying Non-compressed Domain Names

   This implementation does not explicitly check for the value of the
   label length octet: this value can be up to 255 octets, and a single
   label can fill the name buffer.  Depending on the memory layout of
   the target, how the name buffer is allocated, and the size of the
   malformed packet, it is possible to trigger various memory corruption
   issues.

   Both Figures 1 and 4 restrict the size of the name buffer to 255
   octets; however, there are no restrictions on the actual number of
   octets that will be copied into this buffer.  In this particular
   case, a subsequent copy operation (if another label is present in the
   packet) will write past the name buffer, allowing heap or stack
   metadata to be overwritten in a controlled manner.

   Similar examples of vulnerable implementations can be found in the
   code relevant to [CVE-2020-25110], [CVE-2020-15795], and
   [CVE-2020-27009].

   As a general recommendation, a domain label length octet must have a
   value of more than 0 and less than 64 [RFC1035].  If this is not the
   case, an invalid value has been provided within the packet, or a
   value at an invalid position might be interpreted as a domain name
   length due to other errors in the packet (e.g., misplaced Null-
   terminator or invalid compression pointer).

   The number of domain label characters must correspond to the value of
   the domain label octet.  To avoid possible errors when interpreting
   the characters of a domain label, developers may consider
   recommendations for the preferred domain name syntax outlined in
   [RFC1035].

   The domain name length must not be more than 255 octets, including
   the size of decompressed domain names.  The NUL octet ("0x00") must
   be present at the end of the domain name and must be within the
   maximum name length (255 octets).

4.  Null-Terminator Placement Validation

   A domain name must end with a NUL ("0x00") octet, as per [RFC1035].
   The implementations shown in Figures 1 and 4 assume that this is the
   case for the RRs that they process; however, names that do not have a
   NUL octet placed at the proper position within an RR are not
   discarded.

   This issue is closely related to the absence of label and name length
   checks.  For example, the logic behind Figures 1 and 4 will continue
   to copy octets into the name buffer until a NUL octet is encountered.
   This octet can be placed at an arbitrary position within an RR or not
   placed at all.

   Consider the pseudocode function shown in Figure 5.  The function
   returns the length of a domain name ("name") in octets to be used
   elsewhere (e.g., to allocate a name buffer of a certain size): for
   compressed domain names, the function returns 2; for decompressed
   names, it returns their true length using the "strlen(3)" function.

   1: get_name_length(*name) {
   2:
   3:     if (is_compression_pointer(name))
   4:         return 2;
   5:
   6:     name_len = strlen(name) + 1;
   7:     return name_len;
   8: }

      Figure 5: A Broken Implementation of a Function That Returns the
                          Length of a Domain Name

   "strlen(3)" is a standard C library function that returns the length
   of a given sequence of characters terminated by the NUL ("0x00")
   octet.  Since this function also expects names to be explicitly Null-
   terminated, the return value "strlen(3)" may also be controlled by
   attackers.  Through the value of "name_len", attackers may control
   the allocation of internal buffers or specify the number by octets
   copied into these buffers, or they may perform other operations,
   depending on the implementation specifics.

   The absence of explicit checks for placement of the NUL octet may
   also facilitate controlled memory reads and writes.  An example of
   vulnerable implementations can be found in the code relevant to
   [CVE-2020-25107], [CVE-2020-17440], [CVE-2020-24383], and
   [CVE-2020-27736].

   As a general recommendation for mitigating such issues, developers
   should never trust user data to be Null-terminated.  For example, to
   fix/mitigate the issue shown in the code in Figure 5, developers
   should use the function "strnlen(3)", which reads at most X
   characters (the second argument of the function), and ensure that X
   is not larger than the buffer allocated for the name.

5.  Response Data Length Validation

   As stated in [RFC1035], every RR contains a variable-length string of
   octets that contains the retrieved resource data (RDATA) (e.g., an IP
   address that corresponds to a domain name in question).  The length
   of the RDATA field is regulated by the resource data length field
   (RDLENGTH), which is also present in an RR.

