Apache Log4j Cores Rfc5424Layout https://logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layout , in versions 2.21.0 through 2.25.3, is vulnerable to log injection via CRLF sequences due to undocumented renames of security-relevant configuration attributes.
Two distinct issues affect users of stream-based syslog services who configure Rfc5424Layout directly:
- The newLineEscape attribute was silently renamed, causing newline escaping to stop working for users of TCP framing (RFC 6587), exposing them to CRLF injection in log output.
- The useTlsMessageFormat attribute was silently renamed, causing users of TLS framing (RFC 5425) to be silently downgraded to unframed TCP (RFC 6587), without newline escaping.
Users of the SyslogAppender are not affected, as its configuration attributes were not modified.
Users are advised to upgrade to Apache Log4j Core 2.25.4, which corrects this issue.
Weakness
The product constructs a log message from external input, but it does not neutralize or incorrectly neutralizes special elements when the message is written to a log file.
Affected Software
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Log4j | Apache | 2.21.0 (including) | 2.25.4 (excluding) |
| Log4j | Apache | 3.0.0-beta1 (including) | 3.0.0-beta1 (including) |
| Log4j | Apache | 3.0.0-beta2 (including) | 3.0.0-beta2 (including) |
| Log4j | Apache | 3.0.0-beta3 (including) | 3.0.0-beta3 (including) |
| Red Hat build of Apache Camel 4.18.1.P1 for Spring Boot 3.5.16 | RedHat | log4j-core | * |
| Red Hat Data Grid 8.6.1 | RedHat | log4j-core | * |
| Red Hat Data Grid 8.6.1 | RedHat | log4j-core-test | * |
| Red Hat Offline Knowledge Portal 1.2.4 | RedHat | offline-knowledge-portal/rhokp-rhel9:1779996999 | * |
| Red Hat OpenShift AI 2.25 | RedHat | rhoai/odh-modelmesh-rhel9:1783443816 | * |
| Apache-log4j1.2 | Ubuntu | esm-apps/xenial | * |
| Apache-log4j1.2 | Ubuntu | questing | * |
| Apache-log4j2 | Ubuntu | esm-infra/xenial | * |
| Apache-log4j2 | Ubuntu | questing | * |
Potential Mitigations
- Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
- When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
- Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
Related Attack Patterns
- https://cwe.mitre.org/data/definitions/268.html
- https://cwe.mitre.org/data/definitions/81.html
- https://cwe.mitre.org/data/definitions/93.html
References
- https://github.com/apache/logging-log4j2/pull/4074
- https://lists.apache.org/thread/3k1clr2l6vkdnl4cbhjrnt1nyjvb5gwt
- https://logging.apache.org/cyclonedx/vdr.xml
- https://logging.apache.org/log4j/2.x/manual/layouts.html#RFC5424Layout
- https://logging.apache.org/security.html#CVE-2026-34478
- http://www.openwall.com/lists/oss-security/2026/04/10/7