CVE-2025-12642 in lighttpd
Summary
by MITRE • 11/03/2025
lighttpd1.4.80 incorrectly merged trailer fields into headers after http request parsing. This behavior can be exploited to conduct HTTP Header Smuggling attacks.
Successful exploitation may allow an attacker to:
* Bypass access control rules * Inject unsafe input into backend logic that trusts request headers * Execute HTTP Request Smuggling attacks under some conditions
This issue affects lighttpd1.4.80
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Analysis
by VulDB Data Team • 05/31/2026
The vulnerability identified as CVE-2025-12642 represents a critical HTTP header processing flaw in lighttpd version 1.4.80 that fundamentally compromises the integrity of HTTP request parsing mechanisms. This issue stems from an improper handling of trailer fields during HTTP request processing where the web server incorrectly merges trailer headers into the main header structure, creating a condition that can be exploited for sophisticated header manipulation attacks. The flaw specifically manifests during the HTTP request parsing phase where trailer fields that should remain separate from the primary header section are being incorrectly processed and merged, leading to unpredictable behavior in how subsequent request handling components interpret header data.
The technical implementation of this vulnerability creates a dangerous condition where the HTTP header smuggling attack vector becomes viable through the manipulation of trailer field behavior. When lighttpd 1.4.80 processes HTTP requests containing trailer fields, the incorrect merging behavior causes the server to treat trailer data as if it were part of the regular header section, which can result in inconsistent header interpretation by downstream components. This merging process creates a scenario where an attacker can craft requests that appear to contain specific header values to one component while actually containing different values to another, effectively enabling the exploitation of HTTP header smuggling techniques. The vulnerability operates at the HTTP protocol layer and specifically targets the server's request parsing logic, making it particularly dangerous as it can bypass standard security controls that rely on proper header validation.
The operational impact of this vulnerability extends beyond simple header manipulation to encompass significant security implications for web applications relying on lighttpd 1.4.80. Attackers can exploit this weakness to bypass access control mechanisms that depend on specific header values, potentially gaining unauthorized access to protected resources or functionality. The vulnerability also enables injection of unsafe input into backend processing logic that trusts the integrity of request headers, creating opportunities for additional attacks including but not limited to cache poisoning, cross-site scripting, and server-side request forgery. Furthermore, the conditions under which HTTP request smuggling can be executed make this vulnerability particularly dangerous for applications that use proxy servers or load balancers, as the header manipulation can cause inconsistent behavior across different components in the request handling chain. This type of vulnerability is classified under CWE-1244 which specifically addresses improper handling of HTTP headers and trailers, and aligns with ATT&CK technique T1071.004 for application layer protocol manipulation.
Organizations using lighttpd 1.4.80 should immediately implement mitigations including upgrading to a patched version of the web server software, as this vulnerability cannot be effectively addressed through configuration changes alone due to its fundamental nature in the HTTP parsing logic. Network-level mitigations such as proxy configuration adjustments may provide temporary protection but do not address the root cause of the trailer field merging issue. Security teams should also implement monitoring for unusual header patterns and conduct thorough testing of applications that might be affected by header smuggling attacks. The vulnerability demonstrates the critical importance of proper HTTP protocol implementation and the potential for seemingly minor parsing errors to create significant security weaknesses in web infrastructure components. Given the nature of HTTP header smuggling attacks, organizations should also review their existing security controls and validate that access control mechanisms are robust enough to withstand potential header manipulation attacks, as traditional security measures may be insufficient to protect against this specific class of vulnerability.