CVE-2017-7753 in Firefox
Summary
by MITRE
An out-of-bounds read occurs when applying style rules to pseudo-elements, such as ::first-line, using cached style data. This vulnerability affects Thunderbird < 52.3, Firefox ESR < 52.3, and Firefox < 55.
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Analysis
by VulDB Data Team • 11/26/2025
This vulnerability represents a critical out-of-bounds read flaw in Mozilla's browser engine that manifests when processing CSS style rules applied to pseudo-elements. The issue specifically occurs during the handling of cached style data, where the application fails to properly validate memory boundaries when accessing style information for elements like ::first-line. The flaw exists in the rendering engine's CSS processing pipeline and affects multiple Mozilla products including Thunderbird and various Firefox versions. The out-of-bounds read vulnerability falls under the CWE-125 weakness category, which specifically addresses out-of-bounds read conditions that can lead to information disclosure or potential code execution. This vulnerability is particularly concerning because it operates within the browser's core rendering functionality where CSS processing occurs during page rendering.
The technical implementation of this flaw involves the browser's style system maintaining cached data structures for pseudo-elements to improve performance. When these cached structures are reused for subsequent page renderings, the system fails to properly validate the bounds of memory accesses when applying style rules. This allows an attacker to craft malicious CSS content that, when processed, triggers the out-of-bounds read condition. The vulnerability is particularly dangerous because it can be exploited through web content without requiring user interaction beyond visiting a malicious website. The flaw demonstrates poor memory management practices in the CSS engine's handling of cached style data, where proper bounds checking is bypassed during pseudo-element style application.
The operational impact of this vulnerability extends beyond simple information disclosure, potentially enabling more severe attacks including remote code execution. Attackers can leverage this flaw by embedding malicious CSS in web pages that trigger the specific code path involving cached pseudo-element styles. When the browser processes these styles, the out-of-bounds read can expose sensitive memory contents or potentially allow attackers to manipulate memory layout for further exploitation. The vulnerability affects both desktop and mobile browser implementations, making it a widespread concern across Mozilla's product ecosystem. According to ATT&CK framework, this vulnerability maps to T1059.007 for script-based attacks and potentially T1068 for privilege escalation through memory corruption.
Mitigation strategies for this vulnerability require immediate patching of affected software versions, with Mozilla releasing updates for Thunderbird 52.3 and Firefox ESR 52.3 as well as Firefox 55. Organizations should implement comprehensive browser update policies to ensure all systems receive the necessary security patches. Additional defensive measures include implementing content security policies that restrict CSS processing, monitoring for unusual memory access patterns, and deploying web application firewalls that can detect and block malicious CSS content. The vulnerability highlights the importance of proper bounds checking in memory-intensive operations and demonstrates how seemingly benign CSS processing can become a vector for serious security exploits. Security teams should prioritize this vulnerability in their assessment matrices due to its potential for remote code execution and the widespread use of affected browser versions across enterprise environments.