CVE-2025-27809: Arm Mbed TLS Information Disclosure Flaw

CVE-2025-27809 Overview

CVE-2025-27809 is a Certificate Validation Bypass vulnerability affecting Arm Mbed TLS, a widely-used cryptographic library for embedded systems and IoT devices. The vulnerability exists in versions before 2.28.10 and 3.x before 3.6.3, where on the client side, the library accepts servers that have trusted certificates for arbitrary hostnames unless the TLS client application explicitly calls mbedtls_ssl_set_hostname. This Insecure Default Configuration (CWE-1188) could enable man-in-the-middle attacks against applications that fail to properly configure hostname verification.

Critical Impact

Applications using vulnerable Mbed TLS versions without explicit hostname verification may unknowingly accept TLS connections from malicious servers presenting valid certificates for unrelated hostnames, potentially exposing sensitive communications to interception and tampering.

Affected Products

• Arm Mbed TLS versions prior to 2.28.10
• Arm Mbed TLS 3.x versions prior to 3.6.3
• Embedded systems and IoT devices utilizing affected Mbed TLS library versions

Discovery Timeline

• 2025-03-25 - CVE CVE-2025-27809 published to NVD
• 2025-07-17 - Last updated in NVD database

Technical Details for CVE-2025-27809

Vulnerability Analysis

This vulnerability stems from an insecure default configuration in Mbed TLS's certificate validation process. When a TLS client application establishes a connection using Mbed TLS, the library validates that the server's certificate is signed by a trusted Certificate Authority (CA). However, by default, it does not verify that the certificate's Common Name (CN) or Subject Alternative Name (SAN) matches the hostname of the server being connected to.

This means that if an attacker possesses any valid certificate signed by a trusted CA—even one for a completely unrelated domain—they could potentially impersonate any server to vulnerable clients. The attack requires the attacker to position themselves in the network path between the client and the legitimate server (man-in-the-middle position).

Root Cause

The root cause is an Insecure Default Configuration (CWE-1188). The mbedtls_ssl_set_hostname function must be explicitly called by the client application to enable hostname verification. Without this call, the library performs certificate chain validation but skips the critical step of verifying that the certificate actually belongs to the intended server hostname. This design places the burden of security on application developers who may not realize hostname verification is not enabled by default.

Attack Vector

The attack vector is network-based with high complexity requirements. An attacker must:

1. Obtain a valid TLS certificate from a trusted CA for any domain they control
2. Position themselves in the network path between the victim client and the target server
3. Intercept the TLS handshake and present their valid certificate instead of the legitimate server's certificate
4. The vulnerable Mbed TLS client will accept this certificate since it is validly signed, even though it's for the wrong hostname

This allows the attacker to decrypt, view, and potentially modify traffic that the client believes is securely communicated to the legitimate server. The vulnerability affects the confidentiality and integrity of communications, as detailed in the official Mbed TLS Security Advisory.

Detection Methods for CVE-2025-27809

Indicators of Compromise

• TLS connections established without hostname verification logging or errors in application logs
• Client applications connecting to IP addresses or hostnames that don't match presented certificate subjects
• Network traffic analysis revealing certificate hostname mismatches during TLS handshakes
• Presence of Mbed TLS versions prior to 2.28.10 or 3.6.3 in software inventory

Detection Strategies

• Review source code for TLS client implementations to verify mbedtls_ssl_set_hostname is called before establishing connections
• Perform software composition analysis (SCA) to identify applications using vulnerable Mbed TLS versions
• Monitor TLS connections for certificate hostname mismatches using network security monitoring tools
• Implement runtime detection for TLS connections where hostname verification is not configured

Monitoring Recommendations

• Enable verbose TLS logging in applications to capture certificate validation details
• Deploy network monitoring to detect potential man-in-the-middle attack patterns
• Establish baseline TLS connection behavior to identify anomalous certificate presentations
• Integrate vulnerability scanning into CI/CD pipelines to detect vulnerable Mbed TLS dependencies

How to Mitigate CVE-2025-27809

Immediate Actions Required

• Upgrade Mbed TLS to version 2.28.10 or later for the 2.x branch
• Upgrade Mbed TLS to version 3.6.3 or later for the 3.x branch
• Audit all TLS client code to ensure mbedtls_ssl_set_hostname is called with the expected server hostname
• Review firmware and embedded device inventory for affected Mbed TLS versions

Patch Information

Arm has released patched versions addressing this vulnerability. Version 2.28.10 resolves the issue for the 2.x branch, while version 3.6.3 addresses it for the 3.x branch. The patches improve the default behavior and provide clearer guidance on hostname verification requirements. Detailed release information is available in the GitHub Release Notes. Additional context regarding this long-standing issue can be found in the GitHub Issue Discussion.

Workarounds

• Ensure all TLS client applications explicitly call mbedtls_ssl_set_hostname with the expected server hostname before establishing connections
• Implement additional certificate validation at the application layer to verify hostname matches
• Consider implementing certificate pinning for critical connections as an additional security layer
• Restrict network access for affected devices to reduce man-in-the-middle attack surface

// Proper Mbed TLS hostname verification configuration
// Ensure this is called before mbedtls_ssl_handshake()
int ret = mbedtls_ssl_set_hostname(&ssl_context, "expected.hostname.com");
if (ret != 0) {
    // Handle error - hostname verification configuration failed
    // Do not proceed with connection
}