   Implementations that process RRs may not check for the validity of
   the RDLENGTH field value when retrieving RDATA.  Failing to do so may
   lead to out-of-bound read issues, whose impact may vary
   significantly, depending on the implementation specifics.  We have
   observed instances of Denial-of-Service conditions and information
   leaks.

   Therefore, the value of the data length byte in response DNS records
   (RDLENGTH) must reflect the number of bytes available in the field
   that describes the resource (RDATA).  The format of RDATA must
   conform to the TYPE and CLASS fields of the RR.

   Examples of vulnerable implementations can be found in the code
   relevant to [CVE-2020-25108], [CVE-2020-24336], and [CVE-2020-27009].

6.  Record Count Validation

   According to [RFC1035], the DNS header contains four two-octet fields
   that specify the amount of question records (QDCOUNT), answer records
   (ANCOUNT), authority records (NSCOUNT), and additional records
   (ARCOUNT).

   Figure 6 illustrates a recurring implementation anti-pattern for a
   function that processes DNS RRs.  The function
   "process_dns_records()" extracts the value of ANCOUNT ("num_answers")
   and the pointer to the DNS data payload ("data_ptr").  The function
   processes answer records in a loop, decrementing the "num_answers"
   value after processing each record until the value of "num_answers"
   becomes zero.  For simplicity, we assume that there is only one
   domain name per answer.  Inside the loop, the code calculates the
   domain name length ("name_length") and adjusts the data payload
   pointer ("data_ptr") by the offset that corresponds to "name_length +
   1", so that the pointer lands on the first answer record.  Next, the
   answer record is retrieved and processed, and the "num_answers" value
   is decremented.

    1: process_dns_records(dns_header, ...) {
           // ...
    2:     num_answers = dns_header->ancount
    3:     data_ptr = dns_header->data
    4:
    5:     while (num_answers > 0) {
    6:         name_length = get_name_length(data_ptr);
    7:         data_ptr += name_length + 1;
    8:
    9:         answer = (struct dns_answer_record *)data_ptr;
   10:
   11:         // process the answer record
   12:
   13:         --num_answers;
   14:     }
           // ...
   15: }

     Figure 6: A Broken Implementation of a Function That Processes RRs

   If the ANCOUNT number retrieved from the header
   ("dns_header->ancount") is not checked against the amount of data
   available in the packet and it is, for example, larger than the
   number of answer records available, the data pointer ("data_ptr")
   will read outside the bounds of the packet.  This may result in
   Denial-of-Service conditions.

   In this section, we used an example of processing answer records.
   However, the same logic is often reused for implementing the
   processing of other types of records, e.g., the number of question
   (QDCOUNT), authority (NSCOUNT), and additional (ARCOUNT) records.
   The specified numbers of these records must correspond to the actual
   data present within the packet.  Therefore, all record count fields
   must be checked before fully parsing the contents of a packet.
   Specifically, Section 6.3 of [RFC5625] recommends that such malformed
   DNS packets should be dropped and (optionally) logged.

   Examples of vulnerable implementations can be found in the code
   relevant to [CVE-2020-25109], [CVE-2020-24340], [CVE-2020-24334], and
   [CVE-2020-27737].

7.  Security Considerations

   Security issues are discussed throughout this memo; it discusses
   implementation flaws (anti-patterns) that affect the functionality of
   processing DNS RRs.  The presence of such anti-patterns leads to bugs
   that cause buffer overflows, read-out-of-bounds, and infinite-loop
   issues.  These issues have the following security impacts:
   information leaks, Denial-of-Service attacks, and Remote Code
   Execution attacks.

   This document lists general recommendations for the developers of DNS
   record parsing functionality that allow those developers to prevent
   such implementation flaws, e.g., by rigorously checking the data
   received over the wire before processing it.

