EUVD-2026-63490
GHSA-5x87-gf34-8ww7
Severity by source
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
AC:H because the attacker must first capture a valid, still-unexpired assertion (a condition outside their control); PR:N/UI:N since replay needs no credentials or victim action; C:H/I:H for full user impersonation, A:N as availability is unaffected.
Primary rating from Vendor (EEF).
CVSS VectorVendor: EEF
Lifecycle Timeline
2DescriptionCVE.org
Authentication Bypass by Capture-replay vulnerability in dropbox samly allows an attacker to authenticate as the subject of a captured SAML assertion by resubmitting it.
Samly.Helper.decode_idp_auth_resp/3 in lib/samly/helper.ex calls esaml_sp:validate_assertion/2, whose default duplicate detector is a no-op. The /3 arity accepting a DuplicateFun exists in esaml and implements the check, but Samly never calls it and offers no configuration to supply one, so the SAML 2.0 Web Browser SSO Profile requirement that a bearer assertion be used once is unenforced. An attacker holding a valid SAMLResponse obtained from the network, from browser history, or from logs can submit the identical bytes repeatedly until the assertion's NotOnOrAfter passes, each time establishing a session as the assertion's subject.
This issue affects samly: from v0.3.0 onward.
AnalysisAI
Authentication bypass via SAML assertion replay affects Samly, the Elixir/Plug SAML 2.0 Service Provider library from Dropbox, in all releases from v0.3.0 onward. Because Samly.Helper.decode_idp_auth_resp/3 invokes esaml's validate_assertion with a no-op duplicate detector and exposes no way to supply a real one, a bearer assertion is never enforced as single-use; an attacker who captures a single valid SAMLResponse can resubmit the identical bytes to repeatedly log in as that assertion's subject until its NotOnOrAfter expires. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | The attacker must possess one genuine, cryptographically valid SAMLResponse for the victim (obtained from network capture, browser history, or logs) and must replay it before its NotOnOrAfter timestamp elapses - this bounded validity window and the need to first capture a real assertion (reflected as AT:P in the CVSS 4.0 vector) are the primary limiting factors. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 4.0 vector (AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H) rates this 9.1 (Critical), reflecting network reach, no required privileges, and high confidentiality and integrity impact from impersonating a legitimate user. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | Full exploit scenario with step-by-step reproduction available after sign-in. |
| Remediation | No vendor-released patched version is identified in the available data - the references (https://cna.erlef.org/cves/CVE-2026-53424.html and https://osv.dev/vulnerability/EEF-CVE-2026-53424) point to advisory pages rather than a tagged fixed release, so monitor them for an upstream fix that either calls esaml's 3-arity validate_assertion with a real DuplicateFun or exposes configuration to supply one. … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours, audit your infrastructure to identify all applications and services using Samly for SAML authentication, and review your network controls to determine whether SAML assertions could be intercepted or captured by external or internal threats. …
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Same weakness CWE-294 – Authentication Bypass by Capture-replay
View allSame technique Authentication Bypass
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External POC / Exploit Code
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