CVE-2017-15705 in SpamAssassin
Summary
by MITRE
A denial of service vulnerability was identified that exists in Apache SpamAssassin before 3.4.2. The vulnerability arises with certain unclosed tags in emails that cause markup to be handled incorrectly leading to scan timeouts. In Apache SpamAssassin, using HTML::Parser, we setup an object and hook into the begin and end tag event handlers In both cases, the "open" event is immediately followed by a "close" event - even if the tag *does not* close in the HTML being parsed. Because of this, we are missing the "text" event to deal with the object normally. This can cause carefully crafted emails that might take more scan time than expected leading to a Denial of Service. The issue is possibly a bug or design decision in HTML::Parser that specifically impacts the way Apache SpamAssassin uses the module with poorly formed html. The exploit has been seen in the wild but not believed to have been purposefully part of a Denial of Service attempt. We are concerned that there may be attempts to abuse the vulnerability in the future.
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Analysis
by VulDB Data Team • 05/16/2023
The vulnerability identified as CVE-2017-15705 represents a critical denial of service weakness in Apache SpamAssassin versions prior to 3.4.2, fundamentally rooted in how the application processes malformed HTML content through its dependency on the HTML::Parser module. This weakness specifically targets the handling of unclosed HTML tags, creating a scenario where the parsing engine becomes inefficient and consumes excessive processing time during email analysis. The core issue manifests when SpamAssassin encounters HTML markup with improperly closed tags, causing the HTML::Parser to behave in an unexpected manner that disrupts normal processing workflows.
The technical flaw operates through a design interaction between Apache SpamAssassin's use of HTML::Parser and the module's event handling mechanism. When processing HTML content, the system establishes object hooks for begin and end tag events, but the parser's behavior causes an immediate "close" event to follow every "open" event regardless of whether the actual HTML tag is properly closed. This creates a cascading effect where the parser fails to properly handle text events between opening and closing tags, leading to inefficient memory usage and extended processing times. The vulnerability is particularly insidious because it exploits the way the parser manages state transitions, causing legitimate email processing to consume resources far beyond normal expectations.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially compromising entire email processing systems. When attackers craft emails with carefully constructed malformed HTML sequences, they can trigger scan timeouts that effectively render the spam filtering system unavailable for legitimate email processing. This creates a denial of service condition where the system becomes overwhelmed with processing tasks that should be simple text parsing operations. The vulnerability's potential for abuse is heightened by the fact that it can be triggered through normal email traffic without requiring sophisticated attack techniques, making it particularly dangerous for organizations relying on SpamAssassin for email security.
Security practitioners should recognize this vulnerability as mapping to CWE-400, which addresses "Uncontrolled Resource Consumption" and aligns with ATT&CK technique T1499.3, "Resource Hijacking," as the vulnerability enables attackers to consume excessive computational resources. The exploitability of CVE-2017-15705 demonstrates how seemingly minor parsing issues in widely used libraries can create significant security concerns when integrated into critical security infrastructure. Organizations must prioritize updating to Apache SpamAssassin version 3.4.2 or later, as this release contains the necessary patches to address the HTML::Parser integration issues. Additionally, implementing email filtering rules to detect and quarantine malformed HTML content, along with monitoring for unusual processing times, provides additional defense layers against potential exploitation of this vulnerability in environments where immediate patching may not be feasible.