CVE-2015-6291 in Email Security Appliance
Summary
by MITRE
Cisco AsyncOS before 8.5.7-043, 9.x before 9.1.1-023, and 9.5.x and 9.6.x before 9.6.0-046 on Email Security Appliance (ESA) devices mishandles malformed fields during body-contains, attachment-contains, every-attachment-contains, attachment-binary-contains, dictionary-match, and attachment-dictionary-match filtering, which allows remote attackers to cause a denial of service (memory consumption) via a crafted attachment in an e-mail message, aka Bug ID CSCuv47151.
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Analysis
by VulDB Data Team • 11/18/2024
The vulnerability described in CVE-2015-6291 represents a critical denial of service flaw within Cisco Email Security Appliance (ESA) devices running specific versions of AsyncOS software. This issue affects multiple software versions including AsyncOS 8.5.7-043 and earlier, 9.x versions before 9.1.1-023, and 9.5.x and 9.6.x versions before 9.6.0-046. The flaw manifests when the ESA device processes email messages containing malformed fields within various filtering contexts such as body-contains, attachment-contains, every-attachment-contains, attachment-binary-contains, dictionary-match, and attachment-dictionary-match rules. These filtering mechanisms are fundamental to the email security appliance's ability to scan and filter incoming email traffic based on content patterns and attachments.
The technical root cause of this vulnerability lies in the improper handling of malformed data structures during the email filtering process. When an attacker crafts a specially formatted email message with maliciously constructed attachment fields, the ESA device fails to properly validate or sanitize these malformed inputs before processing them within its memory management systems. This inadequate input validation creates a condition where the device's memory consumption grows exponentially or becomes completely consumed, leading to a denial of service state where legitimate email traffic can no longer be processed. The vulnerability specifically targets the memory management subsystem of the AsyncOS, which is responsible for maintaining the state and processing of email filtering operations. This behavior aligns with CWE-129, which describes improper validation of input ranges, and CWE-400, which addresses unchecked resource consumption.
The operational impact of CVE-2015-6291 is severe for organizations relying on Cisco ESA for email security. A successful exploitation can result in complete service disruption of the email security appliance, effectively cutting off email communications for the entire organization. The attack requires only a single malicious email message to be delivered to the targeted ESA device, making it particularly dangerous as it can be executed remotely without requiring authentication or privileged access. The memory exhaustion effect can cause the device to become unresponsive, restart automatically, or require manual intervention to restore normal operations. This vulnerability directly impacts the availability aspect of the CIA triad and can be categorized under the ATT&CK technique T1499.004 for network denial of service attacks. Organizations may experience significant downtime and potential business disruption as email services become unavailable, potentially affecting customer communications, internal collaboration, and critical business processes that depend on email infrastructure.
Mitigation strategies for CVE-2015-6291 should focus on immediate software updates to the affected AsyncOS versions, with the recommended approach being to upgrade to the patched versions mentioned in the advisory. Network administrators should implement additional monitoring and alerting mechanisms to detect unusual memory consumption patterns on ESA devices, which could indicate exploitation attempts. Temporary workarounds may include disabling or modifying the vulnerable filtering rules until the software patches are applied. Organizations should also consider implementing email filtering rules that can detect and block suspicious attachment patterns, though this approach is less effective than the proper software patches. The vulnerability highlights the importance of maintaining up-to-date security software and implementing robust patch management processes. Regular security assessments of email infrastructure and monitoring for anomalous behavior in email processing systems can help detect potential exploitation attempts before they cause significant disruption. This vulnerability serves as a reminder of the critical importance of input validation in security-critical systems and the potential for memory-related flaws to cause complete system outages.