CVE-2026-97731 in MinIO
Summary
by MITRE • 09/25/2026
MinIO through 7aac2a2 does not verify that every x-amz-* header present on a request also appears in the client-supplied X-Amz-SignedHeaders list. extractSignedHeaders() in cmd/signature-v4-utils.go iterates only the claimed list and never enumerates the headers that actually arrived, and thus a header that arrives unsigned is neither hashed into the canonical request nor rejected. Because cmd/api-router.go dispatches CopyObject on the presence of x-amz-copy-source alone, the holder of a presigned PUT URL scoped to a single object can add that header to the unmodified URL and cause a server-side copy, executed as the signer, of any object the signing key can read. A grant to write one object becomes a read of every bucket that key can reach. Amazon S3 rejects the equivalent request with HTTP 403 AccessDenied. The minio/minio GitHub repository was archived in April 2026; pgsty/silo before 1233254 is also affected.
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Analysis
by VulDB Data Team • 09/25/2026
The vulnerability identified in MinIO versions prior to commit 7aac2a2 represents a critical flaw in the implementation of AWS Signature Version 4 authentication, specifically within the header validation logic used for presigned URLs and signed requests. The core technical deficiency lies in the extractSignedHeaders function located in cmd/signature-v4-utils.go, which fails to perform a comprehensive verification of all headers present in an incoming HTTP request against those explicitly listed in the X-Amz-SignedHeaders metadata provided by the client. In a correctly implemented Signature V4 system, every header included in the canonical request string must be both signed and verified for integrity. However, this implementation iterates only over the subset of headers claimed to be signed by the requester, ignoring any additional x-amz-* headers that are physically present on the wire but not listed in the signature scope. Consequently, if a malicious actor includes an unsigned header in their request, the server processes it without hashing it into the canonical request string and does not reject the discrepancy as invalid authentication data.
This architectural oversight creates a severe authorization bypass scenario when combined with how MinIO handles object copy operations via its API router. The cmd/api-router.go module dispatches CopyObject functionality based solely on the presence of the x-amz-copy-source header, without sufficiently validating whether this specific action was authorized by the original signature scope. An attacker possessing a presigned PUT URL scoped to write only a single, specific object can exploit this flaw by appending an unsigned x-amz-copy-source header to that URL. Because the server fails to verify that copy-related headers were part of the signed set, it accepts the request as validly authenticated under the original permissions. The server then executes a server-side copy operation using the credentials associated with the signing key, effectively allowing the attacker to read and duplicate any object within any bucket accessible by that key.
The operational impact of this vulnerability is profound, transforming what should be a limited write-only permission into an unrestricted read access vector for all resources visible to the compromised identity. A grant intended to allow writing to one specific file inadvertently grants the ability to exfiltrate data from every other object in the storage system that the signing key has permissions to list or read. This behavior stands in direct contradiction with Amazon S3, which strictly enforces signature scopes and rejects such requests with an HTTP 403 AccessDenied error if unauthorized headers are introduced into signed URLs. The lack of strict header enumeration allows for a complete compromise of data confidentiality within the affected MinIO deployment, as attackers can leverage simple presigned links to perform mass data exfiltration without needing elevated privileges or additional authentication tokens.
This flaw is categorized under CWE-287, which describes Improper Authentication, and aligns with MITRE ATT&CK technique T1078, specifically the use of valid accounts and credentials for unauthorized access patterns such as impersonation or privilege escalation through parameter manipulation. The vulnerability highlights a common pitfall in cloud storage implementations where partial adherence to industry standards like AWS S3 API specifications leads to security gaps. To mitigate this risk, organizations running affected versions must upgrade immediately to a patched release that enforces strict validation of all incoming headers against the signed header list. Until an official patch is available from MinIO or its successor project pgsty/silo (prior to commit 1233254), administrators should implement reverse proxy rules or WAF policies to reject requests containing x-amz-copy-source headers unless they are explicitly expected and validated, thereby preventing the unauthorized execution of server-side copy operations.