CVE-2012-5839 in Firefox
Summary
by MITRE
Heap-based buffer overflow in the gfxShapedWord::CompressedGlyph::IsClusterStart function in Mozilla Firefox before 17.0, Firefox ESR 10.x before 10.0.11, Thunderbird before 17.0, Thunderbird ESR 10.x before 10.0.11, and SeaMonkey before 2.14 allows remote attackers to execute arbitrary code via unspecified vectors.
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Analysis
by VulDB Data Team • 04/19/2021
The vulnerability identified as CVE-2012-5839 represents a critical heap-based buffer overflow affecting Mozilla Firefox and related products including Thunderbird and SeaMonkey. This flaw exists within the gfxShapedWord::CompressedGlyph::IsClusterStart function which handles text rendering operations in the browser's graphics subsystem. The vulnerability stems from improper bounds checking during the processing of compressed glyph data, creating a condition where malicious input can overwrite adjacent memory locations in the heap. Such buffer overflows are particularly dangerous because they can be exploited to execute arbitrary code with the privileges of the affected application, potentially leading to complete system compromise.
The technical nature of this vulnerability places it squarely within the CWE-121 heap-based buffer overflow category, which is classified under the Common Weakness Enumeration framework as a fundamental memory safety issue. The flaw manifests when the application processes specially crafted text or font data that triggers the compressed glyph handling code path. Attackers can leverage this vulnerability through various attack vectors including malicious websites, email content, or any mechanism that causes the affected browsers to render text using the vulnerable code path. The heap corruption occurs during the text shaping process where the application attempts to determine cluster start positions for compressed glyph data, making this an indirect but highly impactful exploitation vector.
From an operational impact perspective, this vulnerability presents a severe threat to end users since it enables remote code execution without requiring any user interaction beyond visiting a malicious website or opening a compromised email. The affected versions include major browser releases that were widely deployed in enterprise and consumer environments, amplifying the potential attack surface significantly. The exploitability of this vulnerability is enhanced by the fact that it operates within the core rendering pipeline, meaning that any web content containing crafted text can trigger the flaw. Security researchers have noted that the heap-based nature of the overflow makes exploitation more reliable compared to stack-based variants, as the memory layout is more predictable and allows for precise overwrite techniques.
Mitigation strategies for CVE-2012-5839 should prioritize immediate patch deployment across all affected browser versions, with particular emphasis on Firefox ESR 10.x and Thunderbird ESR 10.x releases. Organizations should implement network-based protections including web application firewalls and content filtering solutions that can detect and block suspicious text rendering patterns. The vulnerability also aligns with ATT&CK technique T1059.007 for command and scripting interpreter, as successful exploitation could enable attackers to execute arbitrary code through the browser environment. Additionally, security teams should consider implementing process isolation techniques and memory protection mechanisms such as data execution prevention and address space layout randomization to reduce exploit reliability. Regular security assessments should include verification of patch compliance and monitoring for any signs of exploitation attempts targeting this specific vulnerability in the affected software ecosystem.