The epa4all-client prior to version 1.2.2 does not properly verify cryptographic signatures on discovery documents received from the identity provider (IDP). This allows an attacker who can intercept and modify the TLS connection within the Telematik Infrastruktur network to replace the discovery document with a forged one. The forged document redirects the URIs for encryption and signature public keys to attacker-controlled URLs. Consequently, the client encrypts the signed challenge response to the attacker's key and sends it to the attacker's authentication endpoint, exposing signed authentication material. This flaw is classified under CWE-347 (Improper Verification of Cryptographic Signature) and has a CVSS 3.1 score of 7.4 (high). The issue is resolved in epa4all-client version 1.2.2.

Successful exploitation allows an attacker with MITM capability on the TLS connection within the TI network to capture signed authentication material by redirecting cryptographic key URIs to attacker-controlled endpoints. This compromises the confidentiality and integrity of authentication data, potentially enabling unauthorized access or impersonation within the affected infrastructure. There is no indication of impact on availability. No known exploits in the wild have been reported.

Upgrade the epa4all-client to version 1.2.2 or later, where this vulnerability is fixed. Since the vulnerability requires MITM capability within the TI network, ensuring secure TLS connections and network protections can reduce risk. Patch status is not explicitly stated in vendor advisories, but the description confirms the fix is included in version 1.2.2. No additional vendor mitigation guidance is available.