CVE-2026-63182: Signature Bypass (CVSS 8.1)

Description

LightSAML implements SAML 2.0 data model classes, XML serialization and deserialization, XML security, certificate support, and message bindings. Prior to 4.7.1 and 5.0.1, LightSAML and xmlseclibs disagree about which assertion an inbound SAML signature covers. `Response::deserialize` uses `manyElementsFromXml` to collect only direct-child `saml:Assertion` elements of `samlp:Response`, and `AssertionSignatureValidatorAction` validates their `ds:Signature` using an out-of-band trusted IdP credential resolved by Issuer, not a key from the document. `XMLSecurityDSig::validateReference()` removes the enveloped signature from the shared DOM, but `processRefNode()` resolves `Reference URI="#id"` document-wide through an XPath of the form `//*[@Id]` and takes `item(0)`, which need not be the assertion whose claims LightSAML consumes. An attacker who has captured one genuinely signed assertion, such as an earlier login response, can place its signature-stripped bytes earlier in `samlp:Extensions` and present a forged direct-child assertion with the same ID and copied genuine signature. Validation then covers the hidden genuine bytes while LightSAML surfaces unsigned attacker-controlled `Subject/NameID`, `Conditions`, and `Attributes`, enabling authentication bypass or privilege escalation even when signature validation is required. The behavior was confirmed in versions 5.0.0 and 4.6.1. This issue is fixed in versions 4.7.1 and 5.0.1.

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CVSS v3.1 Score

8.1

HIGH

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

Weakness Type (CWE)

CWE-347 CWE-347

References

Other References

https://github.com/litesaml/lightsaml/commit/230e62d1325f2cbcaac814b78409775f7a8806a2 https://github.com/litesaml/lightsaml/commit/3af82bf2f3865e6aafeeb1a97c97b383b8bb7b00 https://github.com/litesaml/lightsaml/pull/113 https://github.com/litesaml/lightsaml/pull/122 https://github.com/litesaml/lightsaml/releases/tag/4.7.1 https://github.com/litesaml/lightsaml/releases/tag/5.0.1 https://github.com/litesaml/lightsaml/security/advisories/GHSA-w553-pwx6-3mg9

Frequently Asked Questions

What is CVE-2026-63182? +

LightSAML implements SAML 2.0 data model classes, XML serialization and deserialization, XML security, certificate support, and message bindings. Prior to 4.7.1 and 5.0.1, LightSAML and xmlseclibs disagree about which assertion an inbound SAML signature covers. `Response::deserialize` uses `manyElementsFromXml` to collect only direct-child `saml:Assertion` elements of `samlp:Response`, and `AssertionSignatureValidatorAction` validates their `ds:Signature` using an out-of-band trusted IdP credential resolved by Issuer, not a key from the document. `XMLSecurityDSig::validateReference()` removes the enveloped signature from the shared DOM, but `processRefNode()` resolves `Reference URI="#id"` document-wide through an XPath of the form `//*[@Id]` and takes `item(0)`, which need not be the assertion whose claims LightSAML consumes. An attacker who has captured one genuinely signed assertion, such as an earlier login response, can place its signature-stripped bytes earlier in `samlp:Extensions` and present a forged direct-child assertion with the same ID and copied genuine signature. Validation then covers the hidden genuine bytes while LightSAML surfaces unsigned attacker-controlled `Subject/NameID`, `Conditions`, and `Attributes`, enabling authentication bypass or privilege escalation even when signature validation is required. The behavior was confirmed in versions 5.0.0 and 4.6.1. This issue is fixed in versions 4.7.1 and 5.0.1. It has a CVSS v3.1 base score of 8.1 (HIGH).

How severe is CVE-2026-63182? +

CVE-2026-63182 has a CVSS v3.1 score of 8.1 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching.

How do I check if I'm vulnerable to CVE-2026-63182? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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