CVE-2026-86219

What it actually does

Authen::SASL::Perl::DIGEST_MD5 before 2.2100 fails to bind the client DIGEST-MD5 response to the nonce the server issued in server_start. server_step computes the expected response digest from client-supplied parameters, so a captured response verifies whenever its digest matches the nonce it carries. The nonce-count table is keyed on that client-supplied nonce and starts empty in each new server object, so a captured first response with nc=00000001 also passes the count check. The result is a complete SASL authentication bypass: a replay authenticates as the original user without the password.

Why it matters to the business

Any service authenticating users with this Perl DIGEST-MD5 SASL implementation can be accessed as a legitimate user by anyone who previously observed a successful login, enabling account takeover, unauthorized data access, and integrity loss without stealing credentials. CVSS 9.8 reflects network reachability with no privileges or user interaction. The issue is not in CISA KEV, has no reported ransomware use, and exploit maturity is currently none, but it has been public for about 30 days—treat it as a high-priority patch wherever Perl SASL DIGEST-MD5 is in the authentication path.

How an attack unfolds

An attacker who can observe a successful SASL DIGEST-MD5 exchange with qop=auth records the client’s authentication response. Later they open a new session to the same service, host, realm, and user and submit that captured response. Because the server never compares the returned nonce to the challenge it generated, and because nonce-count state does not persist across server objects, the replay is accepted and the attacker is authenticated as the victim without knowing the password.

How to fix it

Upgrade Authen::SASL so Authen::SASL::Perl::DIGEST_MD5 is 2.2100 or later, which is the version that enforces nonce verification per RFC 2831. Inventory Perl applications, mail, LDAP, XMPP, and other SASL clients/servers that pull in this module. If an immediate upgrade is not possible, disable the DIGEST-MD5 SASL mechanism and prefer modern mechanisms such as SCRAM; terminate SASL only inside TLS and add compensating authentication controls until the patched module is deployed.

How to confirm the fix worked

Confirm the installed Authen::SASL / DIGEST_MD5 module version is 2.2100 or newer on every host that performs SASL DIGEST-MD5. Review SASL mechanism lists to ensure DIGEST-MD5 is not offered on unpatched instances. After upgrade, verify that a client response whose nonce does not match the server challenge is rejected and that nonce-count state is not accepted solely from client-supplied values. Search dependency lockfiles and CPAN/OS packages for Authen::SASL versions below 2.2100.

Typical attack path

1. Observe a successful SASL DIGEST-MD5 (qop=auth) exchange for a target user, service, host, and realm
2. Capture the client authentication response (nonce, nonce-count, digest, and related parameters)
3. Open a new SASL DIGEST-MD5 session to the same service, host, realm, and user
4. Submit the captured response in the client authentication step
5. Obtain an authenticated session as that user without the password

How to detect it

This is a credential-replay authentication bypass, so look for evidence of the same DIGEST-MD5 client response succeeding more than once rather than a new implant. Review SASL, application, mail, LDAP, and XMPP authentication logs for DIGEST-MD5 successes where the nonce is reused across sessions or where nc=00000001 appears with a nonce previously seen for that user, realm, and host. Correlate multiple successful authentications for one user from different source addresses or session IDs with identical digest-response fields. On hosts, identify processes still loading Authen/SASL/Perl/DIGEST_MD5.pm from a package older than 2.2100. There are no named threat actors or public exploit artifacts associated with this CVE at publication plus 30 days; absence of malware IOCs does not rule out silent replays.

• Authen::SASL::Perl::DIGEST_MD5
• Authen/SASL/Perl/DIGEST_MD5.pm
• SASL mechanism=DIGEST-MD5
• qop=auth
• nc=00000001
• DIGEST-MD5 nonce reuse across sessions

What your security team should do

• Patch Authen::SASL::Perl::DIGEST_MD5 to 2.2100 or later on all SASL-authenticating Perl services as a priority, despite no KEV listing or known public exploit
• Inventory and disable DIGEST-MD5 where the module cannot be upgraded immediately; prefer SCRAM or other non-DIGEST mechanisms over TLS
• Hunt authentication logs for reused DIGEST-MD5 nonces and repeated nc=00000001 responses for the same user/realm across distinct sessions
• Require TLS for SASL and review privileged accounts that authenticated via DIGEST-MD5 in the last 30 days
• Track CPAN and OS package updates and add version pinning so the module cannot regress below 2.2100