CVE-2025-66578

An authentication bypass vulnerability exists due to a flaw in the libxml2 canonicalization process, which is used by xmlseclibs during document transformation. This weakness allows an attacker to generate a valid signature once and reuse it indefinitely. In practice, a signature created during a previous interaction - or through a misconfigured authentication flow - can be replayed to bypass authentication checks.

Details

When libxml2’s canonicalization is invoked on an invalid XML input, it may return an empty string rather than a canonicalized node. xmlseclibs then proceeds to compute the DigestValue over this empty string, treating it as if canonicalization succeeded.

https://github.com/robrichards/xmlseclibs/blob/f4131320c6dcd460f1b0c67f16f8bf24ce4b5c3e/src/XMLSecurityDSig.php#L296

Suggested remediation

Treat canonicalization failures (exceptions or nil/empty outputs) as fatal and abort validation.
Add explicit checks: reject when canonicalize returns nil/empty or raise

Digest bypass: By crafting input that causes canonicalization to yield an empty string, the attacker can manipulate validation to pass incorrectly.

Signature replay on empty canonical form: If an empty string has been signed once (e.g., in a prior interaction or via a misconfigured flow), that signature can potentially be replayed to bypass authentication.

CVE-2025-66578 has a CVSS score of 6.0 (Medium). The vector is network-reachable, high privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.1.4); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-66578? CVE-2025-66578 is a medium-severity security vulnerability in robrichards/xmlseclibs (composer), affecting versions <= 3.1.3. It is fixed in 3.1.4.
2. How severe is CVE-2025-66578? CVE-2025-66578 has a CVSS score of 6.0 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of robrichards/xmlseclibs are affected by CVE-2025-66578? robrichards/xmlseclibs (composer) versions <= 3.1.3 is affected.
4. Is there a fix for CVE-2025-66578? Yes. CVE-2025-66578 is fixed in 3.1.4. Upgrade to this version or later.
5. Is CVE-2025-66578 exploitable, and should I be worried? Whether CVE-2025-66578 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-66578 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-66578? Upgrade robrichards/xmlseclibs to 3.1.4 or later.