CVE-2018-6337 in HHVM
Summary
by MITRE
folly::secureRandom will re-use a buffer between parent and child processes when fork() is called. That will result in multiple forked children producing repeat (or similar) results. This affects HHVM 3.26 prior to 3.26.3 and the folly library between v2017.12.11.00 and v2018.08.09.00.
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Analysis
by VulDB Data Team • 05/06/2025
The vulnerability described in CVE-2018-6337 represents a critical cryptographic weakness in the folly library's secure random number generation mechanism within Facebook's HHVM runtime environment. This issue stems from improper handling of random number generators during process forking operations, creating a significant security risk that affects systems using affected versions of HHVM and the folly library. The flaw specifically impacts the secureRandom function's behavior when fork() system calls are executed, leading to predictable or duplicated random sequences across multiple child processes.
The technical root cause of this vulnerability lies in the improper buffer management within the folly library's implementation of secure random number generation. When a process calls fork(), the child processes inherit the parent's memory space including any active random number generator states. The folly library fails to properly reinitialize or reset the random number generator buffer between parent and child processes, causing subsequent calls to secureRandom in forked children to produce identical or highly correlated outputs. This behavior directly violates fundamental cryptographic principles that require random number generators to produce independent sequences even when processes are forked, as outlined in the cryptographic standards for secure random generation.
The operational impact of this vulnerability extends beyond simple predictability issues, as it can severely compromise security-sensitive applications that rely on random number generation for cryptographic operations, session management, key generation, and nonce creation. Attackers who can observe or influence the execution flow of forked processes can potentially predict future random values, leading to session hijacking, cryptographic key compromise, and other serious security breaches. The vulnerability affects HHVM versions 3.26.0 through 3.26.2 and folly library versions between v2017.12.11.00 and v2018.08.09.00, making it particularly concerning for web applications and services that utilize these components. This weakness aligns with CWE-330, which addresses the use of insufficiently random values in security contexts, and represents a clear violation of the principle that cryptographic random number generators must maintain their security properties even under concurrent or forked execution conditions.
The security implications of this vulnerability can be exploited through various attack vectors that leverage process forking behavior in web applications. When an application forks multiple processes to handle concurrent requests, the predictable random number generation can lead to identical session tokens, repeated cryptographic nonces, or compromised key material. This vulnerability directly maps to ATT&CK technique T1059.007, which involves the use of scripting languages in attack scenarios, as attackers can exploit the predictable random behavior in scripting environments that rely on HHVM. Additionally, the issue demonstrates characteristics of privilege escalation through weak cryptographic implementations, as the vulnerability can be exploited to bypass security controls that depend on unpredictable random values. Organizations should immediately update to HHVM 3.26.3 or later versions and folly library versions beyond v2018.08.09.00 to remediate this vulnerability, as the affected components are commonly used in high-traffic web applications and microservices architectures where the consequences of predictable random number generation can be severe. The fix implemented in the updated versions properly handles the random number generator state management during fork operations, ensuring that each process maintains independent random number generation capabilities and maintains the cryptographic security properties required for production environments.