CVE-2026-53503 in Thumbor
Summary
by MITRE • 07/31/2026
Thumbor is an open-source photo thumbnail service by globo.com. Prior to 7.8.0, Thumbor's filters:convolution(<matrix>, <columns>, <should_normalize>) filter passes the user-controlled <columns> value to a C extension (thumbor/ext/filters/_convolution.c) where it is used as a divisor (for % and /) without validating columns > 0. When columns=0, the C code triggers undefined behavior; on x86_64 this reliably results in a fatal divide-by-zero trap (SIGFPE) and crashes the Thumbor process, causing a remote denial of service. This issue is fixed in 7.8.0.
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Analysis
by VulDB Data Team • 07/31/2026
The vulnerability resides within Thumbor's convolution filter implementation where user-supplied parameters are not properly validated before being passed to low-level C extensions. This represents a classic case of input validation failure that can be exploited to trigger system instability and denial of service conditions. The specific flaw exists in the thumbor/ext/filters/_convolution.c file where the columns parameter is directly used as a divisor without proper bounds checking, creating an exploitable condition that can be leveraged by remote attackers.
The technical execution of this vulnerability occurs when an attacker crafts a malicious request containing a convolution filter with columns set to zero. This value bypasses any application-level validation and flows directly into the C extension code where it is used in division operations. When columns equals zero, the mathematical operation results in a divide-by-zero condition that generates a SIGFPE signal on x86_64 architectures, causing the Thumbor process to terminate abruptly. This behavior stems from undefined behavior in C programming when division by zero occurs, which the operating system translates into a fatal signal that cannot be recovered from.
This vulnerability directly maps to CWE-369: Divide by Zero and CWE-755: Improper Handling of Exceptional Conditions, both of which are categorized under the broader weakness of improper input validation and exception handling in software systems. The attack surface is particularly concerning as it allows for remote code execution in the form of denial of service, where an attacker can reliably crash the thumbnail service and potentially disrupt all image processing operations for the affected system. From an operational perspective, this vulnerability undermines the availability of the service and can be exploited at scale to cause widespread disruption.
The impact extends beyond simple service disruption as it represents a fundamental security flaw that could potentially be chained with other vulnerabilities or used as part of larger attack campaigns targeting web applications that rely on Thumbor for image processing. The fix implemented in version 7.8.0 addresses the core issue by introducing proper validation to ensure columns parameter values are greater than zero before being passed to the C extension, thereby preventing the division by zero condition from ever occurring and maintaining system stability.
This vulnerability exemplifies the critical importance of input validation at all layers of software architecture, particularly when transitioning between high-level languages like python and low-level C extensions where undefined behaviors can lead to catastrophic failures. The ATT&CK framework categorizes this under T1499.004: Endpoint Denial of Service, as it specifically targets the availability of service endpoints through deliberate exploitation of mathematical errors in processing pipelines. Organizations utilizing Thumbor should prioritize immediate patching to prevent exploitation and maintain the integrity of their image processing infrastructure while also implementing monitoring for similar patterns in other software components that may exhibit similar validation weaknesses.