CVE-2020-11076: 
Ruby vulnerability analysis and mitigation

CVE-2020-11076 affects Puma (RubyGem) versions before 4.3.4 and 3.12.5. The vulnerability allows an attacker to smuggle an HTTP response by using an invalid transfer-encoding header. This security issue was discovered by ZeddYu and was disclosed on May 21, 2020 (GitHub Advisory).

The vulnerability is related to how Puma handles the transfer-encoding header in HTTP requests. The issue occurs when processing invalid transfer-encoding headers, which could lead to HTTP response smuggling. The vulnerability has a CVSS 3.1 base score of 7.5 (High), with the vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N, indicating network accessibility, low attack complexity, no privileges required, and no user interaction needed (Ubuntu).

When exploited, this vulnerability allows an attacker to smuggle HTTP responses, which could potentially lead to response queue poisoning, cache poisoning, or request/response desynchronization. The impact primarily affects the integrity of HTTP communications, with no direct effect on confidentiality or availability (Ubuntu).

The vulnerability is exploitable over the network and requires no special privileges or user interaction. An attacker can trigger the vulnerability by sending specially crafted HTTP requests with invalid transfer-encoding headers (GitHub Advisory).

The vulnerability has been fixed in Puma versions 3.12.5 and 4.3.4. Users should upgrade to these or later versions to mitigate the risk. The fix involves improved handling of transfer-encoding headers and was implemented through a code change that better handles client input (GitHub Commit).

Multiple Linux distributions and package maintainers responded to this vulnerability by releasing security updates, including Ubuntu, Debian, and Fedora. Debian classified it as a moderate security issue and released version 3.6.0-1+deb9u1 to address it (Debian LTS).