GHSA-w74x-v6f9-j3xg CVE-2026-110306
fast-uri vulnerable to URI scheme spoofing via percent-encoded scheme decoding
- Severity
- 6.1 / 10
- Medium · CVSS 3.1
- Exploit chance (EPSS)
- 0.25%
- Chance of exploitation in the next 30 days. Higher than 16.36% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Fix
2.4.8- Upgrade fast-uri to this version or later
Description
Impact
fast-uri percent-decodes the scheme component with the legacy global unescape() before validating it, so an input whose decoded scheme is a valid but dangerous token is emitted as a live scheme. RFC 3986 section 3.1 does not allow percent-encoding in a scheme, so no RFC or WHATWG parser treats such input as having that scheme, but fast-uri does.
For example, normalize('%6Aavascript:alert(1)') returns javascript:alert(1), resolve('https://a.example/', '%6Aavascript:alert(1)') returns javascript:alert(1), and parse('%6Aavascript:alert(1)').scheme returns javascript. The %uXXXX form (%u006Aavascript:) produces the same result, and equal('%68ttp://a/', 'http://a/') returns true.
An application that screens dangerous schemes on the raw input and then stores or renders normalize(input) or resolve(base, input) turns an inert string into an executable javascript:, data:, or vbscript: URL, a stored or reflected cross-site scripting vector in the consuming application. equal() also treats an encoded and a plain scheme as the same, which can bypass equality-based allowlists.
Patches
Upgrade to fast-uri 2.4.8, 3.1.9, or 4.2.3.
Workarounds
Reject input whose scheme part (before the first :, and before any /, ?, or #) contains %. Alternatively, check parse(x).scheme against an allowlist after normalization and use that same parsed result rather than the raw string.
Affected versions
npm
| Package | Vulnerable | Fixed in |
|---|---|---|
| fast-uri | < 2.4.8 |
2.4.8 |
| fast-uri | >= 3.0.0, < 3.1.9 |
3.1.9 |
| fast-uri | >= 4.0.0, < 4.2.3 |
4.2.3 |
| fast-uri | < 2.4.8 |
2.4.8 |
| fast-uri | >= 3.0.0, < 3.1.9 |
3.1.9 |
| fast-uri | >= 4.0.0, < 4.2.3 |
4.2.3 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
fast-uri npm → 2.4.8
- npm
npm install fast-uri@2.4.8- yarn
yarn add fast-uri@2.4.8- pnpm
pnpm add fast-uri@2.4.8
Severity breakdown
CVSS 3.1 6.1
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- Required
- Scope
- Changed
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
Weakness
CWE-172: Encoding Error
The product does not properly encode or decode the data, resulting in unexpected values.
How to prevent it
- Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- Implementation: While it is risky to use dynamically-generated query strings, code, or commands that mix control and data together, sometimes it may be unavoidable. Properly quote arguments and escape any special characters within those arguments. The most conservative approach is to escape or filter all characters that do not pass an extremely strict allowlist (such as everything that is not alphanumeric or white space). If some special characters are still needed, such as white space, wrap each argument in quotes after the escaping/filtering step. Be careful of argument injection (CWE-88).
- Implementation: Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.
CWE-180: Incorrect Behavior Order: Validate Before Canonicalize
The product validates input before it is canonicalized, which prevents the product from detecting data that becomes invalid after the canonicalization step.
How to prevent it
- Implementation: Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.
References
Advisories
Fixes and patches
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-333x-w4r6-6crp fast-uri vulnerable to scheme injection via an unescaped first-segment colon in suffix references | Low | Also affects fast-uri |
| GHSA-wjr9-j795-jxcr fast-uri vulnerable to host confusion via unchecked WHATWG host canonicalization | Medium | Also affects fast-uri |
| GHSA-xhq7-xmqj-769h fast-uri vulnerable to improper path equivalence in relative reference normalization | Low | Also affects fast-uri |
| GHSA-jvvf-x445-j334 fast-uri vulnerable to mailto header injection via percent-encoded field-name desynchronization | Medium | Also affects fast-uri |
| GHSA-hrr3-gc8f-f4qj fast-uri vulnerable to inconsistent host case normalization via percent-encoded octets | Medium | Also affects fast-uri |
| GHSA-58mr-gqgx-xq4g fast-uri vulnerable to host confusion via an unclosed bracket in the URI authority | High | Also affects fast-uri |
| GHSA-qw65-cvwx-89v3 fast-uri vulnerable to authority injection via an unvalidated port in serialize | High | Also affects fast-uri |
| GHSA-jqff-g426-hqxp fast-uri vulnerable to host confusion via percent-encoded scheme normalization | High | Also affects fast-uri |
Credits: UlisesGascon, mcollina
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