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XZ Utils (previously LZMA Utils) is a set of free software command-line lossless data compressors for Unix-like operating systems and, from version 5.0 onwards, Microsoft Windows. For compression/decompression the Lempel–Ziv–Markov chain algorithm (LZMA) is used. XZ Utils consists of two major components:
Malicious code was discovered in the upstream tarballs of XZ Utils, starting with version 5.6.0. On 29 March 2024, a thread was published on Openwall’s oss-security mailing list showing that the code of liblzma was potentially compromised. The thread author identified compressed test files which have been added to the code for setting up a backdoor via additions to the configure script in the tar files. A modified version of build-to-host.m4 was included in the release tar file uploaded on GitHub, which extracts a script that performs the actual injection into liblzma. GitHub has terminated the repository for XZ due to terms of service violation, presumably because of the supply chain attack.
Under the right circumstances this could potentially enable a malicious actor to break sshd authentication and gain unauthorised remote access to the system remotely via protocols including SSH, leading to potential total compromise and sequestration of the system.
CISA recommends developers and users to downgrade XZ Utils to an uncompromised version – such as XZ Utils 5.4.6 Stable. Vendors including Red Hat advise immediately stopping use of the product until it is remediated.
NOTE: Remediation of this vulnerability by patching to a specific version indicated may not be sufficient to secure the product against further vulnerabilities discovered in later versions, subsequent to the publication of this guidance. Unless contra-indicated, customers are therefore advised to always upgrade to the latest version of the product available.
(The vendor has not advised of any alternative temporary mitigation or workarounds)
NOTE: Caution should always be taken in applying any temporary mitigations listed. Mitigations are only recommended in cases where patches to remediate the vulnerability are not available, or cannot safely be applied to a given environment immediately. A given mitigation may not in all cases be recommended officially by the application vendor. The viability of any given temporary mitigation measure may vary, depending on server platform and existing configuration. Mitigations listed may incompletely remediate any given vulnerability. Configuration changes to implement listed mitigations may impact/disrupt required functionality within a given customer application. Care should therefore be taken to carefully analyse any listed mitigations for appropriateness to a given environment. Customers are advised to test any configuration changes prior to their being introduced into a production environment.
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