CVE-2026-65838 in Skipper
Summary
by MITRE • 09/14/2026
Skipper is an HTTP router and reverse proxy for service composition. Prior to 0.27.35, the opaAuthorizeRequestWithBody filter in filters/openpolicyagent/openpolicyagent.go can allow an oversized declared Content-Length request to bypass a deny-on-presence Rego policy because ExtractHttpBodyOptionally leaves OPA with an empty parsed_body while forwarding the complete request body upstream. This incomplete remediation of CVE-2026-50197 affects deployments that authorize request-body content and exceed -open-policy-agent-max-request-body-size, which defaults to 1 MB. Policy logic that does not reject input.attributes.request.http.truncated_body can therefore fail open and permit a forbidden payload to reach the protected service, while small bodies and the previously fixed chunked-body case are evaluated normally. This issue is fixed in version 0.27.35.
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Analysis
by VulDB Data Team • 09/14/2026
Skipper serves as an HTTP router and reverse proxy designed for service composition within modern microservice architectures. A critical security vulnerability exists within its Open Policy Agent integration, specifically affecting the opaAuthorizeRequestWithBody filter located in filters/openpolicyagent/openpolicyagent.go prior to version 0.27.35. This flaw represents a failure in input validation and boundary enforcement that allows attackers to bypass access control mechanisms by manipulating HTTP request headers. The vulnerability is particularly dangerous because it undermines the integrity of policy-based authorization systems that rely on inspecting the body content of incoming requests to determine whether they should be allowed or denied.
The technical root cause lies in how Skipper handles oversized payloads relative to its configured maximum request body size limit, which defaults to one megabyte via the -open-policy-agent-max-request-body-size flag. When a client submits an HTTP request with a Content-Length header indicating a payload larger than this threshold, the ExtractHttpBodyOptionally function truncates or discards the actual body content before passing it to the Open Policy Agent for evaluation. Consequently, OPA receives an empty parsed_body object despite the fact that Skipper continues to forward the complete, unmodified request body upstream to the protected backend service. This creates a dangerous discrepancy where the authorization engine evaluates a sanitized, empty input while the downstream application processes the full malicious payload.
This behavior constitutes a classic access control bypass vulnerability, aligning with CWE-284 Improper Access Control and CWE-798 Use of Hard-coded Credentials if policies rely on static thresholds that are circumvented by size manipulation. Furthermore, it maps to MITRE ATT&CK technique T1059 Command and Scripting Interpreter or T1190 Exploit Public-Facing Application depending on the nature of the payload being injected. The vulnerability effectively allows an attacker to send a forbidden payload that would normally be rejected by Rego policy logic checking for specific patterns in input.attributes.request.http.truncated_body or similar fields. Because the body is truncated from OPA's perspective, these checks fail open, permitting the request through while the backend service remains unaware of the authorization failure and executes the potentially harmful content.
The impact of this vulnerability is severe for deployments that depend on Skipper to enforce security policies based on HTTP request bodies. Attackers can exploit this flaw to inject malicious scripts, execute arbitrary commands via deserialization attacks, or exfiltrate sensitive data by crafting requests with large Content-Length headers. Small body sizes and chunked transfer encoding scenarios are not affected as they fall within the normal processing path where OPA receives accurate input. However, any system relying on content inspection for security decisions is at risk when an adversary exceeds the configured size limit intentionally to trigger this truncation behavior.
To mitigate this vulnerability, organizations must upgrade Skipper to version 0.27.35 or later immediately. This release corrects the logic in ExtractHttpBodyOptionally to ensure that OPA receives a representation of the request body consistent with what is forwarded upstream, thereby maintaining the integrity of authorization decisions. In addition to upgrading, administrators should review their Rego policies to ensure they explicitly handle truncated bodies and enforce strict size limits appropriate for their application context. Implementing Web Application Firewall rules or rate limiting can provide an additional layer of defense by detecting anomalous request sizes before they reach Skipper. Regular security audits and penetration testing focused on access control bypasses are recommended to verify that policy enforcement remains robust against edge cases involving payload manipulation.