2FAuth: Authenticated SSRF via DNS rebinding in the otpauth image= icon fetch (validate-then-fetch TOCTOU)

published 2026-11-06 2 d ago modified 2026-11-06 2 d ago also EUVD-2026-118490

cvss: 5.9; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.2%; EPSS 0.2% chance of exploitation in 30 days, percentile 10%
public exploits: 0; none known

2FAuth is a web app to manage Two-Factor Authentication (2FA) accounts and generate their security codes. Prior to 8.0.0, the otpauth image-fetch path validates a hostname through ValidatesUrls::isExternalHost() and then lets IconService::storeRemoteImage() resolve the same hostname again for the HTTP request. When external otpauth image fetching is enabled, an authenticated user controlling low-TTL authoritative DNS can return a public address during validation and an internal address during the later connection, bypassing the private-address deny list to reach loopback, private-network, link-local, or cloud metadata endpoints. This issue is fixed in version 8.0.0.

Patch / fix

links 2

github.com/Bubka/2FAuth@52e9db9 commit
github.com/Bubka/2FAuth/releases/tag/v8.0.0 release

Exploit prediction

0.20% chance of exploitation in the next 30 days, higher than 10% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	5.9	CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:L/A:N

Affected products

Bubka 2FAuth

Classification

Weakness

CWE-367 Time-of-check Time-of-use (TOCTOU) Race Condition
CWE-918 Server-Side Request Forgery

References

Sources

CVE List CVE-2026-63440
NVD CVE-2026-63440

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary