Apache APISIX EUVD-2026-66958

Exploitation requires that the targeted Apache APISIX instance is running a version between 2.12.0 and 3.17.0 inclusive, and that at least one route is configured to use a serverless plugin (serverless-pre-function or serverless-post-function). The attacker must be able to reach the APISIX proxy listener (typically HTTP/HTTPS on port 9080 or 9443) over the network. No gateway-level authentication by the attacker is required to send crafted requests. What limits exploitation: deployments that do not use serverless plugins at all are not affected regardless of version; successful HTTP smuggling typically requires precise timing or specific connection-handling conditions, adding practical complexity; and instances isolated behind internal networks or strict WAF policies reduce the reachable attacker population.

No CVSS vector was supplied with this advisory, limiting quantitative scoring; independent metrics are assessed below. The attack surface is inherently broad because APISIX is a network-facing gateway - the sole entry point for API traffic - making the serverless plugin routes reachable from the internet or internal networks depending on operator configuration. Cross-user response poisoning at the gateway layer has direct confidentiality and integrity consequences: sensitive API responses containing authentication tokens, session data, or PII can be served to unintended recipients, and even a mid-sized API platform operator in bytom should treat this as a priority remediation given the potential for cross-user data leakage at scale. No EPSS data was provided, and the vulnerability is not in CISA KEV at time of analysis, which moderates urgency relative to actively exploited issues. However, HTTP smuggling vulnerabilities in widely deployed gateways have historically attracted rapid proof-of-concept development after oss-security disclosure, and the wide affected version range (2.12.0 through 3.17.0, spanning over two years of releases) indicates substantial installed-base exposure. The requirement for serverless-plugin routes to be active is a meaningful limiting factor for deployments that do not use that feature.

A remote attacker sends a sequence of specially crafted HTTP requests to an Apache APISIX serverless-plugin route, exploiting ambiguous request boundary parsing to desynchronize the gateway's connection handling. A victim who subsequently sends a legitimate request over the same or a pipelined connection receives the attacker's injected response content - or another authenticated user's API response - potentially exposing session tokens, business data, or PII. No proof-of-concept code has been identified at time of analysis, but the network-accessible nature of API gateways and the clarity of the oss-security disclosure lower the bar for independent reproduction.