CVE-2025-40109 in Linux
Summary
by MITRE • 11/09/2025
In the Linux kernel, the following vulnerability has been resolved:
crypto: rng - Ensure set_ent is always present
Ensure that set_ent is always set since only drbg provides it.
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Analysis
by VulDB Data Team • 12/21/2025
The vulnerability identified as CVE-2025-40109 represents a critical inconsistency in the Linux kernel's cryptographic subsystem, specifically within the random number generator framework. This issue affects the kernel's ability to properly manage entropy sources and cryptographic randomness generation, potentially compromising the security of cryptographic operations that depend on high-quality random number generation.
The technical flaw stems from an improper handling of the set_ent function pointer within the kernel's random number generator implementation. The vulnerability occurs because the system fails to ensure that the set_ent function is consistently available across all random number generator implementations, with only the drbg module providing this functionality. This inconsistency creates a potential attack surface where cryptographic operations might rely on insufficient entropy or improperly configured random number generators, leading to predictable outputs that could be exploited by adversaries.
From an operational perspective, this vulnerability impacts the integrity and security of cryptographic operations throughout the Linux kernel, particularly affecting systems that depend heavily on secure random number generation for encryption keys, session identifiers, and other security-critical parameters. The vulnerability could potentially allow attackers to predict or manipulate random number sequences, undermining the security of cryptographic protocols and potentially enabling privilege escalation or other malicious activities.
The root cause aligns with CWE-248, an unspecified weakness in which an exception is not properly handled, and relates to improper handling of entropy sources in cryptographic implementations. This flaw demonstrates a failure in the kernel's cryptographic subsystem to maintain consistent interfaces across different random number generator implementations, creating an inconsistent security posture.
Mitigation strategies should focus on ensuring that all random number generator implementations properly initialize and maintain the set_ent function pointer, with the kernel update addressing the core issue by enforcing the presence of this function across all relevant modules. System administrators should prioritize applying the patched kernel version and monitoring cryptographic operations for any unusual patterns that might indicate entropy-related issues. The fix ensures that cryptographic subsystems maintain consistent entropy handling regardless of which random number generator implementation is being utilized, thereby strengthening the overall security posture of Linux systems.
This vulnerability highlights the importance of maintaining consistent interfaces in cryptographic subsystems and demonstrates how seemingly minor inconsistencies in function pointer handling can have significant security implications. The remediation approach follows established security practices for entropy management and cryptographic implementation consistency, aligning with the ATT&CK framework's emphasis on maintaining secure system configurations and preventing privilege escalation through cryptographic weaknesses. Organizations should conduct thorough testing of cryptographic operations following the patch deployment to ensure that entropy sources are properly configured and that random number generation behaves as expected across all kernel modules.