Description

Inconsistent Interpretation of HTTP Requests ('HTTP Request Smuggling') (CWE-444) in Elasticsearch can lead to information disclosure via HTTP Request Smuggling (CAPEC-33). Under specific proxy deployment configurations, a network attacker could obtain confidential responses intended for other authenticated users.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.9",
  "pubDate": "2026-09-01T20:17:23.757Z",
  "pubdate": "2026-09-01T20:17:23.757Z",
  "executiveSummary": "This vulnerability involves an Inconsistent Interpretation of HTTP Requests (CWE-444) within Elasticsearch, facilitating HTTP Request Smuggling (CAPEC-33). The flaw arises from how the application parses or interprets ambiguous HTTP request headers—specifically 'Content-Length' and 'Transfer-Encoding'—when deployed behind a reverse proxy or load balancer. An attacker can exploit this discrepancy to desynchronize the connection between the frontend proxy and the backend Elasticsearch instance.\nThe primary risk implication is unauthorized information disclosure. By manipulating the request stream, an attacker can append malicious data to legitimate requests from other authenticated users. Consequently, the backend server may process the injected request as if it originated from the victim's session, potentially returning sensitive data to the attacker. Exploitation requires the attacker to be positioned on the network between the client and the target infrastructure, necessitating specific proxy deployment configurations that permit header ambiguity. This vulnerability poses a severe threat to confidentiality in multi-tenant or protected environments where sensitive data is accessed via HTTP.",
  "technicalDetails": "The root cause of this vulnerability lies in the discrepancy between how a frontend proxy and the backend Elasticsearch server interpret HTTP/1.1 message boundaries. HTTP request smuggling occurs when there is a lack of consensus on the end of a message, typically exploited through conflicting 'Content-Length' (CL) and 'Transfer-Encoding' (TE) headers.\nIn a vulnerable deployment, if the frontend proxy favors one header (e.g., CL) while the backend Elasticsearch server favors the other (e.g., TE), the attacker can craft a payload that defines two requests within a single stream. The proxy treats the entire payload as one request and forwards it to the backend. The backend, parsing the request according to its own logic, terminates the first request prematurely, leaving the remainder of the payload (the smuggled second request) in the buffer.\nWhen a subsequent legitimate user sends a request, the backend appends it to the smuggled request still residing in the buffer. The victim's request is then concatenated with the attacker's smuggled request, causing the victim's request headers or body to be interpreted as part of the attacker's illicit payload. This allows for response queue poisoning, where the server may return a sensitive response intended for the victim directly to the attacker.\nThe attack flow proceeds as follows: 1) The attacker sends a malformed HTTP request to the proxy containing both CL and TE headers. 2) The proxy forwards the request, which the backend reads only partially based on the conflicting interpretation. 3) The backend leaves the 'smuggled' portion of the request in the socket buffer. 4) The next valid user request arrives and is implicitly attached to the smuggled payload. 5) The server processes the combined request, resulting in the leakage of the authenticated user's response to the attacker. The impact is significant, as it bypasses application-level security controls and authentication mechanisms by hijacking the context of legitimate, authorized sessions. This requires precise timing and alignment of the HTTP request pipeline, which is highly dependent on the specific proxy/backend architecture deployed."
}