CVSS Meta Temp Score CVSS is a standardized scoring system to determine possibilities of attacks. The Temp Score considers temporal factors like disclosure, exploit and countermeasures. The unique Meta Score calculates the average score of different sources to provide a normalized scoring system.	Current Exploit Price (≈) Our analysts are monitoring exploit markets and are in contact with vulnerability brokers. The range indicates the observed or calculated exploit price to be seen on exploit markets. A good indicator to understand the monetary effort required for and the popularity of an attack.	CTI Interest Score Our Cyber Threat Intelligence team is monitoring different web sites, mailing lists, exploit markets and social media networks. The CTI Interest Score identifies the interest of attackers and the security community for this specific vulnerability in real-time. A high score indicates an elevated risk to be targeted for this vulnerability.
5.2	$0-$5k	0.00

A vulnerability classified as critical has been found in XenSource Xen (Virtualization Software). This affects the function mod_l2_entry of the file arch/x86/mm.c of the component Mapping Handler. The manipulation with an unknown input leads to a input validation vulnerability. CWE is classifying the issue as CWE-20. The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

The mod_l2_entry function in arch/x86/mm.c in Xen 3.4 through 4.6.x does not properly validate level 2 page table entries, which allows local PV guest administrators to gain privileges via a crafted superpage mapping.

The issue has been introduced in 11/05/2008. The weakness was disclosed 10/29/2015 by Alibaba with Xen Security Team as XSA-148 as confirmed security advisory (Website). The advisory is shared at xenbits.xen.org. This vulnerability is uniquely identified as CVE-2015-7835 since 10/14/2015. An attack has to be approached locally. No form of authentication is needed for exploitation. Technical details are known, but no exploit is available. The reason for this vulnerability is this part of code:

if ( l2e_get_flags(nl2e) & _PAGE_PRESENT )
   {
      if ( unlikely(l2e_get_flags(nl2e) & L2_DISALLOW_MASK) )
         {
            //...
         }

The advisory points out:

We see the attacker might request setting of the (PSE | RW) bits in the L2 PDE (which is possible thanks to L2_DISALLOW_MASK not excluding the PSE bit, something which has been added to support the superpage mappings for the PV guests), thus making the whole L1 table accessible to the guest with R/W rights (now seen as a large 2MB page), and modify one or more of the PTEs there to point to an arbitrary MFN the attacker feels like having access to. Now that would not be all fatal, if the attacker had no way of tricking Xen into treating this (now under the attacker's control) super-page back as a valid table of PTEs. Sadly, there is nothing to stop her from doing that. Thus we end up with Xen now treating the attacker-filled memory as a set of valid PTEs for the (PV) guest. This means the guest, by referencing the addresses mapped by these pages, is now really accessing whatever MFNs the attacker decided to write into the PTEs. In other words, the guest can access now all the system's memory. Reliably. The attack works irrespectively of whether the opt_allow_superpage is true or not.

The vulnerability was handled as a non-public zero-day exploit for at least 2549 days. During that time the estimated underground price was around $25k-$100k. The vulnerability scanner Nessus provides a plugin with the ID 86700 (Debian DSA-3390-1 : xen - security update), which helps to determine the existence of the flaw in a target environment. It is assigned to the family Debian Local Security Checks and running in the context l. The commercial vulnerability scanner Qualys is able to test this issue with plugin 370027 (Citrix XenServer Security Update (CTX202404)).

Applying a patch is able to eliminate this problem. A possible mitigation has been published immediately after the disclosure of the vulnerability.

The vulnerability is also documented in the databases at X-Force (107667), Tenable (86700), SecurityFocus (BID 77366†) and Vulnerability Center (SBV-54040†). Further details are available at blog.xenproject.org. The entries VDB-78959, VDB-78960 and VDB-78963 are pretty similar.

