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fast-uri CVE-2026-18446

| EUVDEUVD-2026-51553 HIGH
Interpretation Conflict (CWE-436)
2026-07-31 openjs GHSA-7p8r-x3mc-p8w7
7.5
CVSS 3.1 · Vendor: openjs
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Severity by source

Vendor (openjs) PRIMARY
7.5 HIGH
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
vuln.today AI
7.5 HIGH

Network vector with no privileges required to submit a crafted URL; integrity-only impact reflects security control bypass without direct confidentiality or availability consequence.

3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
SUSE
5.3 MEDIUM
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Red Hat
7.5 HIGH
qualitative

Primary rating from Vendor (openjs).

CVSS VectorVendor: openjs

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

Lifecycle Timeline

3
Patch available
Jul 31, 2026 - 16:02 EUVD
Analysis Generated
Jul 31, 2026 - 15:21 vuln.today
CVE Published
Jul 31, 2026 - 14:37 cve.org
HIGH 7.5

DescriptionCVE.org

fast-uri before 4.1.2, 3.1.5, and 2.4.4 requires a literal double forward slash to recognize a URI authority, so a reference that uses a backslash based introducer in place of it (backslash backslash, forward slash backslash, or backslash forward slash) is parsed with no authority and folds into the path. Node's native WHATWG URL parser instead treats a backslash as interchangeable with a forward slash for special schemes, so the two parsers extract different hosts from the same input. Applications that use fast-uri to enforce host based policy such as allowlists, SSRF filtering, or redirect validation before passing the same URL into Node's URL or fetch consumers can be steered to an unintended host. Upgrade to fast-uri 4.1.2, 3.1.5, or 2.4.4.

AnalysisAI

Security filter bypass in fast-uri (before versions 4.1.2, 3.1.5, and 2.4.4) enables unauthenticated remote attackers to circumvent host-based access controls - including SSRF filters, allowlists, and redirect validators - by submitting URLs that use backslash-based authority introducers. The parser inconsistency between fast-uri and Node.js's native WHATWG URL handler causes the same crafted URL to resolve to different hosts in each parser, allowing attackers to steer downstream requests to unintended destinations. No public exploit is identified at time of analysis and this CVE does not appear in CISA KEV, though the CVSS 7.5 High rating with AV:N/AC:L/PR:N/UI:N reflects straightforward remote exploitability wherever the vulnerable application pattern exists.

Technical ContextAI

fast-uri is a URI parsing library central to the Fastify Node.js web framework ecosystem, identified by CPE cpe:2.3:a:fast-uri:fast-uri:*:*:*:*:*:*:*:*. The root cause is CWE-436 (Interpretation Conflict) - a vulnerability class where two components processing the same input derive different semantic results. fast-uri strictly requires a literal double forward slash ('//') to recognize a URI authority component; when a backslash-based sequence ('\\', '/\', or '\/') is used instead, fast-uri treats the input as having no authority and folds the remainder into the path. Node.js's native WHATWG URL parser, by contrast, follows the WHATWG URL Living Standard which explicitly treats backslash as equivalent to forward slash for 'special' URI schemes (http, https, ftp, ws, wss). This standard-compliant behavior means Node.js correctly identifies the intended host in such URLs while fast-uri does not - a textbook parser differential, a well-documented attack pattern in SSRF and open-redirect research. Applications that use fast-uri as a security enforcement gate and then pass the same URL string to Node's URL API or fetch() as the actual consumer are exposed to the discrepancy.

RemediationAI

Upgrade fast-uri to version 4.1.2, 3.1.5, or 2.4.4 as appropriate for the major version in use; these are the vendor-released patches confirmed by the OpenJS Foundation CNA via the advisory at https://github.com/fastify/fast-uri/security/advisories/GHSA-7p8r-x3mc-p8w7. If an immediate upgrade is not feasible, a compensating control is to preprocess user-supplied URLs before passing them to fast-uri by normalizing all backslash characters in the authority position to forward slashes - this eliminates the ambiguity the attacker exploits, though it alters URL semantics and must be validated against application behavior. Alternatively, consolidate all URL host extraction and policy enforcement onto Node's native URL parser exclusively, eliminating the parser differential entirely; this requires verifying that fast-uri is removed from the security-critical validation path. As a defense-in-depth measure independent of this fix, enforce network-level egress filtering on outbound HTTP requests so that no application-layer allowlist is the sole SSRF control - bypassing the filter should not be sufficient to reach sensitive internal endpoints.

CVE-2026-13676 HIGH POC
7.5 Jun 29

Host-based security policy bypass in the fast-uri Node.js URI parser (versions 2.3.1 through 3.1.2 and 4.0.0) lets remot

CVE-2026-6321 HIGH POC
7.5 May 04

Path normalization bypass in fast-uri 3.1.0 and earlier allows remote attackers to circumvent path-based access controls

CVE-2026-6322 HIGH POC
7.5 May 05

Authority confusion in fast-uri JavaScript library allows remote attackers to bypass URL validation security controls. T

CVE-2026-75931 HIGH POC
7.5 Aug 24

Host confusion in fast-uri, a Node.js URI parser in the Fastify ecosystem, allows policy bypass by exploiting inconsiste

CVE-2026-75975 HIGH POC
7.5 Aug 24

Server-side request forgery and address-policy bypass in fast-uri, the Node.js URI parser used widely in the fastify eco

CVE-2026-75899 HIGH POC
7.5 Aug 24

Double-decoding of percent-encoded hostnames in fast-uri (Node.js URI parser) creates a server-side request forgery and

CVE-2026-76172 HIGH POC
7.5 Aug 24

Parsing and serialization inconsistency in fast-uri, the Node.js URI parser core to the Fastify ecosystem, allows networ

CVE-2026-84292 HIGH
7.5 Sep 02

URI authority injection in fast-uri allows an attacker-supplied port value containing delimiter characters to demote the

CVE-2026-84394 HIGH
7.5 Sep 02

Host-parsing ambiguity in fast-uri allows attackers to silently bypass SSRF denylists, redirect allowlists, and proxy ro

CVE-2026-16221 HIGH POC
7.5 Jul 19

Host-based policy bypass in the fast-uri Node.js URI parser (versions 2.3.1 through 4.1.0, spanning the 2.x line up to 2

Vendor StatusVendor

SUSE

Severity: Moderate
Product Status
SUSE Linux Enterprise Micro 5.3 Not-Affected
SUSE Linux Enterprise Micro 5.3 Not-Affected
SUSE Linux Enterprise Micro 5.3 Not-Affected
SUSE Linux Enterprise Micro 5.3 Not-Affected
SUSE Linux Enterprise Micro 5.4 Not-Affected

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CVE-2026-18446 vulnerability details – vuln.today

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