Unsigned SAML LogoutRequest Acceptance in gosaml2

Summary

The ValidateEncodedLogoutRequestPOST function in gosaml2 accepts completely unsigned SAML LogoutRequest messages even when SkipSignatureValidation is set to false. When validateElementSignature returns dsig.ErrMissingSignature, the code in decode_logout_request.go:60-62 silently falls through to process the unverified XML element instead of rejecting it. An attacker who can reach the SP's Single Logout endpoint can forge a LogoutRequest for any user, terminating their session without possessing the IdP's signing key.

Affected Version

• Library: github.com/russellhaering/gosaml2
• Version: All versions up to and including the latest commit on main (as of 2026-03-16)
• File: decode_logout_request.go, lines 58-69

Vulnerable Code

// decode_logout_request.go:57-69
var requestSignatureValidated bool
if !sp.SkipSignatureValidation {
    el, err = sp.validateElementSignature(el)
    if err == dsig.ErrMissingSignature {
        // Unfortunately we just blew away our Response
        el = doc.Root()                    // <-- BUG: falls through with unsigned element
    } else if err != nil {
        return nil, err
    } else if el == nil {
        return nil, fmt.Errorf("missing transformed logout request")
    } else {
        requestSignatureValidated = true
    }
}

When ErrMissingSignature is returned, the code resets el to the raw document root and continues. The requestSignatureValidated variable remains false, but no error is returned. The unsigned LogoutRequest is unmarshalled and passed to ValidateDecodedLogoutRequest, which performs attribute/issuer checks but does not verify that a signature was present.

Attack Details

Property	Value
Attack vector	Network (HTTP POST to SLO endpoint)
Authentication required	None
Payload size	~450 bytes (unsigned XML)
User interaction	None
Complexity	Low -- only requires knowledge of the SP's SLO URL and IdP issuer
CVSS estimate	7.5 (High) -- Network/Low/None/None, Availability impact

Impact

• Arbitrary session termination: An attacker can force-logout any user by forging a LogoutRequest with the victim's NameID. This is a targeted denial-of-service.
• Business disruption: Critical users (executives, admins, operators) can be repeatedly logged out, disrupting access to the application during incidents or time-sensitive operations.
• Security control bypass: If session termination triggers downstream effects (e.g., revoking tokens, clearing caches), an attacker can weaponize this to force re-authentication flows and potentially intercept them.
• No cryptographic material needed: The attacker does not need the IdP's private key. The forged request contains zero cryptographic elements.

Suggested Fix

When ErrMissingSignature is returned and SkipSignatureValidation is false, the function should return an error instead of falling through:

// decode_logout_request.go -- fixed version
var requestSignatureValidated bool
if !sp.SkipSignatureValidation {
    el, err = sp.validateElementSignature(el)
    if err == dsig.ErrMissingSignature {
        // FIXED: reject unsigned requests when signature validation is required
        return nil, fmt.Errorf("logout request is not signed: %w", dsig.ErrMissingSignature)
    } else if err != nil {
        return nil, err
    } else if el == nil {
        return nil, fmt.Errorf("missing transformed logout request")
    } else {
        requestSignatureValidated = true
    }
}

This ensures that unsigned LogoutRequest messages are rejected when SkipSignatureValidation is false, matching the behavior that operators expect when they configure signature enforcement.

Attached lab
f1_unsigned_logout.zip