Productinfo

Type

• Virtualization Software

Vendor

• XenSource

Name

• Xen

Version

• 3.4.0
• 3.4.1
• 3.4.2
• 3.4.3
• 3.4.4
• 4.0.0
• 4.0.1
• 4.0.2
• 4.0.3
• 4.0.4
• 4.1.0
• 4.1.1
• 4.1.2
• 4.1.3
• 4.1.4
• 4.1.5
• 4.1.6.1
• 4.2.0
• 4.2.1
• 4.2.2
• 4.2.3
• 4.3.0
• 4.3.1
• 4.3.2
• 4.3.4
• 4.4.0
• 4.4.1
• 4.5.0
• 4.5.1
• 4.6.0

License

• open-source

CPE 2.3info

• 🔍
• 🔍
• 🔍

CPE 2.2info

• 🔍
• 🔍
• 🔍

CVSSv4info

VulDB CVSS-B Score: 🔍
VulDB CVSS-BT Score: 🔍
VulDB Vector: 🔍
VulDB Reliability: 🔍

CVSSv3info

VulDB Meta Base Score: 5.9
VulDB Meta Temp Score: 5.2

VulDB Base Score: 5.9
VulDB Temp Score: 5.2
VulDB Vector: 🔍
VulDB Reliability: 🔍

CVSSv2info

AV	AC	Au	C	I	A
💳	💳	💳	💳	💳	💳
💳	💳	💳	💳	💳	💳
💳	💳	💳	💳	💳	💳

Vector	Complexity	Authentication	Confidentiality	Integrity	Availability
unlock	unlock	unlock	unlock	unlock	unlock
unlock	unlock	unlock	unlock	unlock	unlock
unlock	unlock	unlock	unlock	unlock	unlock

VulDB Base Score: 🔍
VulDB Temp Score: 🔍
VulDB Reliability: 🔍

NVD Base Score: 🔍

Exploitinginfo

Class: Input validation
CWE: CWE-20
CAPEC: 🔍
ATT&CK: 🔍

Local: Yes
Remote: No

Availability: 🔍
Status: Unproven

EPSS Score: 🔍
EPSS Percentile: 🔍

Price Prediction: 🔍
Current Price Estimation: 🔍

0-Day	unlock	unlock	unlock	unlock
Today	unlock	unlock	unlock	unlock

Nessus ID: 86700
Nessus Name: Debian DSA-3390-1 : xen - security update
Nessus File: 🔍
Nessus Risk: 🔍
Nessus Family: 🔍
Nessus Context: 🔍

OpenVAS ID: 703390
OpenVAS Name: Debian Security Advisory DSA 3390-1 (xen - security update)
OpenVAS File: 🔍
OpenVAS Family: 🔍

Qualys ID: 🔍
Qualys Name: 🔍

Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: Patch
Status: 🔍

Reaction Time: 🔍
0-Day Time: 🔍
Exposure Time: 🔍

Timelineinfo

11/05/2008 🔍
10/14/2015 +2534 days 🔍
10/29/2015 +15 days 🔍
10/29/2015 +0 days 🔍
10/29/2015 +0 days 🔍
10/29/2015 +0 days 🔍
10/30/2015 +1 days 🔍
10/30/2015 +0 days 🔍
10/30/2015 +0 days 🔍
11/03/2015 +4 days 🔍
06/25/2022 +2426 days 🔍

Sourcesinfo

Vendor: citrix.com

Advisory: XSA-148
Researcher: Alibaba
Organization: Xen Security Team
Status: Confirmed
Confirmation: 🔍

CVE: CVE-2015-7835 (🔍)
OVAL: 🔍

X-Force: 107667 - Xen mappings privilege escalation
SecurityFocus: 77366 - Xen CVE-2015-7835 Privilege Escalation Vulnerability
SecurityTracker: 1034032
Vulnerability Center: 54040 - Xen Local Privilege Escalation via a PV Guest Access, High

Misc.: 🔍
See also: 🔍

Entryinfo

Created: 10/30/2015 09:38 AM
Updated: 06/25/2022 02:18 PM
Changes: 10/30/2015 09:38 AM (91), 12/02/2018 09:54 AM (2), 06/25/2022 02:18 PM (3)
Complete: 🔍
Cache ID: 18:9FD:40

Discussion

Interested in the pricing of exploits?

See the underground prices here!