app-Authenticator: Improper Verification of Source of a Communication Channel
CVE-2026-33875 is a critical-severity improper verification of source of a communication channel vulnerability in app-Authenticator from Gematik, with a CVSS 3.1 base score of 9.3 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, but only if a user takes part, for example by opening a link or file. Affected app-Authenticator versions: < 4.16.0. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.27% chance of exploitation within 30 days, which is low compared with other CVEs.
Critical severity: a successful attack would have severe impact.
What to do
- Update Authenticator to 4.16.0 or a later version.
- Read the advisory on github.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
- Because the attack needs a user to take part, warn users about unexpected links and files related to this product until it is patched.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Gematik | app-Authenticator | < 4.16.0 | Not stated |
| Gematik | Authenticator | before 4.16.0 | 4.16.0 |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- RequiredA user must take an action such as opening a file or link.
- Scope
- ChangedThe attack can reach beyond the vulnerable component into other systems.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- NoneAvailability is not affected.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N. Scored by CNA (github.com).
Official description
Gematik Authenticator securely authenticates users for login to digital health applications. Versions prior to 4.16.0 are vulnerable to authentication flow hijacking, potentially allowing attackers to authenticate with the identities of victim users who click on a malicious deep link. Update Gematik Authenticator to version 4.16.0 or greater to receive a patch. There are no known workarounds.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Improper Verification of Source of a Communication Channel
The product establishes a communication channel to handle an incoming request that has been initiated by an actor, but it does not properly verify that the request is coming from the expected origin.
How developers prevent this class of flaw
- Use a mechanism that can validate the identity of the source, such as a certificate, and validate the integrity of data to ensure that it cannot be modified in transit using an Adversary-in-the-Middle (AITM) attack. When designing functionality of actions in the URL scheme, consider whether the action should be accessible to all mobile applications, or if an allowlist of applications to interface with is appropriate.
More Improper Verification of Source of a Communication Channel vulnerabilities (CWE-940)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Other references
Other vulnerabilities in app-Authenticator
- 7.8 High severity
Recent Improper Verification of Source of a Communication Channel vulnerabilities
- 7.5 High severity
- 7.5 High severity
- 9.3 Critical severity
- 7.6 High severity
- 7.0 High severity
- 8.4 High severity
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