VulnerabilityPublished 3 Aug 2026

CVE-2026-18446: fast-uri vulnerable to host confusion via backslash authority introducer

Affects fast-uriSource GCVE Database

Compare

Not known to be exploited · CVSS 7.5 High : upgrade to 2.4.4 soon.

Data as of 1 Nov 2026, 22:58 UTC

CVSS 3.1 base score

7.5High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.3% chance of exploitation in 30 days – higher than 32% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.4.4, 3.1.5, 4.1.2

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

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

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 3 Aug 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 1 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade fast-uri to 2.4.4; fast-uri to 3.1.5; fast-uri to 4.1.2.

Affected software

Product	Affected	Fixed in
npmfast-uri	Affected <2.4.4	Fixed in 2.4.4
npmfast-uri	Affected >=3.0.0 <3.1.5	Fixed in 3.1.5
npmfast-uri	Affected >=4.0.0 <4.1.2	Fixed in 4.1.2

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

### Impact `fast-uri` v4.1.1 and earlier require a literal `//` to recognize a URI authority, so a reference that uses `\\`, `/\`, or `\/` as the authority introducer (in place of `//`, after an optional scheme) is parsed with no authority: the sequence and everything after it fold into the path. Node's native WHATWG `URL` (used by `fetch()`, `undici`, and Node's `http`/`https` clients) instead treats `\` as interchangeable with `/` for special schemes (`http`, `https`, `ws`, `wss`, `ftp`, `file`), so the two parsers extract different hosts from the same input. For example, `fast-uri` resolves `\\evil.com/path` against base `https://allowed.com/` to `https://allowed.com/%5C%5Cevil.com/path` (confined to the trusted host), while Node's WHATWG URL resolves the same reference to `https://evil.com/path`. Applications that use `fast-uri` to enforce host-based policy (allowlists, denylists, loopback/SSRF filtering, redirect validation, outbound proxy routing) before passing the same URL into Node's URL or `fetch()` consumers see a policy/use desync and can be steered to an unintended destination. ### Patches Upgrade to `fast-uri` v4.1.2, v3.1.5, v2.4.4. ### Workarounds None. Upgrade to the patched version.

CWE-436Interpretation Conflict (opens in a new tab)
Class weaknessOWASP Top 10 A06:2025: Insecure Design (opens in a new tab)
Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.
OWASP Cheat Sheets
REST Assessment (opens in a new tab)
REST Security (opens in a new tab)
Cross-Site Request Forgery Prevention (opens in a new tab)
Secure Design Principles (opens in a new tab)
OWASP ASVS 4.0.3
V13.2.5 (opens in a new tab)
Attack patterns
HTTP Request Splitting (opens in a new tab)
HTTP Response Smuggling (opens in a new tab)
HTTP Response Splitting (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

WeaknessesCWE-436

Technical details

References

Community reviews

Related threats