CVE-2026-77341 in cpp-httplib
Summary
by MITRE • 08/28/2026
cpp-httplib is a C++ header-only HTTP/HTTPS library. In version 0.49.0, the chunked-response trailer output path writes trailer header names and values directly to the socket without validating them, allowing CRLF sequences in a trailer field to inject additional headers or split the HTTP response. Unlike every other header-writing path in the library, the trailer-writing code applies none of the field-name and field-value checks that reject carriage return and line feed, so an application that places attacker-influenced data into a chunked response trailer emits attacker-controlled CRLF onto the wire. This enables HTTP response splitting, letting an attacker forge response headers or inject a second response. This issue is fixed in version 0.50.0.
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Analysis
by VulDB Data Team • 08/28/2026
The cpp-httplib library serves as a widely adopted header-only solution for implementing HTTP and HTTPS clients and servers within C++ applications. In its release prior to version 0.50.0, specifically affecting versions up through 0.49.0, the library contained a critical security flaw in the handling of chunked transfer encoding responses. This vulnerability arises from an inconsistency in how different types of HTTP headers are processed and validated before being transmitted over the network socket. While standard header fields undergo rigorous sanitization to prevent injection attacks, the code path responsible for writing trailer headers in chunked responses failed to apply these same security controls.
The technical root cause lies in the absence of input validation within the trailer-writing function. When an application utilizes cpp-httplib to construct a response with trailers, any data placed into those trailer fields is written directly to the socket without checking for carriage return or line feed characters. This lack of sanitization allows an attacker who can influence the content of these trailers to inject arbitrary CRLF sequences. By inserting newlines, the attacker effectively breaks the structure of the HTTP response header section, allowing them to terminate the current headers prematurely and append a completely new set of headers or even a second HTTP response entirely under their control.
This flaw constitutes an HTTP Response Splitting vulnerability, which is categorized under CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers. The operational impact of this issue is severe, as it enables attackers to perform various malicious actions including cross-site scripting by injecting scripts into the body via a forged response header, cache poisoning by manipulating caching directives such as Cache-Control or Vary, and session hijacking if Set-Cookie headers can be injected with attacker-controlled values. The ability to split responses also allows for HTTP Smuggling scenarios in certain proxy configurations, potentially leading to further exploitation of downstream infrastructure.
The vulnerability is distinct because it represents a regression from the library's general security posture; every other header-writing path in cpp-httplib correctly rejects CRLF characters, ensuring that only valid ASCII text without control codes reaches the wire. The trailer implementation was an oversight where this critical check was omitted. This inconsistency creates a predictable attack surface for developers who assume all parts of the HTTP response generation are equally protected against injection attacks.
To mitigate this risk, organizations using cpp-httplib must upgrade to version 0.50.0 or later immediately. The fix in the newer release implements proper validation logic that mirrors the checks applied to other header fields, ensuring that any attempt to inject CRLF sequences into trailer headers is rejected before transmission. For applications unable to update immediately due to dependency constraints, developers should implement manual input sanitization on all data passed to trailer functions, explicitly stripping or encoding carriage return and line feed characters prior to inclusion in the response structure. This defensive coding practice aligns with general secure development guidelines for HTTP header manipulation found in industry standards such as OWASP Secure Coding Practices.