Description

openssl_encrypt versions before 1.4.0 use an in-memory rate limiter for TOTP brute-force protection that is not shared across workers and is lost on server restart. Attackers can distribute authentication attempts across multiple server instances or retry immediately after a restart to bypass rate limiting protections.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "9.8",
  "pubDate": "2026-08-17T11:16:42.330Z",
  "pubdate": "2026-08-17T11:16:42.330Z",
  "executiveSummary": "An architectural security flaw exists in openssl_encrypt versions before 1.4.0 regarding Time-based One-Time Password (TOTP) brute-force protection mechanisms.\nThe vulnerability involves the implementation of an in-memory rate limiter that lacks synchronization across multiple server worker processes and state persistence across server restarts.\nThe primary impact of this vulnerability is the complete bypass of rate-limiting protections designed to mitigate brute-force and credential stuffing attacks against multi-factor authentication (MFA) endpoints.\nAffected systems are applications utilizing openssl_encrypt versions prior to 1.4.0 running in clustered environments or worker-based architectures.\nThe risk implication is high, as malicious actors can systematically guess TOTP tokens without triggering defensive lockouts or throttling mechanisms.\nAttacker capabilities include the ability to execute high-velocity enumeration attacks by distributing authentication attempts across distinct worker instances or by re-initiating brute-force campaigns immediately following server reboots.\nNo specific authentication or specialized privileges are required to target the vulnerable endpoint, lowering the overall barrier to exploitation.",
  "technicalDetails": "The root cause of the vulnerability stems from the reliance on a localized, volatile in-memory data structure within openssl_encrypt versions before 1.4.0 to track and enforce TOTP verification attempt thresholds.\nIn multi-worker web server deployments, each worker maintains an independent memory space, preventing the rate limiter from aggregating global attempt counters across instances.\nFurthermore, because the state is stored exclusively in volatile memory rather than a centralized, persistent data store (such as a distributed cache or database), any server restart completely flushes the attempt history.\nThe vulnerable component is the internal rate-limiting subsystem handling TOTP validation within openssl_encrypt.\nExploitation occurs methodically through distributed request routing and state-reset manipulation.\nDuring an attack flow, an adversary initiates a brute-force attack targeting the TOTP validation mechanism.\nBecause the application utilizes multiple worker processes, the attacker routes individual HTTP requests across different worker threads or server instances.\nEach worker evaluates the incoming authentication attempt against its own isolated rate limiter instance, failing to recognize the cumulative number of failed attempts occurring across sibling workers.\nConsequently, the threshold configured to trigger a lockout or delay is never reached on any single worker.\nAlternatively, if a rate-limiting threshold is momentarily triggered on a specific worker, the attacker or an automated orchestration script can induce or wait for a server restart, which purges the volatile in-memory state and instantly restores full authentication capacity.\nThis architectural limitation completely neutralizes the defensive posture of the TOTP implementation, allowing adversaries to exhaust the limited keyspace (typically 1,000,000 possible 6-digit codes) within an operationally feasible timeframe.\nNetwork exposure is defined by the public-facing accessibility of the authentication endpoint consuming the vulnerable component.\nNo pre-existing privileges or authenticated sessions are required to initiate the attack flow, as the vulnerability resides within the pre-authentication verification phase of the multi-factor authentication challenge."
}