CVE-2019-6802 in pypiserver
Summary
by MITRE
CRLF Injection in pypiserver 1.2.5 and below allows attackers to set arbitrary HTTP headers and possibly conduct XSS attacks via a %0d%0a in a URI.
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Analysis
by VulDB Data Team • 07/03/2023
The vulnerability CVE-2019-6802 represents a critical CRLF (Carriage Return Line Feed) injection flaw discovered in pypiserver versions 1.2.5 and earlier. This vulnerability stems from insufficient input validation and sanitization within the server's handling of URI parameters, specifically when processing encoded newline characters. The issue manifests when attackers exploit the lack of proper sanitization to inject malicious CRLF sequences into HTTP headers, which can then be leveraged to manipulate the server's response behavior and potentially execute cross-site scripting attacks. The vulnerability is particularly dangerous because it affects the core HTTP header processing functionality of the pypiserver, which is used to serve python package repositories.
The technical flaw occurs at the application layer where URI parameters are not adequately filtered or escaped before being incorporated into HTTP headers. When a user submits a URI containing encoded CRLF sequences such as %0d%0a, the server fails to properly sanitize this input, allowing these sequences to be interpreted as actual line terminators within HTTP header fields. This creates a condition where attackers can inject additional headers or manipulate existing ones, potentially leading to header injection attacks that can redirect traffic, modify response content, or enable more sophisticated attack vectors including XSS exploitation. The vulnerability maps directly to CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers and CWE-79: Cross-site Scripting, as it enables both header manipulation and XSS payload delivery through the same attack vector.
The operational impact of this vulnerability extends beyond simple header injection, as it provides attackers with a foothold for more complex attacks within the package repository environment. An attacker who successfully exploits this vulnerability could potentially inject malicious JavaScript into HTTP responses, leading to session hijacking, credential theft, or redirection to malicious sites. The attack surface is particularly concerning for organizations that rely on pypiserver for internal package distribution, as it could enable attackers to compromise the integrity of their software supply chain. Additionally, the vulnerability affects the server's ability to maintain secure and predictable HTTP responses, potentially disrupting normal operations while providing attackers with opportunities to establish persistent access or escalate privileges within the affected environment.
Mitigation strategies for CVE-2019-6802 should focus on immediate version upgrades to pypiserver 1.2.6 or later, which contain the necessary patches to address the input sanitization issues. Organizations should also implement strict input validation at multiple layers, including URI parameter sanitization and HTTP header filtering, to prevent similar vulnerabilities from manifesting in other components of their infrastructure. Network-level mitigations such as web application firewalls can provide additional protection by detecting and blocking known CRLF injection patterns, though these should be considered supplementary rather than primary defenses. Security teams should also conduct thorough audits of all URI handling code within their applications to identify potential similar vulnerabilities, as this type of injection attack often occurs in contexts where input validation is insufficient or improperly implemented. The vulnerability demonstrates the critical importance of proper input sanitization in HTTP applications and aligns with ATT&CK technique T1071.004: Application Layer Protocol - DNS to understand how protocol-level vulnerabilities can be exploited to manipulate application behavior and compromise security controls.