CVE-2026-79515 in stb
Summary
by MITRE • 09/09/2026
An out-of-bounds read in the stbtt_GetGlyphShape component of nothings stb commit 31c1ad3 allows attackers to cause a Denial of Service (DoS) via sending a crafted TTF file.
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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability identified as an out-of-bounds read within the stb_truetype library, specifically in the stbtt_GetGlyphShape function, represents a critical memory safety flaw that can be exploited to disrupt service availability. This single-header C library is widely adopted across various software ecosystems for rendering TrueType fonts due to its simplicity and ease of integration. The specific commit referenced indicates that despite ongoing maintenance efforts, certain edge cases in font parsing logic were not adequately guarded against malformed input data. When an attacker provides a specially crafted TTF file containing maliciously constructed glyph shape definitions, the library fails to perform sufficient bounds checking before accessing memory locations associated with those shapes. This lack of validation allows the application to read from memory regions that are outside the intended buffer boundaries, leading to undefined behavior that typically manifests as a crash or segmentation fault in standard execution environments.
From a technical perspective, this flaw aligns closely with CWE-125, which describes an out-of-bounds read vulnerability where software reads data past the end of a buffer. In the context of font processing, glyph shapes are often represented by complex arrays of points and contours that define the visual outline of characters. The stbtt_GetGlyphShape function is responsible for parsing these structures to generate renderable geometry. If the input TTF file contains corrupted or intentionally malformed data regarding contour counts or point indices, the parser may attempt to access array elements that do not exist within the allocated memory space. This results in reading garbage values from adjacent memory sectors, which can corrupt internal state variables or trigger immediate termination of the process if the operating system detects an illegal memory access. The severity lies in the fact that font rendering is often a core functionality for many applications, including web browsers, document viewers, and graphic design tools, making this vulnerability highly impactful for end-users who encounter maliciously designed documents or web pages containing embedded fonts.
The operational impact of this vulnerability is primarily centered on Denial of Service conditions. Since the exploit does not require authentication and can be triggered simply by loading a compromised file or visiting a webpage with a hostile font resource, it poses a significant risk to service continuity. An attacker could distribute such files through email attachments, malicious websites, or software update mechanisms that fail to validate third-party assets thoroughly. Upon execution, the application hosting the vulnerable library will likely crash unexpectedly, causing data loss for unsaved work and requiring manual intervention to restart services. In server-side applications where font rendering is performed on demand, this could lead to resource exhaustion if multiple concurrent requests trigger crashes simultaneously, effectively taking down the service until administrators can patch or mitigate the issue. This scenario reflects common attack patterns found in ATT&CK technique T1496, which involves environmental preparation for denial of service attacks by leveraging software vulnerabilities to degrade system performance and availability without necessarily compromising data integrity or confidentiality.
Mitigation strategies must focus on both immediate remediation and long-term defensive coding practices. The most effective solution is to update the stb_truetype library to a version that includes patches addressing this specific out-of-bounds read condition, ensuring that all array accesses are validated against buffer limits before execution. Developers integrating this header file should verify their dependency versions regularly through automated supply chain security tools. Additionally, implementing input validation at the application level can provide an additional layer of defense by sanitizing font files before they reach the rendering engine. This includes checking for maximum allowable sizes and verifying structural integrity of TTF components prior to parsing. Security teams should also consider deploying runtime protection mechanisms such as Address Sanitizer during testing phases to detect similar memory errors early in development cycles, thereby preventing such vulnerabilities from reaching production environments where they could be exploited by adversaries seeking to disrupt critical operations.