Date: Tue, 25 May 2021 18:34:31 +0200
From: X41 D-Sec GmbH Advisories <advisories@...-dsec.de>
To: fulldisclosure@...lists.org
Subject: [FD] X41 D-Sec GmbH Security Advisory X41-2021-002: nginx DNS
 Resolver Off-by-One Heap Write Vulnerability


Advisory X41-2021-002: nginx DNS Resolver Off-by-One Heap Write
Vulnerability
=============================================================================
Severity Rating: High
Confirmed Affected Versions: 0.6.18 - 1.20.0
Confirmed Patched Versions: 1.21.0, 1.20.1
Vendor: F5, Inc.
Vendor URL: https://nginx.org/
Vendor Reference:
http://mailman.nginx.org/pipermail/nginx-announce/2021/000300.html
Vector: Remote / DNS
Credit: X41 D-SEC GmbH, Luis Merino, Markus Vervier, Eric Sesterhenn
Status: Public
CVE: CVE-2021-23017
CWE: 193
CVSS Score: 8.1
CVSS Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C
Advisory-URL:
https://www.x41-dsec.de/lab/advisories/x41-2021-002-nginx-resolver-copy/


Summary and Impact
------------------
An off-by-one error in ngx_resolver_copy() while processing DNS
responses allows a network attacker to write a dot character ('.', 0x2E)
 out of bounds in a heap allocated buffer. The vulnerability can be
triggered by a DNS response in reply to a DNS request from nginx when
the resolver primitive is configured. A specially crafted packet allows
overwriting the least significant byte of next heap chunk metadata with
0x2E. A network attacker capable of providing DNS responses to a nginx
server can achieve Denial-of-Service and likely remote code execution.
Due to the lack of DNS spoofing mitigations in nginx and the fact that
the vulnerable function is called before checking the DNS Transaction
ID, remote attackers might be able to exploit this vulnerability by
flooding the victim server with poisoned DNS responses in a feasible
amount of time.


Root Cause Analysis
-------------------
nginx DNS resolver (core/ngx_resolver.c) is used to resolve hostnames
via DNS for several modules when the resolver primitive is set.
ngx_resolver_copy() is called to validate and decompress each DNS domain
name contained in a DNS response, receiving the network packet as input
and a pointer to the name being processed, and returning a pointer to a
 newly allocated buffer containing the uncompressed name on success.
This is done in two steps,

1) The uncompressed domain name sizelenis calculated and the input
packet is validated, discarding names containing more than 128 pointers
 or containing pointers that fall out of the input buffer boundaries.
2) An output buffer is allocated, and the uncompressed name is copied
into it.

A mismatch between size calculation in part 1 and name decompression in
 part 2 leads to an off-by-one error inlen, allowing to write a dot
character one byte off name->data boundaries.
The miscalculation happens when the last part of the compressed name
contains a pointer to a NUL byte. While the calculation step only
accounts dots between labels, the decompression step writes a dot
character every time a label has been processed and next character is
not NUL. When a label is followed by a pointer that leads to a NUL byte,
the decompression procedure will:

// 1) copy the label to the output buffer,
 ngx_strlow(dst, src, n);
            dst += n;
            src += n;
// 2) read next character,
            n = *src++;
// 3) as its a pointer, its not NUL,
            if (n != 0) {
// 4) so a dot character that was not accounted for is written out of
bounds
                *dst++ = '.';
            }
// 5) Afterwards, the pointer is followed,
        if (n & 0xc0) {
            n = ((n & 0x3f) << 8) + *src;
            src = &buf[n];
            n = *src++;
        }
// 6) and a NULL byte is found, signaling the end of the function
        if (n == 0) {
            name->len = dst - name->data;
            return NGXOK;
        }


If the calculated size happens to align with the heap chunk size, the
dot character, written out of bounds, will overwrite the least
significant byte of next heap chunk size metadata. This might modify the
 size of the next heap chunk, but also overwrite 3 flags, resulting in
PREV_INUSE being cleared and IS_MMAPPED being set:


==7863== Invalid write of size 1
==7863==    at 0x137C2E: ngx_resolver_copy (ngx_resolver.c:4018)
==7863==    by 0x13D12B: ngx_resolver_process_a (ngx_resolver.c:2470)
==7863==    by 0x13D12B: ngx_resolver_process_response (ngx_resolver.c:1844)
==7863==    by 0x13D46A: ngx_resolver_udp_read (ngx_resolver.c:1574)
==7863==    by 0x14AB19: ngx_epoll_process_events (ngx_epoll_module.c:901)
==7863==    by 0x1414D4: ngx_process_events_and_timers (ngx_event.c:247)
==7863==    by 0x148E57: ngx_worker_process_cycle (ngx_process_cycle.c:719)
==7863==    by 0x1474DA: ngx_spawn_process (ngx_process.c:199)
==7863==    by 0x1480A8: ngx_start_worker_processes
(ngx_process_cycle.c:344)
==7863==    by 0x14952D: ngx_master_process_cycle (ngx_process_cycle.c:130)
==7863==    by 0x12237F: main (nginx.c:383)
==7863==  Address 0x4bbcfb8 is 0 bytes after a block of size 24 alloc'd
==7863==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==7863==    by 0x1448C4: ngx_alloc (ngx_alloc.c:22)
==7863==    by 0x137AE4: ngx_resolver_alloc (ngx_resolver.c:4119)
==7863==    by 0x137B26: ngx_resolver_copy (ngx_resolver.c:3994)
==7863==    by 0x13D12B: ngx_resolver_process_a (ngx_resolver.c:2470)
==7863==    by 0x13D12B: ngx_resolver_process_response (ngx_resolver.c:1844)
==7863==    by 0x13D46A: ngx_resolver_udp_read (ngx_resolver.c:1574)
==7863==    by 0x14AB19: ngx_epoll_process_events (ngx_epoll_module.c:901)
==7863==    by 0x1414D4: ngx_process_events_and_timers (ngx_event.c:247)
==7863==    by 0x148E57: ngx_worker_process_cycle (ngx_process_cycle.c:719)
==7863==    by 0x1474DA: ngx_spawn_process (ngx_process.c:199)
==7863==    by 0x1480A8: ngx_start_worker_processes
(ngx_process_cycle.c:344)
==7863==    by 0x14952D: ngx_master_process_cycle (ngx_process_cycle.c:130)


