The Socket Appender in Apache Log4j Core versions 2.0-beta9 through 2.25.2 does not perform TLS hostname verification of the peer certificate, even when the verifyHostName https://logging.apache.org/log4j/2.x/manual/appenders/network.html#SslConfiguration-attr-verifyHostName configuration attribute or the log4j2.sslVerifyHostName https://logging.apache.org/log4j/2.x/manual/systemproperties.html#log4j2.sslVerifyHostName system property is set to true.
This issue may allow a man-in-the-middle attacker to intercept or redirect log traffic under the following conditions:
- The attacker is able to intercept or redirect network traffic between the client and the log receiver.
- The attacker can present a server certificate issued by a certification authority trusted by the Socket Appender’s configured trust store (or by the default Java trust store if no custom trust store is configured).
Users are advised to upgrade to Apache Log4j Core version 2.25.3, which addresses this issue.
As an alternative mitigation, the Socket Appender may be configured to use a private or restricted trust root to limit the set of trusted certificates.
Weakness
The product communicates with a host that provides a certificate, but the product does not properly ensure that the certificate is actually associated with that host.
Affected Software
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Log4j | Apache | 2.0.1 (including) | 2.25.3 (excluding) |
| Log4j | Apache | 2.0 (including) | 2.0 (including) |
| Log4j | Apache | 2.0-beta9 (including) | 2.0-beta9 (including) |
| Log4j | Apache | 2.0-rc1 (including) | 2.0-rc1 (including) |
| Log4j | Apache | 2.0-rc1-rc1 (including) | 2.0-rc1-rc1 (including) |
| Log4j | Apache | 2.0-rc2 (including) | 2.0-rc2 (including) |
| Apache-log4j2 | Ubuntu | esm-infra/xenial | * |
| Apache-log4j2 | Ubuntu | plucky | * |
| Apache-log4j2 | Ubuntu | questing | * |
Extended Description
Even if a certificate is well-formed, signed, and follows the chain of trust, it may simply be a valid certificate for a different site than the site that the product is interacting with. In order to ensure data integrity, the certificate must be valid, and it must pertain to the site that is being accessed. Even if the product attempts to check the hostname, it is still possible to incorrectly check the hostname. For example, attackers could create a certificate with a name that begins with a trusted name followed by a NUL byte, which could cause some string-based comparisons to only examine the portion that contains the trusted name.
Potential Mitigations
References
- https://github.com/apache/logging-log4j2/pull/4002
- https://lists.apache.org/thread/xr33kyxq3sl67lwb61ggvm1fzc8k7dvx
- https://logging.apache.org/cyclonedx/vdr.xml
- https://logging.apache.org/log4j/2.x/manual/appenders/network.html#SslConfiguration-attr-verifyHostName
- https://logging.apache.org/log4j/2.x/manual/systemproperties.html#log4j2.sslVerifyHostName
- https://logging.apache.org/security.html#CVE-2025-68161
- http://www.openwall.com/lists/oss-security/2025/12/18/1
- https://lists.debian.org/debian-lts-announce/2026/01/msg00015.html