CVE-2017-5436 in Firefox
Summary
by MITRE
An out-of-bounds write in the Graphite 2 library triggered with a maliciously crafted Graphite font. This results in a potentially exploitable crash. This issue was fixed in the Graphite 2 library as well as Mozilla products. This vulnerability affects Thunderbird < 52.1, Firefox ESR < 45.9, Firefox ESR < 52.1, and Firefox < 53.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability identified as CVE-2017-5436 represents a critical out-of-bounds write flaw within the Graphite 2 font processing library, which serves as a core component in rendering complex scripts and typography across various software applications. This vulnerability specifically manifests when the Graphite 2 library processes maliciously crafted font files that contain malformed data structures or unexpected parameter values. The flaw occurs during the font rendering process where the library fails to properly validate input parameters before performing memory operations, leading to unauthorized memory access patterns that can overwrite adjacent memory locations beyond the intended buffer boundaries.
The technical nature of this vulnerability places it squarely within the CWE-787 category of out-of-bounds write conditions, which are particularly dangerous because they can lead to arbitrary code execution when exploited. The Graphite 2 library, designed to handle complex text rendering for scripts that require sophisticated shaping algorithms, processes font data through a series of internal functions that manipulate memory structures without adequate bounds checking. When a specially crafted font file is loaded and processed, the library's parsing routines fail to validate the size or structure of various data elements, causing the program to write data beyond allocated memory regions. This memory corruption can potentially be leveraged by attackers to redirect program execution flow or inject malicious code.
The operational impact of this vulnerability extends across multiple software platforms that rely on the Graphite 2 library for text rendering capabilities. The affected software ecosystem includes Mozilla Thunderbird versions prior to 52.1, Firefox Extended Support Release versions before 45.9 and 52.1, and Firefox versions before 53, all of which are susceptible to this memory corruption issue. When exploited, this vulnerability can result in application crashes that may be leveraged for denial-of-service attacks or more severe exploitation techniques. The vulnerability's exploitation potential aligns with ATT&CK technique T1059.007 for application layer attacks, as it targets the font rendering subsystem that operates at the application layer of software execution.
The mitigation strategy for this vulnerability involves updating to patched versions of the affected software components, specifically requiring users to upgrade to Thunderbird 52.1, Firefox ESR 45.9 or 52.1, and Firefox 53 or later versions. Additionally, system administrators should implement proper patch management protocols to ensure all affected applications receive the necessary security updates. The fix implemented in the Graphite 2 library addresses the root cause by introducing proper input validation and bounds checking mechanisms before any memory operations are performed. Organizations should also consider implementing security measures such as application whitelisting, sandboxing, and monitoring for suspicious font file handling activities. The vulnerability serves as a reminder of the importance of validating all external input data, particularly in libraries that handle complex data structures like font rendering engines, and demonstrates how seemingly benign text processing functionality can become a vector for serious security exploits.