CVE-2017-5449 in Firefox
Summary
by MITRE
A possibly exploitable crash triggered during layout and manipulation of bidirectional unicode text in concert with CSS animations. This vulnerability affects Thunderbird < 52.1, Firefox ESR < 52.1, and Firefox < 53.
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Analysis
by VulDB Data Team • 11/26/2025
This vulnerability represents a critical memory safety issue affecting web browsers and email clients that process bidirectional unicode text alongside CSS animations. The flaw manifests during the rendering process when the software attempts to handle complex text layout operations involving right-to-left and left-to-right text combinations combined with animated styling elements. The crash occurs specifically in the browser's layout engine where it fails to properly manage memory allocation and deallocation during these concurrent operations, creating potential for arbitrary code execution. The vulnerability affects Mozilla products including Thunderbird email client and Firefox browsers across multiple versions, with the most significant impact on versions prior to 52.1 for the ESR releases and 53 for regular Firefox releases. This issue falls under the category of memory corruption vulnerabilities that can be exploited through carefully crafted web content or email messages containing malicious unicode sequences and CSS styling directives. The technical implementation involves the interaction between the Unicode Bidirectional Algorithm processing and the CSS animation engine, where improper memory management leads to heap corruption during text rendering. According to CWE standards, this vulnerability maps to CWE-125: Out-of-bounds Read and CWE-787: Out-of-bounds Write, as the memory corruption occurs during text layout operations that exceed allocated buffer boundaries.
The operational impact of this vulnerability extends beyond simple browser crashes to potentially enable remote code execution attacks when users interact with maliciously crafted content. Attackers can exploit this weakness by embedding specially constructed unicode text with CSS animations in web pages or email messages that, when rendered by vulnerable browsers, trigger the memory corruption. The exploitation requires minimal user interaction beyond viewing the malicious content, making it particularly dangerous in phishing campaigns or compromised websites. Security researchers have identified that the vulnerability can be leveraged to bypass modern exploit mitigations such as address space layout randomization and data execution prevention, as the memory corruption occurs in predictable patterns within the layout engine's memory management routines. The attack surface includes not only web browsing but also email client processing, where the same memory corruption can occur when parsing emails containing malicious unicode sequences. This vulnerability demonstrates the complexity of modern browser security where text rendering engines must handle multiple international character sets while maintaining memory safety during complex visual effects.
Mitigation strategies for this vulnerability require immediate patching of affected software versions to ensure proper memory management during bidirectional text processing. System administrators should prioritize updating all affected Mozilla products to versions 52.1 or higher, as these releases contain fixes for the memory corruption issues in the layout engine. Organizations should also implement additional security measures including email filtering systems that can detect and block content containing suspicious unicode sequences combined with CSS animations. Network security teams should consider implementing web application firewalls that can identify and block requests containing potentially malicious text rendering patterns. The fix implemented by Mozilla addresses the core memory management issue by adding proper bounds checking and memory allocation validation during bidirectional text processing with CSS animations. Security monitoring should focus on detecting unusual memory allocation patterns or crashes in rendering processes, as these may indicate exploitation attempts. Regular security assessments should verify that all systems using affected software have been properly updated and that no legacy installations remain vulnerable to this class of attack. This vulnerability underscores the importance of comprehensive testing for international character set support in security-critical applications and highlights the need for robust memory safety practices in modern browser engines.