CVE-2020-11076 in Puma Gem
Summary
by MITRE
In Puma (RubyGem) before 4.3.4 and 3.12.5, an attacker could smuggle an HTTP response, by using an invalid transfer-encoding header. The problem has been fixed in Puma 3.12.5 and Puma 4.3.4.
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Analysis
by VulDB Data Team • 05/11/2026
The vulnerability CVE-2020-11076 represents a critical HTTP response smuggling issue within the Puma web server implementation for Ruby applications. This flaw exists in versions prior to 3.12.5 and 4.3.4 of the Puma RubyGem, creating a pathway for malicious actors to manipulate HTTP responses through carefully crafted invalid transfer-encoding headers. The vulnerability stems from insufficient validation of HTTP headers during response processing, allowing attackers to inject or alter response content in ways that can bypass normal security controls and potentially execute unauthorized operations.
The technical root cause of this vulnerability lies in the improper handling of HTTP transfer-encoding headers within Puma's response processing pipeline. When an invalid transfer-encoding header is present in an HTTP request, the server fails to properly validate or reject such headers, enabling an attacker to craft malicious requests that can cause the server to process multiple responses or manipulate the response structure. This behavior creates opportunities for HTTP response splitting attacks where attackers can inject additional HTTP responses or manipulate existing ones. The vulnerability specifically relates to CWE-1247, which addresses improper handling of HTTP headers and their potential for causing response manipulation. The flaw operates at the application layer of the OSI model, affecting the HTTP protocol implementation within the Ruby web server environment.
The operational impact of CVE-2020-11076 extends beyond simple response manipulation to potentially enable more sophisticated attacks including cache poisoning, session hijacking, and cross-site scripting exploitation. When exploited, this vulnerability allows attackers to inject malicious content into responses that might be cached by intermediate proxies or browsers, affecting multiple users who access the same resources. The vulnerability can be particularly dangerous in environments where Puma serves applications with sensitive data or authentication mechanisms, as it may enable attackers to bypass security controls and gain unauthorized access to protected resources. Additionally, the attack vector requires minimal sophistication and can be automated, making it particularly attractive to threat actors who seek to exploit widely deployed web server implementations.
Mitigation strategies for CVE-2020-11076 focus primarily on immediate version upgrades to Puma 3.12.5 or 4.3.4, which contain the necessary patches to properly validate transfer-encoding headers. Organizations should conduct comprehensive vulnerability assessments to identify all systems running affected Puma versions and prioritize their remediation. Network administrators should implement additional monitoring for suspicious HTTP header patterns and consider deploying web application firewalls that can detect and block malformed transfer-encoding headers. The fix implemented in the patched versions follows ATT&CK technique T1071.004 for Application Layer Protocol, specifically addressing the manipulation of HTTP headers through proper input validation and sanitization. Organizations should also implement regular security scanning procedures to identify other potential vulnerabilities in their Ruby application stack and maintain updated security baselines for their web server configurations.