CVE-2021-45105

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DescriptionStatementMitigationAdditional informationCVSS Score DetailsWeakness (CWE)FAQ

Description

A flaw was found in the Apache Log4j logging library 2.x. when the logging configuration uses a non-default Pattern Layout with a Context Lookup. Attackers with control over Thread Context Map (MDC) input data can craft malicious input data that contains a recursive lookup and can cause Denial of Service.

A flaw was found in the Apache Log4j logging library 2.x. when the logging configuration uses a non-default Pattern Layout with a Context Lookup. Attackers with control over Thread Context Map (MDC) input data can craft malicious input data that contains a recursive lookup and can cause Denial of Service.

Statement

Red Hat Product Security has performed an analysis of this flaw and has classified the Attack Complexity(AC) as High because there are multiple factors involved which are beyond attacker's control: - The application has to use the logging configuration using a Context Map Lookup (for example, $${ctx:loginId}) which is a non-default Pattern Layout. - The application developer has to use the map org.apache.logging.log4j.ThreadContext in the application code and save at-least one key (for example, ThreadContext.put("loginId", "myId");) in the ThreadContext map object. - Attackers must also know this saved key name in order to exploit this flaw. Note that saving keys in this map is a non-essential usage of log4j and just an optional feature provided. Refer to https://logging.apache.org/log4j/2.x/manual/lookups.html#ContextMapLookup to know more about the Context Map Lookup feature of Log4j. Log4j 1.x is not impacted by this vulnerability. Note that only the log4j-core JAR file is impacted by this vulnerability. Applications using ONLY the log4j-api JAR file without the log4j-core JAR file are NOT impacted by this vulnerability. Despite including a vulnerable version of Log4j 2.x, this vulnerability is not exploitable in Elasticsearch[0], as shipped in OpenShift Container Platform and OpenShift Logging. OpenShift 3.11 specifically does not contain any context lookups: https://github.com/openshift/openshift-ansible/blob/release-3.11/roles/openshift_logging_elasticsearch/templates/log4j2.properties.j2 This vulnerability is therefore rated Low for Elasticsearch in OpenShift Container Platform and OpenShift Logging. [0] https://discuss.elastic.co/t/apache-log4j2-remote-code-execution-rce-vulnerability-cve-2021-44228-esa-2021-31/291476#update-december-18-4

Red Hat Product Security has performed an analysis of this flaw and has classified the Attack Complexity(AC) as High because there are multiple factors involved which are beyond attacker's control:

  • The application has to use the logging configuration using a Context Map Lookup (for example, $${ctx:loginId}) which is a non-default Pattern Layout.
  • The application developer has to use the map org.apache.logging.log4j.ThreadContext in the application code and save at-least one key (for example, ThreadContext.put("loginId", "myId");) in the ThreadContext map object.
  • Attackers must also know this saved key name in order to exploit this flaw.

Note that saving keys in this map is a non-essential usage of log4j and just an optional feature provided. Refer to https://logging.apache.org/log4j/2.x/manual/lookups.html#ContextMapLookup to know more about the Context Map Lookup feature of Log4j.

Log4j 1.x is not impacted by this vulnerability. Note that only the log4j-core JAR file is impacted by this vulnerability. Applications using ONLY the log4j-api JAR file without the log4j-core JAR file are NOT impacted by this vulnerability.

Despite including a vulnerable version of Log4j 2.x, this vulnerability is not exploitable in Elasticsearch[0], as shipped in OpenShift Container Platform and OpenShift Logging. OpenShift 3.11 specifically does not contain any context lookups:

https://github.com/openshift/openshift-ansible/blob/release-3.11/roles/openshift_logging_elasticsearch/templates/log4j2.properties.j2

This vulnerability is therefore rated Low for Elasticsearch in OpenShift Container Platform and OpenShift Logging.

[0] https://discuss.elastic.co/t/apache-log4j2-remote-code-execution-rce-vulnerability-cve-2021-44228-esa-2021-31/291476#update-december-18-4

Mitigation

For Log4j 2 versions up to and including 2.16.0, this flaw can be mitigated by:
- In PatternLayout in the Log4j logging configuration, replace Context Lookups like ${ctx:loginId} or $${ctx:loginId} with Thread Context Map patterns (%X, %mdc, or %MDC) like %X{loginId}.
- Otherwise, in the Log4j logging configuration, remove references to Context Lookups like ${ctx:loginId} or $${ctx:loginId} where they originate from sources external to the application such as HTTP headers or user input.

Additional information

  • Bugzilla 2034067: log4j-core: DoS in log4j 2.x with Thread Context Map (MDC) input data contains a recursive lookup and context lookup pattern
  • CWE-20->CWE-674: Improper Input Validation leads to Uncontrolled Recursion
  • FAQ: Frequently asked questions about CVE-2021-45105

Common Vulnerability Scoring System (CVSS) Score Details

Important note

CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).

