CVE-2026-75899: fast-uri vulnerable to server-side request forgery via repeated hostname percent-decoding
Impact
fast-uri decodes a hostname's percent escapes twice in a single normalize() or resolve() call: once during parsing and again during authority recomposition. A nested percent-encoded host therefore survives the first decode and is turned into a live destination by the second, so normalize('http://%256c%256f%2563%2561%256c%2568%256f%2573%2574/') returns http://localhost/. Applications that normalize or resolve an untrusted URI before an SSRF check, redirect validation, or host allowlist can be steered to a different destination, including internal addresses such as loopback or a cloud metadata endpoint, than the encoded input appeared to contain. This is an incomplete-fix variant of CVE-2026-6322, whose encoded-authority-delimiter fix introduced the second decode.
Patches
Fixed in fast-uri 2.4.5, 3.1.6, and 4.1.3.
Workarounds
Reject untrusted URIs whose host component contains an encoded percent sign (%25) before passing them to normalize() or resolve().
Other sources
fast-uri is a URI parser for Node.js. It decodes percent escapes in a hostname during parsing and then decodes the parsed hostname a second time during authority recomposition, so a single call to normalize or resolve can turn nested percent-encoded input into a different network destination such as a loopback hostname or address. For example, a doubly encoded host that spells out a loopback name decodes to that live host in one operation, which contradicts RFC 3986 section 2.4 that an implementation must not decode the same string more than once. An application that normalizes or resolves an untrusted HTTP-family URI before outbound routing, redirect validation, or a host-policy check can receive a destination different from the one the original encoded host represented, giving a server-side request forgery and host-policy bypass primitive. This is an incomplete-fix variant of CVE-2026-6322. The affected versions are 2.4.1 up to but not including 2.4.5, 3.1.2 up to but not including 3.1.6, and 4.0.0 up to but not including 4.1.3. The issue is fixed in 2.4.5, 3.1.6, and 4.1.3, which normalize percent escapes once and preserve encoded percent signs. Users should upgrade to a patched version.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/fast-urito a version that resolves this vulnerability.Fixed in 4.1.3 - Upgrade
Upgrade
npm/fast-urito a version that resolves this vulnerability.Fixed in 3.1.6 - Upgrade
Upgrade
npm/fast-urito a version that resolves this vulnerability.Fixed in 2.4.5 - Upgrade
Upgrade
fast-urito a version that resolves this vulnerability.Fixed in 2.4.5 - Upgrade
Upgrade
fast-urito a version that resolves this vulnerability.Fixed in 3.1.6 - Upgrade
Upgrade
fast-urito a version that resolves this vulnerability.Fixed in 4.1.3 - Configuration
Before passing any untrusted HTTP-family URI to fast-uri normalize() or resolve(), check the URI’s host component and reject the request if the host contains an encoded percent sign (%25), to prevent nested percent-encoded hostnames from being decoded into a different destination (e.g., loopback/internal endpoints).
fast-uri (Node.js URI parser) Input validation for URI host = Reject untrusted URIs whose host component contains encoded percent sign (%25) before calling normalize() or resolve()
Event History
Frequently Asked Questions
Which fast-uri versions need to be remediated?
Affected releases are 2.4.1 through 2.4.4, 3.1.2 through 3.1.5, and 4.0.0 through 4.1.2. Upgrade to 2.4.5, 3.1.6, or 4.1.3, respectively.
What application patterns make this issue exploitable?
Exposure exists when an application passes an untrusted HTTP-family URI through fast-uri normalize or resolve before making an outbound request, validating a redirect, or enforcing a host policy. The normalized or resolved result can identify a different destination than the originally supplied encoded hostname.
Does exploitation require authentication or user interaction?
No. The supplied severity vector indicates network-based exploitation with low complexity, no privileges required, and no user interaction.
How can teams identify potentially affected deployments?
Inventory direct and transitive fast-uri dependencies for versions in the affected ranges. Prioritize applications that normalize or resolve attacker-controlled HTTP-family URIs as part of outbound routing, redirect handling, or hostname allowlist/blocklist checks.