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@@ -46,6 +46,82 @@ If you have encountered an unlisted security vulnerability or other unexpected b
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The threat model that Log4j uses considers configuration files as safe input controlled by the programmer; **potential vulnerabilities that require the ability to modify a configuration are not considered vulnerabilities** as the required access to do so implies the attacker can execute arbitrary code.
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====
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[#model]
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== Threat Model
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Log4j is a low level library where configuration inputs and the classpath are expected to be controlled by the programmer.
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Configurations have the ability to execute arbitrary code.
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While specific Log4j plugins (such as a JNDI lookup) may use constraint validators or conditionals to require additional settings to opt in to functionality, this is not universally required by custom plugins.
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Specific security considerations involved in our threat model are detailed below.
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=== Parameterized Logging
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When using a log message containing template parameters like `{}`, only the format string is evaluated for parameters to be substituted.
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The message parameters themselves are not evaluated for parameters; they are only included in the format string corresponding to their template position.
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The conversion of message parameters into a string is done on-demand depending on the layout being used.
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When structure-preserving transformations of log message data are required, the `Message` API should be used for logging structured data combined with a structured layout (e.g., `JsonTemplateLayout`).
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Format strings should be compile-time constants, and under no circumstances should format strings be built using user-controlled input data.
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=== Unstructured Logging
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When using an unstructured layout such as `PatternLayout`, no guarantees can be made about the output format.
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This layout is mainly useful for development purposes and should not be relied on in production applications.
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For example, if a log message contains new lines, these are not escaped or encoded specially unless the configured pattern uses the `%encode{pattern}{CRLF}` wrapper pattern converter (which will encode a carriage return as the string `\r` and a line feed as the string `\n`) or some other `%encode` option.
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Note that `%xEx` is appended to the pattern unless already present.
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Similarly, other encoding options are available for other formats, but pattern layouts cannot make assumptions about the entire output.
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As such, when using unstructured layouts, no user-controlled input should be included in logs.
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It is strongly recommended that a structured layout (e.g., `JsonTemplateLayout`) is used instead for these situations.
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Note that `StrLookup` plugins (those referenced by `${...}` templates in configuration files) that contain user-provided input should not be referenced by layouts.
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=== Structured Logging
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When using a structured layout (most layouts besides pattern layout), log messages are encoded according to various output formats.
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These safely encode the various fields included in a log message.
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For example, the `JsonTemplateLayout` can be configured to output log messages in various JSON structures where all log data is properly encoded into safely parseable JSON.
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This is the recommended mode of operation for use with log parsing and log collection tools that rely on log files or arbitrary output streams.
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=== Code Signing
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Log4j artifacts are all signed using PGP using a key from the Logging Services PMC https://downloads.apache.org/logging/KEYS[KEYS file].
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Information on how to verify releases signed with PGP is https://httpd.apache.org/dev/verification.html[documented here].
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Thus, PGP signatures should be validated in your build process.
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=== Java Security Manager
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Log4j no longer supports running in or using a custom `SecurityManager`.
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This Java feature has been deprecated for removal in Java 21.
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Previous versions of Log4j 2.x include partial support for running with a security manager.
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=== Log Masking
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Log4j, like any other generic logging library, cannot generically support log masking of sensitive data.
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While custom plugins may be developed to attempt to mask various regular expressions (such as a string that looks like a credit card number), the general problem of log masking is equivalent to the halting problem in computer science where sensitive data can always be obfuscated in such a way as to avoid detection by log masking.
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As such, it is the responsibility of the developer to properly demarcate sensitive data such that it can be consistently masked by log masking plugins.
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This sort of use case should make use of the `Message` API for better control over the output of such data.
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=== Availability
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Log4j goes to great lengths to minimize performance overhead along with options for minimizing latency or maximizing throughput.
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However, we cannot guarantee availability of the application if the appenders cannot keep up with the logs being written.
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Synchronous logging can cause applications to block and wait for a log message to be written.
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Asynchronous logging can also cause applications to block and wait depending on the wait strategy and queue full policy configured.
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Configuring too large or too many buffers in Log4j can also result in out of memory errors.
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=== Configuration Sources
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All configuration sources to an application must be trusted by the programmer.
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When loading a configuration file from disk (especially when a monitor interval is configured to reload the file periodically), the location of the configuration file must be kept safe from unauthorized modifications.
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Similarly, when loading a configuration file over the network such as through HTTP, this should be configured to use TLS or a secure connection in general with strong authentication guarantees.
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This remote location must be kept safe from unauthorized modifications.
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When configurations are modified through JMX, the JMX server should be safely configured to require authentication and a secure connection if being accessed over the network.
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When configurations are provided through JNDI, these should only use the `java` scheme for sharing configurations in a JavaEE or JakartaEE application service.
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JNDI-sourced configurations should not use other JNDI providers such as LDAP, DNS, or RMI, as all these providers are difficult to properly secure.
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=== Compressing Logs
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If log compression is used along with custom encryption where logs contain user-controlled input, then this can lead to a https://en.wikipedia.org/wiki/CRIME[CRIME attack] style vulnerability where a chosen-plaintext attack is combined with information leakage caused by how the compression algorithm handles different inputs.
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The simplest way to avoid this problem is to never combine compression with encryption when encoding user-controlled input.
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