8.  IANA Considerations

   This document has no IANA actions.  Please see [RFC6895] for a
   complete review of the IANA considerations introduced by DNS.

9.  References

9.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines",
              BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,
              <https://www.rfc-editor.org/info/rfc5625>.

9.2.  Informative References

   [CVE-2000-0333]
              Common Vulnerabilities and Exposures, "CVE-2000-0333: A
              denial-of-service vulnerability in tcpdump, Ethereal, and
              other sniffer packages via malformed DNS packets", 2000,
              <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-
              2000-0333>.

   [CVE-2017-9345]
              Common Vulnerabilities and Exposures, "CVE-2017-9345: An
              infinite loop in the DNS dissector of Wireshark", 2017,
              <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-
              2017-9345>.

   [CVE-2020-15795]
              Common Vulnerabilities and Exposures, "CVE-2020-15795: A
              denial-of-service and remote code execution vulnerability
              DNS domain name label parsing functionality of Nucleus
              NET", 2021, <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=CVE-2020-15795>.

   [CVE-2020-17440]
              Common Vulnerabilities and Exposures, "CVE-2020-17440 A
              denial-of-service vulnerability in the DNS name parsing
              implementation of uIP", 2020, <https://cve.mitre.org/cgi-
              bin/cvename.cgi?name=CVE-2020-17440>.

   [CVE-2020-24334]
              Common Vulnerabilities and Exposures, "CVE-2020-24334: An
              out-of-bounds read and denial-of-service vulnerability in
              the DNS response parsing functionality of uIP", 2020,
              <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-
              2020-24334>.

   [CVE-2020-24335]
              Common Vulnerabilities and Exposures, "CVE-2020-24335: A
              memory corruption vulnerability in domain name parsing
              routines of uIP", 2020, <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=CVE-2020-24335>.

   [CVE-2020-24336]
              Common Vulnerabilities and Exposures, "CVE-2020-24336: A
              buffer overflow vulnerability in the DNS implementation of
              Contiki and Contiki-NG", 2020, <https://cve.mitre.org/cgi-
              bin/cvename.cgi?name=CVE-2020-24336>.

   [CVE-2020-24338]
              Common Vulnerabilities and Exposures, "CVE-2020-24338: A
              denial-of-service and remote code execution vulnerability
              in the DNS domain name record decompression functionality
              of picoTCP", 2020, <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=CVE-2020-24338>.

   [CVE-2020-24339]
              Common Vulnerabilities and Exposures, "CVE-2020-24339: An
              out-of-bounds read and denial-of-service vulnerability in
              the DNS domain name record decompression functionality of
              picoTCP", 2020, <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=CVE-2020-24339>.

   [CVE-2020-24340]
              Common Vulnerabilities and Exposures, "CVE-2020-24340: An
              out-of-bounds read and denial-of-service vulnerability in
              the DNS response parsing functionality of picoTCP", 2020,
              <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-
              2020-24340>.

   [CVE-2020-24383]
              Common Vulnerabilities and Exposures, "CVE-2020-24383: An
              information leak and denial-of-service vulnerability while
              parsing mDNS resource records in FNET", 2020,
              <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-
              2020-24383>.

   [CVE-2020-25107]
              Common Vulnerabilities and Exposures, "CVE-2020-25107: A
              denial-of-service and remote code execution vulnerability
              in the DNS implementation of Ethernut Nut/OS", 2020,
              <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-
              2020-25107>.

   [CVE-2020-25108]
              Common Vulnerabilities and Exposures, "CVE-2020-25108: A
              denial-of-service and remote code execution vulnerability
              in the DNS implementation of Ethernut Nut/OS", 2020,
              <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-
              2020-25108>.

   [CVE-2020-25109]
              Common Vulnerabilities and Exposures, "CVE-2020-25109: A
              denial-of-service and remote code execution vulnerability
              in the DNS implementation of Ethernut Nut/OS", 2020,
              <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-
              2020-25109>.

