Stack-Based Out-of-Bounds Read in stunnel

Executive Summary

This is a stack-based out-of-bounds read vulnerability in stunnel's 's_vlog' function. It occurs when handling oversized log messages longer than 1024 bytes via 'vsnprintf'. A remote attacker can send specially crafted protocol inputs that trigger these long log messages, causing the program to read beyond its allocated memory buffer. In some cases, this could also replace trailing newline characters with null bytes.

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Detection Guidance

Monitor stunnel logs for crashes or unusual behavior when handling long protocol inputs. Check for IMAP protocol configurations with server-side negotiation. Use network traffic analysis tools to detect oversized log messages exceeding 1024 bytes.

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Impact Analysis

The main impact is a denial-of-service where the stunnel service crashes. An attacker could send malicious inputs that cause the service to become unstable or stop responding. In specific configurations like IMAP protocol negotiation, an unauthenticated attacker might remotely crash the service without needing privileges or user interaction.

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Mitigation Strategies

Restrict network access to stunnel services to trusted sources only. Disable server-side IMAP protocol negotiation if configured. Monitor for crashes and apply vendor patches once available.

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