CVE-2026-53504 in Thumbor
Summary
by MITRE • 07/31/2026
Thumbor is an open-source photo thumbnail service by globo.com. Prior to 7.8.0, the convolution filter regular expression performs exponential backtracking on crafted repeated numeric input, allowing a URL request to exhaust processing time. This issue is fixed in 7.8.0.
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Analysis
by VulDB Data Team • 07/31/2026
The Thumbor image processing service presents a critical security vulnerability through its convolution filter implementation that enables denial of service attacks via carefully crafted input sequences. This flaw exists in versions prior to 7.8.0 where the regular expression used for parsing convolution filter parameters exhibits exponential backtracking behavior when processing repeated numeric input patterns. The vulnerability stems from insufficient input validation and inadequate regex design that fails to account for catastrophic backtracking scenarios.
The technical implementation of this vulnerability occurs within the convolution filter's parameter parsing logic where a maliciously constructed URL request containing repeated numeric sequences can cause the regular expression engine to perform an exponential number of matching attempts. When the convolution filter processes such inputs, the regex engine must explore multiple possible match combinations due to overlapping quantifiers and alternation patterns, leading to dramatic performance degradation and eventual system resource exhaustion. This behavior directly corresponds to CWE-1333 which specifically addresses inefficient regular expression complexity that can be exploited for denial of service attacks.
The operational impact of this vulnerability extends beyond simple resource exhaustion as it represents a significant threat to service availability and system stability. An attacker can construct a single malicious URL request that consumes excessive CPU cycles and memory resources, potentially causing the entire Thumbor service to become unresponsive or crash entirely. This makes the vulnerability particularly dangerous in production environments where service uptime is critical, as even a single malicious request can effectively bring down the image processing service. The attack vector requires minimal sophistication since it only involves crafting specific URL parameters that trigger the exponential backtracking behavior.
Security practitioners should implement immediate mitigations including upgrading to version 7.8.0 or later which contains the patched regular expression implementation that prevents exponential backtracking scenarios. Additional protective measures include implementing rate limiting on image processing requests, validating input parameters against expected patterns before processing, and monitoring for unusual processing times or resource consumption patterns. The fix implemented in version 7.8.0 likely involves either rewriting the vulnerable regex pattern to eliminate backtracking opportunities or implementing proper input length limits that prevent the triggering of catastrophic backtracking conditions. Organizations should also consider deploying web application firewalls that can detect and block suspicious URL patterns before they reach the Thumbor service, following ATT&CK technique T1496 for resource exhaustion attacks.
This vulnerability demonstrates the critical importance of proper regex design in security-sensitive applications and aligns with common attack patterns found in the OWASP Top Ten 2021 under A05:2021 - Security Misconfiguration. The exponential backtracking issue represents a classic example of how seemingly benign input validation can create significant security risks when not properly implemented, particularly in systems that process untrusted user input through complex pattern matching operations. Organizations processing user-supplied parameters should conduct regular security reviews of their regex implementations and consider implementing automated tools to detect potential catastrophic backtracking scenarios in their codebase.