CVE-2026-34715 Overview
CVE-2026-34715 is an HTTP Response Splitting vulnerability affecting ewe, a Gleam web server. Prior to version 3.0.6, the encode_headers function in src/ewe/internal/encoder.gleam directly interpolates response header keys and values into raw HTTP bytes without validating or stripping CRLF (\r\n) sequences. An application that passes user-controlled data into response headers (e.g., setting a Location redirect header from a request parameter) allows an attacker to inject arbitrary HTTP response content.
This vulnerability enables response splitting attacks, which can lead to cache poisoning and potential cross-site scripting (XSS). Notably, ewe does validate CRLF in incoming request headers via validate_field_value() in the HTTP/1.1 parser, but provides no equivalent protection for outgoing response headers in the encoder.
Critical Impact
Attackers can inject arbitrary HTTP headers and response content through user-controlled data passed to response headers, enabling response splitting, cache poisoning, and cross-site scripting attacks.
Affected Products
- ewe Gleam web server versions prior to 3.0.6
Discovery Timeline
- 2026-04-02 - CVE CVE-2026-34715 published to NVD
- 2026-04-02 - Last updated in NVD database
Technical Details for CVE-2026-34715
Vulnerability Analysis
The vulnerability exists in the HTTP response encoding mechanism of the ewe web server. When generating HTTP responses, the encode_headers function processes header keys and values without sanitizing CRLF sequences. In HTTP/1.1, CRLF (\r\n) serves as the delimiter between headers and between the header section and response body. When user-controlled input containing these sequences is interpolated into response headers, an attacker can inject additional headers or even control the response body.
The asymmetric validation approach creates a security gap: while incoming request headers are properly validated through validate_field_value(), outgoing response headers lack this protection. This design flaw allows attackers to exploit applications that dynamically construct response headers from user input, such as redirect endpoints that use URL parameters to set the Location header.
Root Cause
The root cause is improper input validation (CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers) in the response header encoder. The encode_headers function in src/ewe/internal/encoder.gleam fails to sanitize or reject header values containing CRLF sequences before interpolating them into the raw HTTP response bytes.
Attack Vector
The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by supplying crafted input to any application endpoint that reflects user data into HTTP response headers. Common attack scenarios include:
- Manipulating redirect endpoints by injecting CRLF into URL parameters used for Location headers
- Poisoning intermediate caches by injecting cache-control headers
- Injecting malicious content into the response body to achieve XSS
The following patch was applied to address this vulnerability:
-module(ewe_ffi).
-export([close_file/1, decode_packet/3, init_clock_storage/0, lookup_http_date/0, now/0,
- now_microseconds/0, open_file/1, set_http_date/1, validate_field_value/1,
+ now_microseconds/0, open_file/1, set_http_date/1, validate_lowercase_field/1,
+ validate_field_value/1, validate_lowercase_field_value/1, sanitize_header_value/1,
coerce_tcp_message/1, parse_path/1]).
% Socket
Source: GitHub Commit
The fix introduces new validation functions including sanitize_header_value/1, validate_lowercase_field/1, and validate_lowercase_field_value/1 to properly sanitize outgoing response headers.
Detection Methods for CVE-2026-34715
Indicators of Compromise
- HTTP response logs showing unexpected additional headers or malformed responses
- Web application firewall (WAF) alerts detecting CRLF sequences (%0d%0a or \r\n) in user input destined for header values
- Cache servers storing poisoned responses with injected headers
- Reports of XSS or unexpected content rendering from users
Detection Strategies
- Monitor application logs for redirect URLs or header values containing encoded CRLF sequences (%0d, %0a, %0D%0A)
- Implement WAF rules to detect and block requests containing CRLF injection patterns in parameters used for response headers
- Audit application code for endpoints that pass user-controlled data to response headers without sanitization
- Review ewe server version deployed in production environments
Monitoring Recommendations
- Enable detailed HTTP response logging to capture full header content
- Configure alerting for anomalous response patterns such as unexpected header counts or content-type mismatches
- Monitor cache hit/miss ratios for signs of cache poisoning attacks
- Implement integrity checks on cached responses for critical endpoints
How to Mitigate CVE-2026-34715
Immediate Actions Required
- Upgrade ewe web server to version 3.0.6 or later immediately
- Review all application code that sets response headers from user-controlled input
- Implement input validation at the application layer to reject or sanitize CRLF sequences
- Deploy WAF rules to block HTTP response splitting attack patterns
Patch Information
The vulnerability has been patched in ewe version 3.0.6. The fix adds proper validation and sanitization functions for outgoing response headers, ensuring CRLF sequences are properly handled before being included in HTTP responses.
Workarounds
- Implement application-level input validation to strip or reject CRLF sequences from any user data before passing to response headers
- Use URL encoding validation on redirect parameters and reject values containing newline characters
- Deploy a reverse proxy or WAF in front of the application to filter malicious input patterns
- Avoid passing user-controlled data directly into response headers where possible
# Example: Update ewe to patched version
gleam update ewe@3.0.6
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
