Attacker Value
High
(1 user assessed)
Exploitability
Moderate
(1 user assessed)
User Interaction
None
Privileges Required
None
Attack Vector
Network
3

CVE-2021-26897

Disclosure Date: March 11, 2021
Add MITRE ATT&CK tactics and techniques that apply to this CVE.

Description

Windows DNS Server Remote Code Execution Vulnerability This CVE ID is unique from CVE-2021-26877, CVE-2021-26893, CVE-2021-26894, CVE-2021-26895.

Add Assessment

3
Ratings
  • Attacker Value
    High
  • Exploitability
    Medium
Technical Analysis

Vulnerability Overview

0patch released a blog article about their micro patch concerning CVE-2021-26897. It describes the root cause as

CVE-2021-26897 is a buffer overflow issue, whereby a series of oversized “dynamic update” DNS queries with SIG (signature) records causes writing beyond the buffer boundary when these records are saved to file.

According to the blog article the record saves happen

  • periodically or
  • when the DNS service stops

The analysis of 0patch was based on an article from the McAfee Labs. They provided enough information to enable 0patch to gain understanding were the vulnerability is located.

Successful exploitation of this vulnerability results in running code with Local System privileges. A attacker does need a domain joined computer and have access to a DNS server. The configuration of the DNS server needs to have Dynamic Updates activated.

In an Active Directory environment Dynamic Updates are enabled by default. The default setting secure dynamic updates only allows domain joined computers to update a DNS zone.

Score reasoning

I rated the Attack Value pretty high. Successful exploitation provides adversaries with high privileged access to domain controllers.
The Exploitability score is based on the fact, that the vulnerability can be reversed through public resources and seems to be a buffer overflow. Nevertheless the broader mass of adversaries might be waiting for a detailed writeup or P-o-C and act opportunistic.

Sources:

CVSS V3 Severity and Metrics
Base Score:
9.8 Critical
Impact Score:
5.9
Exploitability Score:
3.9
Vector:
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Attack Vector (AV):
Network
Attack Complexity (AC):
Low
Privileges Required (PR):
None
User Interaction (UI):
None
Scope (S):
Unchanged
Confidentiality (C):
High
Integrity (I):
High
Availability (A):
High

General Information

Vendors

  • Microsoft

Products

  • Windows Server, version 20H2 (Server Core Installation),
  • Windows Server,
  • Windows Server, version 1909 (Server Core installation),
  • Windows Server, version 2004 (Server Core installation)

Additional Info

Technical Analysis