CVE-2026-85008 in undici
Summary
by MITRE • 09/04/2026
undici's cache interceptor documents that only safe HTTP methods are cached, but its logic to skip caching is built by subtracting the configured methods from the set of safe methods, so an unsafe method such as POST, PUT, or DELETE is never placed in the skip list and instead falls through to the full cache-read path. The response-storage gate also lacked a method check, so a response to an unsafe request that is heuristically cacheable or carries an explicit Cache-Control directive is stored and later replayed from cache. Because response headers from a remote origin are untrusted, an origin can answer once with a cacheable status and then have the client's own subsequent state-changing requests to that path served from the stale cache entry without ever reaching the origin, an integrity failure that occurs under the interceptor's default configuration. This affects undici versions from 7.0.0 up to 7.29.1 and from 8.0.0 up to 8.10.2. Users should upgrade to undici 7.29.1 or 8.10.2.
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Analysis
by VulDB Data Team • 09/05/2026
The vulnerability in the undici HTTP client library represents a critical logic error within its cache interceptor mechanism, specifically affecting versions from 7.0.0 through 7.29.1 and 8.0.0 through 8.10.2. The core issue stems from an incorrect implementation of the method-based caching exclusion list. While the documentation correctly states that only safe HTTP methods should be cached, the underlying code logic attempts to enforce this by subtracting a configured set of unsafe methods from the broader set of safe methods. This approach is fundamentally flawed because it relies on the assumption that any method not explicitly listed as unsafe must therefore be safe for caching. Consequently, if an unsafe method such as POST, PUT, or DELETE is not present in the exclusion list configuration, it falls through to the standard cache-read path rather than being correctly identified and skipped. This design flaw means that state-changing requests are inadvertently treated as idempotent operations eligible for storage and retrieval from the local cache.
The operational impact of this vulnerability is severe due to the lack of a secondary validation gate at the response-storage stage. The mechanism responsible for deciding whether to store an incoming HTTP response in the cache fails to verify the method used in the original request. As a result, if a server returns a response that is heuristically considered cacheable or includes explicit Cache-Control directives indicating it should be cached, undici will store this response regardless of whether the triggering request was unsafe. This creates a scenario where an attacker-controlled origin can manipulate caching behavior by initially serving a cacheable response to a safe method, such as GET. Subsequently, when the client issues state-changing requests using methods like POST or DELETE to the same resource path, these requests are served from the stale local cache entry rather than being forwarded to the remote server. This bypasses network communication entirely for those specific requests, leading to significant integrity failures where application logic proceeds based on outdated or manipulated data without any interaction with the backend service.
From a security architecture perspective, this flaw aligns closely with CWE-20 Improper Input Validation and CWE-359 Exposure of Private Information Through Sensitivity of System Data if cached responses contain sensitive details. Furthermore, it relates to ATT&CK technique T1657 Fabricated Credentials or T1498 Network Denial of Service depending on the specific exploitation context, as it allows an attacker to disrupt normal application flow by forcing stale data delivery. The vulnerability exploits the trust model between the client and server, assuming that cacheable responses are benign regardless of how they were triggered. In practice, this can lead to unauthorized state changes being suppressed or executed incorrectly, potentially bypassing authentication checks, audit logs, or business logic constraints that rely on actual request transmission. For instance, a delete operation might be silently ignored because the client receives a cached success response from a previous unrelated GET request, leaving resources in an inconsistent state.
To mitigate this risk, organizations must immediately upgrade undici to version 7.29.1 or 8.10.2, where the logic for determining cache eligibility has been corrected to explicitly check HTTP methods against a definitive list of unsafe operations rather than relying on subtraction from safe methods. Until an upgrade is possible, developers should consider implementing custom interceptors that enforce strict method-based caching policies independent of undici's default behavior. This includes manually filtering out POST, PUT, DELETE, and PATCH requests from any cache storage or retrieval paths. Additionally, application-level validation should be reinforced to ensure critical state-changing operations are not reliant solely on cached responses without verifying the freshness and origin of the data through explicit network checks for high-risk endpoints. Regular security audits of HTTP client configurations are recommended to prevent similar logic errors in caching mechanisms across other libraries used within the ecosystem.