CVE-2026-116127 in Starlet
Summary
by MITRE • 10/30/2026
Starlet versions before 0.33 for Perl send a body in responses to HEAD requests, corrupting the next response on a persistent or pipelined connection.
The request method is not passed to the response writer, so whatever body the application returns for a HEAD request is sent. With Plack::Middleware::Head the body is empty, but an HTTP/1.1 response without a Content-Length header still gets the chunked encoding terminator. Responses with status 204 or 304 are also sent with their body.
Pipelined GET and HEAD requests keep the connection open even when keep-alive is disabled, which is the default. A client or proxy reading the connection takes the extra bytes as the start of the next response, which may desynchronize a proxy that shares the connection across clients.
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Analysis
by VulDB Data Team • 10/30/2026
The vulnerability in Starlet versions prior to 0.33 represents a significant protocol compliance failure within the Perl web server ecosystem, specifically affecting how HTTP HEAD requests are handled during response generation. The core technical flaw stems from an architectural oversight where the request method is not correctly propagated or utilized by the underlying response writer component. Consequently, when an application processes a HEAD request, which by definition should return only headers and no body content, Starlet erroneously transmits the full message body intended for GET requests. This behavior violates RFC 7231, which explicitly mandates that servers must not send a payload in responses to HEAD methods. The issue is exacerbated by specific middleware configurations; while Plack::Middleware::Head attempts to strip the body, it fails to prevent the transmission of HTTP/1.1 chunked encoding terminators for responses lacking an explicit Content-Length header. Furthermore, status codes that inherently imply no content, such as 204 No Content and 304 Not Modified, are also incorrectly transmitted with their associated bodies, further deviating from standard protocol expectations.
The operational impact of this vulnerability is severe due to its interaction with persistent connections and HTTP pipelining features common in modern web infrastructure. When Starlet sends the extraneous body data for HEAD requests, it leaves residual bytes on the TCP connection stream that were not consumed by the client expecting only headers. In environments where keep-alive is disabled or when multiple requests are pipelined over a single persistent connection, these leftover bytes remain unread in the buffer. A subsequent request sent down the same connection will encounter this garbage data at its start position. For clients and intermediate proxies that rely on strict byte-level synchronization to parse HTTP responses, this results in immediate desynchronization. The proxy or client interprets the tail end of the previous response's body as the beginning of a new status line or header block, leading to parsing errors, connection resets, or potentially malformed request processing.
This misalignment poses substantial security and reliability risks beyond simple connectivity issues. In shared hosting environments or reverse proxy setups where connections are multiplexed across multiple clients, an attacker could exploit this desynchronization to inject malicious data into a subsequent client's response stream. This scenario creates potential vectors for HTTP Response Splitting attacks or Cross-Site Scripting if the injected bytes contain interpretable content that bypasses standard filtering mechanisms due to the broken parsing state. Additionally, the reliability of web services is compromised as legitimate users experience intermittent failures when their requests are corrupted by residual data from previous HEAD operations on pipelined connections. The failure to adhere to HTTP standards also undermines interoperability with strict conformance checking tools and can cause instability in load balancers that depend on accurate header parsing for health checks or routing decisions based on response codes like 204 or 304.
Mitigation strategies must prioritize immediate software updates alongside defensive architectural adjustments. The primary remediation is to upgrade Starlet to version 0.33 or later, where the developers have corrected the logic to ensure that HEAD requests do not transmit body content and properly handle chunked encoding terminators without leaving residual data on the connection stream. For environments unable to patch immediately, implementing a reverse proxy such as Nginx or Apache in front of Starlet can provide an additional layer of protection by enforcing strict HTTP compliance before traffic reaches the vulnerable application server. These proxies typically strip bodies from HEAD responses correctly and manage connection states more robustly than the raw Perl handler. Furthermore, developers should audit their Plack middleware configurations to ensure that body stripping occurs at a stage where it effectively prevents any data transmission for non-body methods. Monitoring logs for unusual connection resets or parsing errors can also help identify instances of this vulnerability in active production environments, allowing for targeted intervention before widespread service degradation occurs.