Overview
The Linux kernel, a key component of many global server infrastructures, has been found to contain a critical vulnerability in its key management subsystem. The vulnerability, tracked as CVE-2025-21893, pertains to the ‘key_put()’ function, which is responsible for reducing a key’s reference count. When mishandled, this can lead to the premature destruction of keys and consequently, potential system compromise or data leakage. Given the widespread adoption of Linux-based systems, this vulnerability poses a significant risk to organizations and their data, making immediate mitigation a top priority.
Vulnerability Summary
CVE ID: CVE-2025-21893
Severity: High (7.8 CVSS Score)
Attack Vector: Network
Privileges Required: Low
User Interaction: None
Impact: Potential system compromise or data leakage
Affected Products
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Product | Affected Versions
Linux Kernel | Versions prior to patch release
How the Exploit Works
The vulnerability at hand concerns the use-after-free (UAF) flaw in the ‘key_put()’ function of the Linux kernel. This function, responsible for reducing a key’s reference count, can potentially lead to the key’s premature destruction once its count reaches zero. The garbage collector thread may destroy the key at any point after this, making any subsequent touches on the key unauthorized. However, to speed up quota reclamation, the key is looked at in ‘key_put()’ after its usage reaches zero, which is a prohibited operation. This oversight can potentially be exploited by malicious actors to compromise the system or lead to data leakage.
Conceptual Example Code
The following pseudocode illustrates a conceptual example of how the vulnerability might be exploited:
struct key *key;
/*...*/
key = key_get(some_key);
/*...*/
// reduce the reference counter to 0
for (int i = 0; i < key->usage; i++) {
key_put(key);
}
/*...*/
// Now, a malicious actor may exploit the use-after-free vulnerability
malicious_operation(key);It’s important to note that this is a simplified, hypothetical example and actual exploitation would require a more complex understanding of the key management subsystem and the kernel’s memory management.
Impact of Exploit
A successful exploit of this vulnerability could allow an attacker to execute arbitrary code in the context of the kernel, leading to potential system compromise or data leakage. This could have far-reaching impacts, from unauthorized access to sensitive data, to the potential for further system-level exploits.
Recommendations
In light of this vulnerability, users are strongly advised to apply the vendor patch as soon as possible. In the event that immediate patching is not feasible, the use of a Web Application Firewall (WAF) or Intrusion Detection System (IDS) can serve as a temporary mitigation measure. However, these should not be seen as long-term solutions and patching should be undertaken as soon as practical.