better-auth before 1.4.2 basePath Modification DoS

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

better-auth (npm) before 1.4.2 allows an external request to configure baseURL when it is not otherwise defined (e.g., BETTER_AUTH_URL is unset). An attacker able to make the very first request to the server after startup can poison the router's base path, causing all routes to return 404 for all users (denial of service). The issue is not reachable when baseURL is explicitly configured or on typical managed hosting platforms.

Source published

2 Aug 2026 12:15:23 UTC

Source modified

3 Aug 2026 14:29:35 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · 9.3 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

CVSS 3.1 · 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

better-auth (npm) before 1.4.2 allows an external request to configure baseURL when it is not otherwise defined (e.g., BETTER_AUTH_URL is unset). An attacker able to make the very first request to the server after startup can poison the router's base path, causing all routes to return 404 for all users (denial of service). The issue is not reachable when baseURL is explicitly configured or on typical managed hosting platforms.

Source published

Aug 2, 2026, 12:15:23 PM (timezone not stated)

Source modified

Aug 3, 2026, 2:29:35 PM (timezone not stated)

Observed here

11 Nov 2026 04:37:50 UTC

Source terms

ENISA EUVD · source terms and attribution apply

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 4.0 · 9.3 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

better-auth (npm) before 1.4.2 allows an external request to configure baseURL when it is not otherwise defined (e.g., BETTER_AUTH_URL is unset). An attacker able to make the very first request to the server after startup can poison the router's base path, causing all routes to return 404 for all users (denial of service). The issue is not reachable when baseURL is explicitly configured or on typical managed hosting platforms.

Source published

2 Aug 2026 13:16:52 UTC

Source modified

6 Oct 2026 22:10:00 UTC

Observed here

25 Oct 2026 04:17:18 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · [email protected] · Secondary · 9.3 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

CVSS 3.1 · [email protected] · Secondary · 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

## Duplicate Advisory This advisory has been withdrawn because it is a duplicate of GHSA-569q-mpph-wgww. This link is maintained to preserve external references. ## Original Description better-auth (npm) before 1.4.2 allows an external request to configure baseURL when it is not otherwise defined (e.g., BETTER_AUTH_URL is unset). An attacker able to make the very first request to the server after startup can poison the router's base path, causing all routes to return 404 for all users (denial of service). The issue is not reachable when baseURL is explicitly configured or on typical managed hosting platforms.

Source published

2 Aug 2026 15:30:21 UTC

Source modified

1 Sep 2026 19:02:53 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Publisher rating · CRITICAL

# Summary Affected versions of Better Auth allow an external request to configure `baseURL` when it isn’t defined through any other means. This can be abused to poison the router’s base path, causing all routes to return 404 for all users. This issue is only exploitable when `baseURL` is not explicitly configured (e.g., `BETTER_AUTH_URL` is missing) *and* the attacker is able to make the very first request to the server after startup. In properly configured environments or typical managed hosting platforms, this fallback behavior cannot be reached. # Details A combination of `X-Forwarded-Host` and `X-Forwarded-Proto` is implicitly trusted. This allows the first request to configure baseURL whenever it is not explicitly configured. Here's the code that reads the headers: <img width="631" height="219" alt="headers" src="https://github.com/user-attachments/assets/b3fb0078-a62f-4058-9d0b-4afbd30c4953" /> Here's the call to `getBaseURL()`, the result is assigned to `ctx.baseURL`. <img width="838" height="414" alt="write" src="https://github.com/user-attachments/assets/a7b4dd17-75c3-49ef-9d08-6a2079d6a0ea" /> Here's the router receiving the poisoned `basePath`: <img width="594" height="372" alt="router" src="https://github.com/user-attachments/assets/5fdf2862-9cd1-4b96-b146-18e67d904157" /> `X-Forwarded-Host` and `X-Forwarded-Proto` can be used to modify the pathname of a parsed URL object which forms `baseURL`. `basePath` is then derived from the pathname of `baseURL`. Once the router `basePath` is poisoned it fails to match & route incoming requests. # Repro Start a better-auth server with no `baseURL` configuration. Send the following request as the first request to the server: ```curl curl -i --location 'https://example.com/api/auth/ok' \ --header 'X-Forwarded-Proto: some:' \ --header 'X-Forwarded-Host: junk' ``` The better-auth API check endpoint returns 404. Now send a regular request without the `X-Forwarded-Proto` and `X-Forwarded-Host` headers. ```curl curl -i --location 'https://example.com/api/auth/ok' ``` The better-auth API check endpoint still returns 404. _Example result_ <img width="662" height="307" alt="attack" src="https://github.com/user-attachments/assets/5a9cfdb5-3db7-4504-9f0a-b3c32a6dc823" /> We have modified the `basePath` for the router until the server is restarted. An attacker can repeatedly send these attack requests aiming to persistently exploit the vulnerability.

Source published

1 Dec 2025 21:29:48 UTC

Source modified

1 Sep 2026 19:02:52 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:P

Publisher rating · HIGH