Authen::SASL::Perl::DIGEST_MD5 versions before 2.2100 for Perl accept replayed authentication responses via unverified nonce in server_step.
server_start generates a fresh nonce and sends it in the challenge, and nothing later compares that value against the nonce the client returns. server_step derives the expected digest from the clients own parameters, so a response verifies whenever its digest matches the nonce it carries. The count table it also checks is keyed on the client-supplied nonce and starts empty in each new server object, so a captured first response, carrying nc=00000001, passes that too. RFC 2831 defines the nonce in the response as the value the server sent in the preceding challenge.
An attacker who observes one successful qop=auth exchange can replay the captured response against a later session for the same service, host, realm and user, and authenticate as that user without knowing the password.
Weakness
A capture-replay flaw exists when the design of the product makes it possible for a malicious user to sniff network traffic and bypass authentication by replaying it to the server in question to the same effect as the original message (or with minor changes).
Affected Software
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Libauthen-sasl-perl | Ubuntu | devel | * |
| Libauthen-sasl-perl | Ubuntu | esm-infra-legacy/trusty | * |
| Libauthen-sasl-perl | Ubuntu | esm-infra-legacy/xenial | * |
| Libauthen-sasl-perl | Ubuntu | esm-infra/bionic | * |
| Libauthen-sasl-perl | Ubuntu | esm-infra/focal | * |
| Libauthen-sasl-perl | Ubuntu | jammy | * |
| Libauthen-sasl-perl | Ubuntu | noble | * |
| Libauthen-sasl-perl | Ubuntu | resolute | * |
| Libauthen-sasl-perl | Ubuntu | stonking | * |
Potential Mitigations
Related Attack Patterns
- https://cwe.mitre.org/data/definitions/102.html
- https://cwe.mitre.org/data/definitions/509.html
- https://cwe.mitre.org/data/definitions/555.html
- https://cwe.mitre.org/data/definitions/561.html
- https://cwe.mitre.org/data/definitions/60.html
- https://cwe.mitre.org/data/definitions/644.html
- https://cwe.mitre.org/data/definitions/645.html
- https://cwe.mitre.org/data/definitions/652.html
- https://cwe.mitre.org/data/definitions/701.html
- https://cwe.mitre.org/data/definitions/94.html
References
- https://datatracker.ietf.org/doc/html/rfc2831#section-2.1.2
- https://github.com/perl-authen-sasl/perl-authen-sasl/commit/94337367030612842924f697cead29964a96448d.patch
- https://metacpan.org/release/EHUELS/Authen-SASL-2.2000/source/lib/Authen/SASL/Perl/DIGEST_MD5.pm#L203-222
- https://metacpan.org/release/EHUELS/Authen-SASL-2.2000/source/lib/Authen/SASL/Perl/DIGEST_MD5.pm#L410-414
- https://metacpan.org/release/EHUELS/Authen-SASL-2.2100/changes
- https://www.cve.org/CVERecord?id=CVE-2025-40918