CVE-2026-123318 in Ktor
Summary
by MITRE • 11/06/2026
In JetBrains Ktor before 3.6.1 hTTP request smuggling was possible in the CIO engine
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Analysis
by VulDB Data Team • 11/06/2026
A critical security vulnerability has been identified within the Common IO (CIO) engine of JetBrains Ktor, a popular asynchronous framework for building connected systems in Kotlin. This flaw manifests as an HTTP request smuggling issue, specifically affecting versions prior to 3.6.1. The CIO engine serves as one of the primary network backends for handling incoming and outgoing HTTP traffic within applications built using this framework. When processing requests, the underlying implementation failed to correctly parse or validate certain aspects of the HTTP protocol headers, creating a discrepancy between how the server interprets request boundaries and how downstream components or reverse proxies might interpret them. This misalignment is the fundamental technical flaw that enables the vulnerability.
HTTP request smuggling occurs when there is an inconsistency in how two different layers of software process HTTP requests. In this specific instance within Ktor's CIO engine, improper handling of content length headers or chunked transfer encoding allows an attacker to inject malicious payloads into subsequent legitimate requests. By carefully crafting a malformed HTTP request that exploits the parsing logic errors in the CIO handler, an adversary can trick the server into treating part of one user's request as the start of another. This breaks the isolation between different client sessions and disrupts the expected sequential processing of network traffic. The vulnerability stems from insufficient validation mechanisms when determining where a request ends and the next begins, particularly under conditions involving complex header combinations or edge cases in body parsing logic.
The operational impact of this vulnerability is severe for any production environment relying on Ktor's CIO engine without upgrading to version 3.6.1 or later. Successful exploitation allows attackers to bypass access controls by injecting their own requests into the context of an authenticated user, potentially leading to unauthorized data access or modification. Furthermore, it can be used to poison caches if a reverse proxy is involved, causing malicious content to be served to other users. In more advanced scenarios, this could facilitate cross-site scripting attacks against other victims who receive the smuggled payload as part of their legitimate session response. The ability to manipulate request boundaries undermines the integrity and confidentiality guarantees provided by standard web application architectures, making it a high-risk issue for systems handling sensitive user data or performing critical business transactions over HTTP.
From a classification perspective, this vulnerability aligns with CWE-436, which refers to Interpretation Conflict, as well as CWE-757, concerning the selection of algorithm for integrity protection. In terms of offensive security frameworks, it maps directly to MITRE ATT&CK technique T1071, specifically sub-technique 9.2 (Application Layer Protocol: Web Protocols), which covers HTTP request smuggling tactics used by adversaries to evade detection and manipulate backend systems. Defense-in-depth strategies are essential here; while the immediate fix is upgrading Ktor, organizations should also ensure that their reverse proxies and load balancers enforce strict header validation rules independent of the application server's behavior. Regular security audits focusing on network protocol implementations and continuous monitoring for anomalous request patterns can further mitigate risks associated with such parsing discrepancies until patches are fully deployed across all affected infrastructure components.