CVE-2026-25834 Overview
A cryptographic vulnerability has been identified in Mbed TLS affecting versions 3.3.0 through 3.6.5 and version 4.0.0 that allows algorithm downgrade attacks. This vulnerability (CWE-295: Improper Certificate Validation) enables attackers to manipulate TLS handshakes and force the use of weaker cryptographic algorithms, potentially compromising the integrity and availability of secure communications.
Critical Impact
Algorithm downgrade vulnerabilities in TLS implementations can allow attackers to weaken the cryptographic protections of encrypted communications, enabling man-in-the-middle attacks and potential data manipulation.
Affected Products
- Mbed TLS v3.3.0 through v3.6.5
- Mbed TLS v4.0.0
Discovery Timeline
- 2026-04-01 - CVE CVE-2026-25834 published to NVD
- 2026-04-01 - Last updated in NVD database
Technical Details for CVE-2026-25834
Vulnerability Analysis
This vulnerability stems from improper validation of signature algorithm parameters during the TLS handshake process. The flaw allows a network-based attacker to inject or manipulate signature algorithm selections, potentially forcing a downgrade to weaker cryptographic algorithms than those originally negotiated between client and server.
The vulnerability is accessible over the network without requiring authentication or user interaction. When exploited, it can impact both the integrity and availability of the affected system. The signature algorithm injection technique allows attackers positioned in the network path to interfere with the secure channel establishment process.
According to the Mbed TLS Advisory 2026-03, this vulnerability specifically targets the signature algorithm negotiation mechanism in affected versions.
Root Cause
The root cause is classified as CWE-295 (Improper Certificate Validation), indicating that the vulnerability exists in how Mbed TLS validates and processes signature algorithm parameters during TLS session establishment. The implementation fails to properly enforce constraints on algorithm selection, allowing malicious actors to influence the negotiation process.
Attack Vector
The attack is network-based and can be executed remotely without requiring any privileges or user interaction. An attacker positioned as a man-in-the-middle can intercept and modify TLS handshake messages to inject preferred signature algorithms, potentially downgrading the connection to use weaker cryptographic primitives.
The vulnerability manifests during the TLS handshake when processing signature algorithm extensions. An attacker can manipulate these extensions to force the selection of weaker algorithms. For detailed technical information regarding the signature algorithm injection technique, refer to the Mbed TLS Security Advisories documentation.
Detection Methods for CVE-2026-25834
Indicators of Compromise
- Unexpected TLS handshake failures or renegotiations in application logs
- Connections using deprecated or weak signature algorithms (e.g., SHA-1 with RSA) when stronger algorithms should be negotiated
- Network traffic anomalies during TLS handshake phases, particularly in ClientHello/ServerHello exchanges
Detection Strategies
- Monitor TLS handshake traffic for signature algorithm mismatches between client capabilities and server selections
- Implement deep packet inspection to detect manipulation of signature algorithm extensions in TLS handshakes
- Deploy network-based intrusion detection rules to identify anomalous TLS negotiation patterns
Monitoring Recommendations
- Enable detailed TLS handshake logging in Mbed TLS applications to track algorithm negotiation
- Audit production systems to identify Mbed TLS versions within the affected range (3.3.0 - 3.6.5 and 4.0.0)
- Monitor for applications establishing connections with unexpectedly weak cryptographic parameters
How to Mitigate CVE-2026-25834
Immediate Actions Required
- Inventory all systems and applications using Mbed TLS and identify those running affected versions (3.3.0 - 3.6.5 or 4.0.0)
- Consult the Mbed TLS Security Advisories for the latest patch information
- Prioritize patching internet-facing services that rely on Mbed TLS for TLS/SSL connections
- Consider implementing network-level protections such as TLS inspection to detect and block algorithm downgrade attempts
Patch Information
Organizations should consult the official Mbed TLS Security Advisories page for the latest patched versions and update instructions. The vendor has published Mbed TLS Advisory 2026-03 with specific remediation guidance for this signature algorithm injection vulnerability.
Workarounds
- Restrict the allowed signature algorithms in Mbed TLS configuration to only permit strong, modern algorithms
- Implement network segmentation to limit exposure of vulnerable TLS endpoints
- Deploy network-based TLS inspection to detect and block suspicious handshake manipulation attempts
# Configuration example - Verify Mbed TLS version
# Check installed Mbed TLS version to determine if patching is required
grep -r "MBEDTLS_VERSION" /path/to/mbedtls/include/mbedtls/build_info.h
# Review TLS configuration for weak algorithms
# Ensure your application's Mbed TLS configuration restricts signature algorithms
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
