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🛡️ CVE-2026-16221 — fast-uri

🟠 CVSS 8.0 — High ✅ No Known Exploit CWE-436 OSV
8.0
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

fast-uri vulnerable to host confusion via literal backslash authority delimiter

Impact

fast-uri v4.1.0 and earlier do not treat a literal backslash (U+005C) as an authority delimiter. Node's native WHATWG URL (used by fetch(), undici, and Node's http/https clients) normalizes \ to / for special schemes (http, https, ws, wss, ftp, file), so the two parsers extract different hosts from the same input string.

For example, http://evil.com\@allowed.com is treated by fast-uri as host allowed.com with userinfo evil.com\, while Node's WHATWG URL parser and fetch() see host evil.com with path /@allowed.com.

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, including cloud metadata endpoints, loopback, or internal hosts.

Patches

Upgrade to fast-uri v4.1.1, v3.1.4, or v2.4.3.

Workarounds

None. Upgrade to the patched version.

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is low, an attacker needs no privileges on the target. No user interaction is required. The scope is unchanged, so the impact stays within the vulnerable component. Rated impact: confidentiality none, integrity high, availability none.

CVSS metrics in full

The score comes from this vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

  • Attack vector: Network — reachable from anywhere that can route to the service.
  • Attack complexity: Low — the attack works reliably, with no preparation.
  • Privileges required: None — an unauthenticated stranger can try it.
  • User interaction: None — nobody has to be tricked into anything.
  • Scope: Unchanged — the damage stays inside the vulnerable component.
  • Confidentiality impact: None.
  • Integrity impact: High — total loss, or loss the attacker controls.
  • Availability impact: None.

Weakness class

CVE-2026-16221 is classified as CWE-436: Interpretation Conflict. Two components parse the same data differently, so a check in one is bypassed in the other.

Affected software

CVE-2026-16221 is recorded against 1 package.

  • fast-uri

Timeline and source

Published on 21 July 2026 and last revised on 5 August 2026. No public exploit is currently recorded for this entry. Record sourced from OSV.

References

github.com (Web)
nvd.nist.gov (Advisory)
github.com (Web)
github.com (Web)
github.com (Web)
cna.openjsf.org (Web)
github.com (Package)
github.com (Web)
github.com (Web)
github.com (Web)

Other advisories for this package

fast-uri has other advisories on record. If you are patching this one, these are worth checking on the same host:

Same weakness in other software

These advisories are the same class of weakness (CWE-436: Interpretation Conflict) in other software:

Details

Severity HIGH
CVSS Score 8.0
CVSS Vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
CWE CWE-436
Public Exploit ✅ No
Source OSV
Published 2026-07-21
Updated 2026-08-20
Modified 2026-08-05
Fix URL N/A

Affected Packages

Software From version Fixed in
fast-uri — —

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ⓘ Data Notice: The information presented above has been compiled from publicly available internet sources. Boteraser aggregates this data solely for informational purposes and does not independently classify, evaluate, or endorse any findings about the vulnerabilities listed. The accuracy and completeness of this information is the sole responsibility of the original publishers. Boteraser and its operators accept no liability for any decisions made based on this data.

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