   [CVE-2020-25110]
              Common Vulnerabilities and Exposures, "CVE-2020-25110: A
              denial-of-service and remote code execution vulnerability
              in the DNS implementation of Ethernut Nut/OS", 2020,
              <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-
              2020-25110>.

   [CVE-2020-25767]
              Common Vulnerabilities and Exposures, "CVE-2020-25767: An
              out-of-bounds read and denial-of-service vulnerability in
              the DNS name parsing routine of HCC Embedded NicheStack",
              2021, <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-
              2020-25767>.

   [CVE-2020-27009]
              Common Vulnerabilities and Exposures, "CVE-2020-27009: A
              denial-of-service and remote code execution vulnerability
              DNS domain name record decompression functionality of
              Nucleus NET", 2021, <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=CVE-2020-27009>.

   [CVE-2020-27736]
              Common Vulnerabilities and Exposures, "CVE-2020-27736: An
              information leak and denial-of-service vulnerability in
              the DNS name parsing functionality of Nucleus NET", 2021,
              <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-
              2020-27736>.

   [CVE-2020-27737]
              Common Vulnerabilities and Exposures, "CVE-2020-27737: An
              information leak and denial-of-service vulnerability in
              the DNS response parsing functionality of Nucleus NET",
              2021, <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-
              2020-27737>.

   [CVE-2020-27738]
              Common Vulnerabilities and Exposures, "CVE-2020-27738: A
              denial-of-service and remote code execution vulnerability
              DNS domain name record decompression functionality of
              Nucleus NET", 2021, <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=CVE-2020-27738>.

   [DNS-COMPRESSION]
              Koch, P., "A New Scheme for the Compression of Domain
              Names", Work in Progress, Internet-Draft, draft-ietf-
              dnsind-local-compression-05, 30 June 1999,
              <https://datatracker.ietf.org/doc/html/draft-ietf-dnsind-
              local-compression-05>.

   [DNSPOOQ]  Kol, M. and S. Oberman, "DNSpooq: Cache Poisoning and RCE
              in Popular DNS Forwarder dnsmasq", JSOF Technical Report,
              January 2021, <https://www.jsof-tech.com/wp-
              content/uploads/2021/01/DNSpooq-Technical-WP.pdf>.

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <https://www.rfc-editor.org/info/rfc6895>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

   [SADDNS]   Man, K., Qian, Z., Wang, Z., Zheng, X., Huang, Y., and H.
              Duan, "DNS Cache Poisoning Attack Reloaded: Revolutions
              with Side Channels", Proc. 2020 ACM SIGSAC Conference on
              Computer and Communications Security, CCS '20,
              DOI 10.1145/3372297.3417280, November 2020,
              <https://dl.acm.org/doi/pdf/10.1145/3372297.3417280>.

   [SIGRED]   Common Vulnerabilities and Exposures, "CVE-2020-1350: A
              remote code execution vulnerability in Windows Domain Name
              System servers", 2020, <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=CVE-2020-1350>.

Acknowledgements

   We would like to thank Shlomi Oberman, who has greatly contributed to
   the research that led to the creation of this document.

Authors' Addresses

   Stanislav Dashevskyi
   Forescout Technologies
   John F. Kennedylaan, 2
   5612AB Eindhoven
   Netherlands
   Email: stanislav.dashevskyi@forescout.com


   Daniel dos Santos
   Forescout Technologies
   John F. Kennedylaan, 2
   5612AB Eindhoven
   Netherlands
   Email: daniel.dossantos@forescout.com


   Jos Wetzels
   Forescout Technologies
   John F. Kennedylaan, 2
   5612AB Eindhoven
   Netherlands
   Email: jos.wetzels@forescout.com


   Amine Amri
   Forescout Technologies
   John F. Kennedylaan, 2
   5612AB Eindhoven
   Netherlands
   Email: amine.amri@forescout.com