jsrsasign DSA Signature Forgery Vulnerability (CVE-2026-4600)

A critical vulnerability, CVE-2026-4600, affects jsrsasign versions before 11.1.1. This flaw stems from insufficient validation of DSA domain parameters within the KJUR.crypto.DSA.setPublic function and related DSA/X509 verification processes in src/dsa-2.0.js. The vulnerability allows an attacker to forge DSA signatures or X.509 certificates. By supplying malicious domain parameters (e.g., g=1, y=1, fixed r=1), an attacker can manipulate the verification equation to evaluate as true, irrespective of the actual hash value. This can lead to successful verification of a forged signature by X509.verifySignature(). This poses a significant risk, as it can be exploited to bypass authentication mechanisms or install malicious software that appears to be legitimately signed.

Attack Chain

1. An attacker crafts a malicious DSA key pair, manipulating the domain parameters g, y, and r to specific values (e.g., g=1, y=1, r=1).
2. The attacker uses this maliciously crafted key to forge a DSA signature for arbitrary data.
3. The forged signature, along with the manipulated DSA public key, is presented to an application using a vulnerable version of jsrsasign.
4. The application calls KJUR.crypto.DSA.setPublic with the attacker-controlled public key, which contains the malicious domain parameters.
5. The application attempts to verify the forged signature using X509.verifySignature().
6. Due to the improper validation of DSA domain parameters, the verification equation evaluates to true regardless of the hash of the signed data.
7. The application incorrectly accepts the forged signature as valid.
8. The attacker gains unauthorized access to resources or executes arbitrary code, relying on the successful verification of the forged signature.

Impact

Successful exploitation of CVE-2026-4600 can have severe consequences. An attacker can forge digital signatures, potentially allowing them to bypass security controls, impersonate legitimate entities, or distribute malicious software that appears to be trusted. This can lead to widespread compromise, data breaches, and significant reputational damage. The vulnerability impacts any application using the vulnerable jsrsasign library for DSA signature verification, particularly those relying on X.509 certificates for authentication and authorization.

Recommendation

• Upgrade the jsrsasign package to version 11.1.1 or later to remediate CVE-2026-4600.
• Deploy the Sigma rule "Detect DSA Public Key with Suspicious Parameters" to identify attempts to use DSA public keys with g=1 and y=1 in your environment.
• Monitor web server logs (category: webserver, product: linux) for suspicious requests involving DSA signature verification, particularly those using parameters like g=1, y=1, and r=1.