CVE-2015-8540 in libpng
Summary
by MITRE
Integer underflow in the png_check_keyword function in pngwutil.c in libpng 0.90 through 0.99, 1.0.x before 1.0.66, 1.1.x and 1.2.x before 1.2.56, 1.3.x and 1.4.x before 1.4.19, and 1.5.x before 1.5.26 allows remote attackers to have unspecified impact via a space character as a keyword in a PNG image, which triggers an out-of-bounds read.
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Analysis
by VulDB Data Team • 06/29/2026
The vulnerability identified as CVE-2015-8540 represents a critical integer underflow condition within the libpng library's png_check_keyword function located in the pngwutil.c source file. This flaw affects multiple versions of the widely-used PNG image library, spanning from version 0.90 through 0.99 up to various 1.x series releases including 1.0.x before 1.0.66, 1.1.x, 1.2.x before 1.2.56, 1.3.x, 1.4.x before 1.4.19, and 1.5.x before 1.5.26. The vulnerability specifically manifests when processing PNG images containing a space character as part of a keyword field, creating a scenario where the library fails to properly validate integer values during keyword processing operations.
The technical nature of this vulnerability stems from an integer underflow condition that occurs during the png_check_keyword function execution. When a PNG image contains a space character within a keyword field, the function performs calculations that result in an integer underflow, subsequently triggering an out-of-bounds read operation. This memory access violation can lead to unpredictable behavior including application crashes, data corruption, or potentially more severe consequences depending on the execution environment. The vulnerability operates at the boundary between valid and invalid memory access patterns, exploiting the library's insufficient input validation mechanisms for keyword fields in PNG metadata.
From an operational perspective, this vulnerability presents significant risks to systems that process PNG images from untrusted sources, which is common in web applications, image processing services, and content management systems. Remote attackers can exploit this flaw by crafting malicious PNG files containing space characters in keyword fields, potentially leading to denial of service conditions or information disclosure. The unspecified impact mentioned in the vulnerability description indicates that depending on the system configuration and memory layout, attackers might be able to leverage this condition for more advanced exploitation techniques. The widespread adoption of libpng across numerous applications and operating systems amplifies the potential attack surface, making this vulnerability particularly dangerous in enterprise environments where PNG image processing is prevalent.
The vulnerability maps to CWE-191 Integer Underflow (Wrap) which is classified as a fundamental weakness in software design that occurs when an integer value is decremented below its minimum representable value. This weakness often leads to unpredictable behavior and can be exploited to bypass security checks or cause memory corruption. From an ATT&CK framework perspective, this vulnerability could be leveraged as part of a reconnaissance phase to identify vulnerable systems, potentially leading to privilege escalation or remote code execution depending on the exploitation chain. The vulnerability's impact is particularly concerning in web applications where users can upload PNG files, as it allows for direct exploitation without requiring complex attack vectors. Mitigation strategies should include immediate patching of affected libpng versions, implementing input validation for image metadata, and deploying application firewalls to filter potentially malicious PNG files. Organizations should also consider implementing sandboxed image processing environments and regular vulnerability assessments to identify similar integer overflow and underflow conditions in their software dependencies, as these types of vulnerabilities often indicate broader issues in memory management and input validation practices within software libraries.