CVE-2026-16158 in reply-from
Summary
by MITRE • 07/18/2026
Impact: @fastify/reply-from versions from 8.3.1 up to but not including 12.6.4 build the internal URL cache key by concatenating the destination and source path without a delimiter. Different destination and source pairs can therefore produce the same key while resolving to different upstream URLs. When getUpstream selects an upstream from request data, a URL cached for one upstream can be reused for a request intended for another upstream, causing cross-upstream data access and modification. The default configuration is affected. Setting disableCache to true prevents the behavior. Patches: upgrade to @fastify/reply-from 12.6.4. Workarounds: pass disableCache: true when registering the plugin.
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Analysis
by VulDB Data Team • 07/18/2026
The vulnerability in @fastify/reply-from versions 8.3.1 through 12.6.3 represents a critical cache collision issue that enables cross-upstream data access and modification. This flaw stems from an inadequate URL cache key generation mechanism where destination and source paths are concatenated without proper delimiters, creating potential hash collisions that can lead to unauthorized data exposure and manipulation. The vulnerability operates at the application layer and directly impacts the integrity of proxy operations within fastify-based systems.
The technical implementation of this vulnerability manifests through improper cache key construction that lacks sufficient entropy to distinguish between different upstream URL combinations. When the getUpstream function selects an upstream from request data, it relies on these improperly constructed cache keys which can map multiple distinct upstream configurations to identical cache entries. This fundamental flaw in the caching algorithm creates a path for attackers to potentially access or modify data intended for different upstream services, effectively breaking the isolation between separate upstream endpoints.
The operational impact of this vulnerability extends beyond simple data leakage to encompass full data modification capabilities across different upstream systems. Attackers can exploit this weakness to achieve unauthorized cross-upstream access where cached responses meant for one destination may be served to requests intended for another upstream service. This creates a scenario where sensitive information could be exposed to unintended recipients while also allowing potential modification of data through carefully crafted requests that leverage the cache collision behavior. The default configuration remains vulnerable, making this issue particularly dangerous as organizations may not even realize they are affected.
Security implications align with CWE-20 and CWE-798 categories, representing improper input validation and hard-coded credentials issues respectively, while also mapping to ATT&CK technique T1566 for credential access through proxy manipulation. The vulnerability essentially creates a man-in-the-middle scenario where cache entries can be misused across different upstream contexts, potentially enabling privilege escalation or data exfiltration attacks against systems relying on this plugin. Organizations using fastify proxy configurations without explicit cache disabling are at risk of having their upstream service isolation compromised.
The recommended remediation involves upgrading to @fastify/reply-from version 12.6.4 which includes proper cache key delimiter implementation that prevents the collision conditions. Alternatively, administrators can implement a temporary workaround by explicitly setting disableCache: true when registering the plugin, which effectively disables the problematic caching mechanism until the upgrade can be completed. This dual approach allows organizations to either immediately address the vulnerability through patching or temporarily mitigate risk while planning for the proper upgrade cycle. The fix ensures proper separation between upstream configurations and prevents the cache key collisions that enable the cross-upstream access behavior.