CVE-2017-17846 in Enigmail
Summary
by MITRE
An issue was discovered in Enigmail before 1.9.9. Regular expressions are exploitable for Denial of Service, because of attempts to match arbitrarily long strings, aka TBE-01-003.
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Analysis
by VulDB Data Team • 01/19/2023
The vulnerability identified as CVE-2017-17846 represents a critical denial of service flaw within Enigmail, a popular email encryption plugin for Mozilla Thunderbird and SeaMonkey. This issue affects versions prior to 1.9.9 and stems from improper handling of regular expressions during string matching operations. The vulnerability operates under the threat model of a malicious actor who can craft specially designed input strings that cause the regular expression engine to consume excessive computational resources during pattern matching. This flaw specifically manifests when the plugin attempts to match arbitrarily long strings against regular expressions, creating a condition where the system's processing time and memory consumption grow exponentially with input length. The vulnerability is categorized under CWE-400, which deals with Uncontrolled Resource Consumption, and more specifically relates to CWE-1321, which addresses Regular Expression Denial of Service. From an operational perspective, this vulnerability can be exploited by attackers who send specially crafted emails containing malformed content that triggers the vulnerable regular expression patterns. The impact extends beyond simple service disruption as it can lead to complete system unresponsiveness or crash the email client entirely, potentially affecting users who rely on encrypted email communications for sensitive data protection.
The technical exploitation of this vulnerability occurs through the manipulation of regular expression patterns that are used for parsing and validating email content within the Enigmail plugin. When the plugin processes incoming emails, it performs various string matching operations using regular expressions to identify and extract specific data patterns for encryption and decryption purposes. The flaw arises when these regular expressions are designed without proper bounds checking or resource limitations, allowing attackers to craft inputs that cause the regular expression engine to enter into computationally expensive backtracking operations. This type of attack falls under the ATT&CK technique T1499.004, which covers Network Denial of Service, and more specifically aligns with T1059.007 for Command and Scripting Interpreter. The vulnerability is particularly dangerous because it can be triggered by routine email processing without requiring any special privileges or user interaction beyond receiving an email. The exponential growth in processing time means that even relatively small malicious inputs can cause significant system impact, making this a particularly effective vector for resource exhaustion attacks. The regular expression engine in question likely employs backtracking algorithms that can exponentially increase processing time as input length increases, especially when dealing with nested quantifiers or alternation patterns that create multiple possible match paths.
Mitigation strategies for CVE-2017-17846 focus primarily on upgrading to Enigmail version 1.9.9 or later, which includes patches that address the vulnerable regular expression patterns. Organizations should implement comprehensive patch management procedures to ensure all instances of Enigmail are updated promptly. Additional defensive measures include configuring email filtering systems to scrutinize incoming messages for suspicious content patterns that might trigger the vulnerability, though this approach is less reliable than direct patching. Network administrators should also consider implementing rate limiting and resource monitoring to detect unusual processing patterns that might indicate exploitation attempts. The vulnerability demonstrates the importance of input validation and resource management in security-critical applications, particularly those involving regular expression processing. From a security architecture standpoint, this vulnerability highlights the need for implementing proper bounds checking and resource allocation limits when using regular expressions in security-sensitive contexts. Organizations should also consider implementing automated vulnerability scanning tools that can detect the presence of vulnerable regular expression patterns in their software components. The incident underscores the necessity of following secure coding practices and conducting thorough security testing, particularly for applications that process untrusted input data, as outlined in the OWASP Secure Coding Practices guidelines. Implementation of these mitigations requires careful coordination between security teams and system administrators to ensure complete remediation across all affected systems while minimizing operational disruption.