Overview
CVE-2025-12889 describes a vulnerability affecting TLS 1.2 connections. The vulnerability allows a client to select a weaker digest for authentication, even if the server supports stronger digests as indicated in the CertificateRequest message. This can potentially weaken the security of the TLS connection, making it more susceptible to certain types of attacks.
Technical Details
In a standard TLS 1.2 handshake, the server sends a CertificateRequest message to the client, specifying the acceptable certificate types and signature algorithms. Ideally, the client should adhere to these constraints and select a signature algorithm (digest) from the offered list. However, CVE-2025-12889 highlights a scenario where the client, when using TLS 1.2, might disregard these server-specified preferences and choose a weaker digest that it supports, regardless of whether it’s in the CertificateRequest’s acceptable algorithms.
This issue potentially stems from implementation flaws in TLS libraries, where the server-specified constraints are not rigorously enforced during the client’s certificate selection process. A client could potentially select a digest algorithm, like MD5 or SHA1 (if still supported), even when the server explicitly prefers SHA256 or higher.
CVSS Analysis
Currently, the CVE entry has a CVSS score of N/A, which means that the severity has not been determined yet. The actual impact depends on several factors, including:
- The specific TLS library implementation affected.
- The availability of weaker digest algorithms in the client’s configuration.
- The attack scenario and attacker capabilities.
A more detailed CVSS score and severity level are expected to be assigned as more information becomes available.
Possible Impact
The ability for a client to choose a weaker digest than the server prefers could lead to the following potential impacts:
- Weakened Authentication: Using weaker digest algorithms makes the connection more susceptible to collision attacks or other cryptographic weaknesses associated with those algorithms.
- Downgrade Attacks: An attacker might be able to manipulate the handshake process to force the client to use a weaker digest, even if both client and server support stronger algorithms.
- Compromised Confidentiality and Integrity: A successful attack exploiting this vulnerability could potentially compromise the confidentiality and integrity of data transmitted over the TLS connection.
Mitigation or Patch Steps
The primary mitigation involves patching the affected TLS library to ensure strict adherence to the server’s CertificateRequest parameters. Specifically, the client-side implementation should be updated to only select digest algorithms that are explicitly specified by the server in the CertificateRequest message.
Based on the provided reference, a potential fix has been identified for wolfSSL:
- Apply the patch available at: wolfSSL Pull Request #9395
General recommendations for addressing similar vulnerabilities include:
- Keep TLS libraries up-to-date with the latest security patches.
- Disable or deprecate support for weaker digest algorithms (e.g., MD5, SHA1) whenever possible.
- Monitor TLS connections for the use of unexpectedly weak digest algorithms.
