Bug 2535831 (CVE-2026-90982)

Summary: CVE-2026-90982 @fastify/static: @fastify/static: Information disclosure via route guard bypass on case-insensitive filesystems
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: bbrownin, sdawley
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in @fastify/static, a Fastify plugin for serving static files. On case-insensitive filesystems, such as Windows or default macOS volumes, a route guard or allowedPath restriction can be bypassed. This occurs because the route matcher is case-sensitive while the filesystem is not, allowing an attacker to alter the letter case of a path segment to bypass protection. Consequently, an unauthenticated attacker can read files that were configured to be protected by a route guard or allowedPath rule, leading to information disclosure.
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Description OSIDB Bzimport 2026-09-17 07:08:37 UTC 
@fastify/static is a Fastify plugin that serves static files from a configured root directory. In versions before 10.1.4, on a case-insensitive filesystem such as Windows or the default macOS volume, a route guard or allowedPath restriction can be bypassed by altering the letter case of a path segment. The route matcher is case-sensitive while the filesystem is not, so a request that changes the case of a protected segment does not match the guarded route and falls through to the static handler, yet the filesystem resolves it to the same protected file. As a result, an unauthenticated request can read a file that a route guard or allowedPath was configured to protect. The issue does not affect case-sensitive filesystems and is not a directory traversal, since nothing is served from outside the configured root. The issue is fixed in @fastify/static 10.1.4, which validates the requested path against its actual on-disk spelling and rejects case-aliased paths before authorization. As a workaround, serve static files from a case-sensitive filesystem, or ensure route guards and allowedPath rules account for every letter-case variant of the protected paths.