CVSS v3 Score Breakdown
Red HatNVD

CVSS v3 Base Score

5.9

5.9

Attack Vector

Network

Network

Attack Complexity

High

High

Privileges Required

None

None

User Interaction

None

None

Scope

Unchanged

Unchanged

Confidentiality Impact

None

None

Integrity Impact

None

None

Availability Impact

High

High

CVSS v3 Vector

Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

NVD: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Red Hat CVSS v3 Score Explanation

Proposed changes to NIST's current CVSSv3 scoring: AC:L -> AC:H AC should be High because there are sufficient factors beyond the attacker's control: - The application has to use the logging configuration using a Context Map Lookup (for example, $${ctx:loginId}) which is a non-default Pattern Layout. - The application developer has to use the map org.apache.logging.log4j.ThreadContext in the application code and save at-least one key (for example, ThreadContext.put("loginId", "myId");) in the ThreadContext map object. - Attackers must also know this saved key name in order to exploit this flaw. Note that saving keys in this map is a non-essential usage of log4j and just an optional feature provided. Refer to https://logging.apache.org/log4j/2.x/manual/lookups.html#ContextMapLookup to know more about the Context Map Lookup feature of Log4j. The vulnerability can be exploited only if all of these conditions are true. This set of conditions, we feel reflects AC:H rather than AC:L. Note that the upstream Apache Log4j Security Vulnerabilities https://logging.apache.org/log4j/2.x/security.html also has changed to AC:H after having a discussion with us.

Proposed changes to NIST's current CVSSv3 scoring: AC:L -> AC:H

AC should be High because there are sufficient factors beyond the attacker's control:

  • The application has to use the logging configuration using a Context Map Lookup (for example, $${ctx:loginId}) which is a non-default Pattern Layout.
  • The application developer has to use the map org.apache.logging.log4j.ThreadContext in the application code and save at-least one key (for example, ThreadContext.put("loginId", "myId");) in the ThreadContext map object.
  • Attackers must also know this saved key name in order to exploit this flaw.

Note that saving keys in this map is a non-essential usage of log4j and just an optional feature provided. Refer to https://logging.apache.org/log4j/2.x/manual/lookups.html#ContextMapLookup to know more about the Context Map Lookup feature of Log4j.

The vulnerability can be exploited only if all of these conditions are true. This set of conditions, we feel reflects AC:H rather than AC:L.

Note that the upstream Apache Log4j Security Vulnerabilities https://logging.apache.org/log4j/2.x/security.html also has changed to AC:H after having a discussion with us.

Frequently Asked Questions

Why is Red Hat's CVSS v3 score or Impact different from other vendors?

For open source software shipped by multiple vendors, the CVSS base scores may vary for each vendor's version depending on the version they ship, how they ship it, the platform, and even how the software is compiled. This makes scoring of vulnerabilities difficult for third-party vulnerability databases such as NVD that only provide a single CVSS base score for each vulnerability. Red Hat scores reflect how a vulnerability affects our products specifically.

For more information, see https://access.redhat.com/solutions/762393.

My product is listed as "Under investigation" or "Affected", when will Red Hat release a fix for this vulnerability?

  • "Under investigation" doesn't necessarily mean that the product is affected by this vulnerability. It only means that our Analysis Team is still working on determining whether the product is affected and how it is affected.
  • The term "Affected" means that our Analysis team has determined that this product, such as Red Hat Enterprise Linux 8 or OpenShift Container Platform 4, is affected by this vulnerability and a fix may be released to address this issue in the near future. This includes all minor releases of this product unless noted otherwise in the Statement text.

What can I do if my product is listed as "Will not fix"?

A "will not fix" status means that a fix for an affected product version is not planned or not possible due to complexity, which may create additional risk.

Available options depend mostly on the Impact of the vulnerability and the current Life Cycle phase of your product. Overall, you have the following options:
  • Upgrade to a supported product version that includes a fix for this vulnerability (recommended).
  • Apply a mitigation (if one exists).
  • Open a support case to request a prioritization of releasing a fix for this vulnerability.

What can I do if my product is listed as "Fix deferred"?

A deferred status means that a fix for an affected product version is not guaranteed due to higher-priority development work.

Available options depend mostly on the Impact of the vulnerability and the current Life Cycle phase of your product. Overall, you have the following options:
  • Apply a mitigation (if one exists).
  • Open a support case to request a prioritization of releasing a fix for this vulnerability.
  • Red Hat Engineering focuses on addressing high-priority issues based on their complexity or limited lifecycle support. Therefore, lower-priority issues will not receive immediate fixes.

What is a mitigation?

A mitigation is an action that can be taken to reduce the impact of a security vulnerability, without deploying any fixes.

I have a Red Hat product but it is not in the above list, is it affected?

The listed products were found to include one or more of the components that this vulnerability affects. These products underwent a thorough evaluation to determine their affectedness by this vulnerability. Note that layered products (such as container-based offerings) that consume affected components from any of the products listed in this table may be affected and are not represented.

Why is my security scanner reporting my product as vulnerable to this vulnerability even though my product version is fixed or not affected?

In order to maintain code stability and compatibility, Red Hat usually does not rebase packages to entirely new versions. Instead, we backport fixes and new features to an older version of the package we distribute. This can result in some security scanners that only consider the package version to report the package as vulnerable. To avoid this, we suggest that you use an approved vulnerability scanner from our Red Hat Vulnerability Scanner Certification program.

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