CVE-2011-1947 in fetchmail
Summary
by MITRE
fetchmail 5.9.9 through 6.3.19 does not properly limit the wait time after issuing a (1) STARTTLS or (2) STLS request, which allows remote servers to cause a denial of service (application hang) by acknowledging the request but not sending additional packets.
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Analysis
by VulDB Data Team • 12/28/2024
The vulnerability identified as CVE-2011-1947 affects fetchmail versions 5.9.9 through 6.3.19 and represents a significant denial of service weakness in email retrieval software. This flaw specifically impacts the application's handling of secure communication protocols during the STARTTLS and STLS handshake processes, creating a condition where remote servers can manipulate the application into indefinite waiting states. The vulnerability stems from insufficient timeout mechanisms that fail to properly terminate connection attempts when remote servers acknowledge handshake requests without completing the necessary data exchange. This behavior creates a window where fetchmail remains in a suspended state, effectively hanging the application and rendering it unable to process additional email requests until manually restarted.
The technical implementation of this vulnerability involves the fetchmail application's failure to enforce proper timeout constraints during secure protocol negotiation phases. When fetchmail initiates either a STARTTLS or STLS request to establish encrypted communication with remote mail servers, the application enters a waiting state expecting subsequent protocol data packets. However, malicious or misconfigured remote servers can acknowledge the initial handshake request with a positive response while simultaneously failing to transmit the required follow-up data. This creates a scenario where fetchmail waits indefinitely for data that will never arrive, causing the application to hang and consume system resources without processing additional email messages. The flaw operates at the network protocol level, specifically targeting the application's inability to distinguish between legitimate protocol delays and malicious resource exhaustion attempts.
From an operational impact perspective, this vulnerability enables attackers to perform effective denial of service attacks against systems running affected fetchmail versions. The hanging behavior can be sustained indefinitely, potentially exhausting system resources such as process memory, file descriptors, and network connections. Organizations relying on fetchmail for email retrieval operations may experience complete service disruption, with email processing ceasing entirely until the affected application is manually terminated and restarted. The vulnerability is particularly concerning because it can be exploited remotely without requiring authentication or special privileges, making it accessible to any attacker who can establish connections to the target system. This characteristic aligns with attack patterns documented in the attack mitigation framework where simple network-based attacks can cause significant operational disruption.
The vulnerability maps directly to CWE-400, which describes "Uncontrolled Resource Consumption" and specifically addresses situations where applications fail to properly manage resource allocation during network operations. The flaw also relates to CWE-617, "Reachable Assertion," as the application's failure to properly handle timeout conditions represents an assertion failure that leads to resource exhaustion. From an ATT&CK framework perspective, this vulnerability corresponds to technique T1499.004 "Endpoint Denial of Service" and T1595.001 "Network Denial of Service," demonstrating how network-based protocols can be exploited to create service disruption. The vulnerability's impact extends beyond simple application hanging to potentially affect system availability, as the hanging processes may prevent other email-related services from functioning properly and could trigger system-level resource exhaustion if multiple connections are simultaneously affected.
Mitigation strategies for CVE-2011-1947 should focus on implementing proper timeout configurations and protocol handling within fetchmail. The most effective immediate solution involves upgrading to fetchmail version 6.3.20 or later, which contains the necessary patches to address the timeout handling issue. Organizations should also implement network-level protections such as connection timeouts, maximum wait time limits, and rate limiting to prevent exploitation attempts. System administrators should configure fetchmail with explicit timeout parameters to ensure that connection attempts do not remain indefinitely in waiting states. Additionally, implementing network monitoring and alerting systems can help detect unusual connection patterns that may indicate exploitation attempts. The vulnerability underscores the importance of proper timeout implementation in network applications and serves as a reminder of the critical need for robust resource management in security-sensitive software components.