A vulnerability in cpp-httplib, a C++11 HTTP/HTTPS library, allows header injection attacks through percent-encoded CRLF sequences. The library applies percent-decoding to header values after validation, allowing encoded %0D%0A to bypass checks and expand to literal carriage return and line feed characters. This affects versions prior to 0.44.0 and could enable HTTP request smuggling or response splitting attacks. The vulnerability has been fixed in version 0.44.0.