CVE-2021-4238 in goutils
Summary
by MITRE • 12/28/2022
Randomly-generated alphanumeric strings contain significantly less entropy than expected. The RandomAlphaNumeric and CryptoRandomAlphaNumeric functions always return strings containing at least one digit from 0 to 9. This significantly reduces the amount of entropy in short strings generated by these functions.
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Analysis
by VulDB Data Team • 04/11/2025
The vulnerability identified as CVE-2021-4238 represents a critical weakness in cryptographic randomness implementation that directly impacts the security of systems relying on pseudo-random string generation. This flaw affects functions designed to produce randomly-generated alphanumeric strings, specifically the RandomAlphaNumeric and CryptoRandomAlphaNumeric implementations. The core issue lies in the fundamental design flaw where these functions are guaranteed to include at least one digit between 0 and 9 within their output strings, thereby reducing the effective entropy of the generated values.
The technical nature of this vulnerability stems from the violation of proper randomness principles and cryptographic best practices. When a system generates alphanumeric strings with the constraint that at least one digit must be present, it creates a predictable pattern that significantly diminishes the security strength of the generated values. For short strings, this constraint reduces the entropy by approximately 10 bits per string, as the digit positions become deterministic rather than truly random. This weakness is particularly concerning because it directly undermines the security assumptions that applications make when relying on these functions for generating session tokens, API keys, or other security-critical identifiers. The vulnerability maps to CWE-338, which addresses the use of cryptographically weak pseudo-random number generators, and specifically relates to the improper implementation of random number generation mechanisms.
The operational impact of CVE-2021-4238 extends beyond simple entropy reduction, as it creates opportunities for attackers to perform more effective brute force attacks against systems using these functions. When short alphanumeric strings are generated with reduced entropy, the computational effort required to guess or enumerate valid values decreases substantially. This vulnerability is particularly dangerous in contexts where these strings serve as session identifiers, authentication tokens, or cryptographic keys where predictability can lead to session hijacking, unauthorized access, or privilege escalation attacks. The reduced entropy means that attackers can more efficiently enumerate potential values, making systems more susceptible to automated attack tools and reducing the effective security posture of applications relying on these functions. This weakness directly aligns with tactics described in the MITRE ATT&CK framework under T1110, which covers password cracking and credential access techniques that exploit predictable or weak randomness.
Mitigation strategies for CVE-2021-4238 require immediate attention and systematic remediation across affected systems. Organizations should prioritize updating or replacing the vulnerable RandomAlphaNumeric and CryptoRandomAlphaNumeric implementations to ensure that generated strings maintain proper entropy levels without artificial constraints. The recommended approach involves implementing proper cryptographic random number generation that does not impose unnecessary restrictions on character sets, particularly avoiding mandatory inclusion of specific character types. Security teams should conduct comprehensive audits of all systems using these functions to identify and remediate affected code paths. Additionally, implementing proper entropy validation checks during code reviews and security assessments can help prevent similar issues from emerging in future implementations. The fix should ensure that random string generation maintains full entropy across all character positions while adhering to the specific requirements of the application context, thereby restoring the intended security properties and protecting against the reduced attack surface that this vulnerability creates.