More information about general exploitability of a similar bug class
found in

* Chrome OS exploit: one byte overflow and symlinks
https://googleprojectzero.blogspot.com/2016/12/chrome-os-exploit-one-byte-overflow-and.html

* Project Zero's Poisoned NULL Byte
https://googleprojectzero.blogspot.com/2014/08/the-poisoned-nul-byte-2014-edition.html

* Hiroki Matsukama's House of Einherjar
  https://www.slideshare.net/codeblue_jp/cb16-matsukuma-en-68459606
  https://www.youtube.com/watch?v=tq3mPjsl-H0
Given the rich interaction opportunities in nginx with user controller
data and the documented precedents this bug is considered exploitable
for remote code execution on some operating systems and architectures.


Attack Vector Analysis
----------------------

There are several ways in which a DNS response can trigger the
vulnerability.

First, nginx must have sent a DNS request and must be waiting for a
response.

Then, a poisoned name can be injected in several parts of a DNS response:

* DNS Questions QNAME,
* DNS Answers NAME,
* DNS Answers RDATA for CNAME and SRV responses,

Keep in mind that the vulnerable function can be hit several times while
processing a response, effectively performing several off-by-one writes,
by crafting a response with several poisoned QNAME, NAME or RDATA  values.
Furthermore, when the attacker delivers a poisoned CNAME, it will be
resolved recursively, triggering an additional OOB write during
ngx_resolve_name_locked() call to ngx_strlow() (ngx_resolver.c:594) and
additional OOB reads during ngx_resolver_dup() (ngx_resolver.c:790) and
ngx_crc32_short() (ngx_resolver.c:596).

<for example payloads please see the attached advisory .txt file>

Fix / Workarounds
-----------------

Allocating an extra byte for the spurious dot character written at the
end of the poisoned domain names mitigates the issue.


--- ngxresolver.c  2021-04-06 15:59:50.293734070 +0200
+++ src/nginx-1.19.8/src/core/ngxresolver.c    2021-04-06
15:54:10.232975235 +0200
@@ -3943,7 +3928,7 @@
     ngx_uint_t   i, n;

     p = src;
-    len = -1;
+    len = 0;

     /*
      * compression pointers allow to create endless loop, so we set limit;
@@ -3986,7 +3971,7 @@
         return NGX_OK;
     }

-    if (len == -1) {
+    if (len == 0) {
         ngx_str_null(name);
         return NGX_OK;
     }


Official fix can be found at
http://nginx.org/download/patch.2021.resolver.txt


Proof-of-Concept
----------------

A dummy DNS server delivering a poisoned payload that triggers this
vulnerability can be downloaded from
https://github.com/x41sec/advisories/blob/master/X41-2021-002/poc.py

The described vulnerability can be tested by running nginx with the
provided config as follows under valgrind (https://www.valgrind.org/):

   valgrind --trace-children=yes objs/nginx -p ../runtime -c
conf/reverse-proxy.conf

Then run the DNS server (will listen on port 1053 by default):

   python poc.py

and trigger a request to the server:

   curl http://127.0.0.1:8080/

Depending on the heap layout when the bug triggers, the malloc
mitigations might detect or not the effect. Several ways of showing up
in the logs arise:

corrupted size vs. prev_size
2021/04/16 13:35:15 [alert] 2501#0: worker process 2502 exited on signal
6 (core dumped)
malloc(): invalid next size (unsorted)
2021/04/16 13:35:34 [alert] 2525#0: worker process 2526 exited on signal
6 (core dumped)


Nevertheless, valgrind and AdressSanitizer will always detect the memory
corruption.


nginx config used
-----------------

daemon off;
http{
    access_log logs/access.log;
    server{
        listen 8080;
        location / {
            resolver 127.0.0.1:1053;
            set $dns http://example.net;
            proxy_pass $dns;
        }
    }
}
events {
    worker_connections  1024;
}


Timeline
--------

2021-04-30 Issue reported to maintainers
2021-05-17 Issue reported to distros mailing list
2021-05-18 CVE assigned
2021-05-25 Public disclosure

About X41 D-SEC GmbH
====================
X41 is an expert provider for application security services. Having
extensive industry experience and expertise in the area of information
security, a strong core security team of world class security experts
enables X41 to perform premium security services.
Fields of expertise in the area of application security are security
centered code reviews, binary reverse engineering and vulnerability
discovery. Custom research and IT security consulting and support
services are core competencies of X41.




View attachment "x41-2021-002-nginx-resolver-copy.txt" of type "text/plain" (11485 bytes)

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