CVE-2026-34715 is a vulnerability classified under CWE-113 (Improper Neutralization of CRLF Sequences in HTTP Headers) affecting the 'ewe' web server, a Gleam language-based HTTP server. In versions prior to 3.0.6, the encode_headers function in src/ewe/internal/encoder.gleam fails to sanitize CRLF sequences in HTTP response header keys and values. This lack of validation allows an attacker who can influence header content (for example, by controlling a Location redirect header parameter) to inject CRLF sequences, effectively splitting the HTTP response. This can lead to several attack vectors including HTTP response splitting, which can poison caches and proxies, and facilitate cross-site scripting (XSS) by injecting malicious scripts into responses. Notably, the server does validate CRLF in incoming request headers using validate_field_value() in its HTTP/1.1 parser, but this validation is absent in the response header encoding path. The vulnerability is remotely exploitable without authentication or user interaction, increasing its risk. The issue was addressed and fixed in version 3.0.6 by adding proper validation or stripping of CRLF sequences in outgoing headers. The CVSS v3.1 base score is 5.3, reflecting medium severity with network attack vector, low attack complexity, no privileges required, no user interaction, and impact limited to integrity (no confidentiality or availability impact). No known exploits have been reported in the wild as of the publication date.

The vulnerability allows attackers to inject arbitrary HTTP response content by exploiting improper CRLF neutralization in response headers. This can lead to HTTP response splitting, enabling cache poisoning attacks that may cause clients or intermediary caches to store malicious or incorrect content. Such cache poisoning can degrade service integrity and mislead users or downstream systems. Additionally, attackers can leverage this to perform cross-site scripting (XSS) attacks by injecting malicious scripts into HTTP responses, potentially compromising user sessions, stealing credentials, or performing unauthorized actions on behalf of users. Since the vulnerability can be exploited remotely without authentication or user interaction, it poses a risk to any publicly accessible web applications using affected versions of the ewe server that incorporate user-controlled data into response headers. However, the impact is limited to integrity; confidentiality and availability are not directly affected. The absence of known exploits reduces immediate risk but does not eliminate potential future exploitation. Organizations relying on affected versions may face reputational damage, user trust erosion, and potential regulatory consequences if exploited.

Organizations should immediately upgrade the ewe web server to version 3.0.6 or later, where the vulnerability has been patched. Until upgrading, as a temporary mitigation, developers should ensure that any user-controlled data incorporated into HTTP response headers is properly sanitized to remove or encode CRLF sequences, preventing injection. Implement strict input validation and output encoding for header values, especially for headers like Location, Set-Cookie, or any custom headers that may include user input. Employ web application firewalls (WAFs) with rules designed to detect and block HTTP response splitting attempts by monitoring for suspicious CRLF injection patterns. Conduct thorough code reviews and security testing to verify that no other parts of the application or server code allow unsanitized header injection. Monitor logs for unusual HTTP responses or cache poisoning indicators. Educate developers about the risks of improper header handling and enforce secure coding practices regarding HTTP header construction. Finally, maintain an up-to-date inventory of software components to quickly identify and remediate vulnerable versions.