Missing InResponseTo validation in Samly allows acceptance of unsolicited SAML responses

Summary

CVE	CVE-2026-53425
State	PUBLISHED
Assigner	EEF
Source Priority	CVE Program / NVD first with legacy fallback
Published	2026-08-20 18:16:27 UTC
Updated	2026-08-20 19:16:54 UTC
Description	Insufficient Verification of Data Authenticity vulnerability in dropbox samly allows an attacker to establish an authenticated session using a SAML response the service provider never requested. Samly.SPHandler.validate_authresp/3 in lib/samly/sp_handler.ex validates a SAML response for the SP-initiated flow by comparing only the RelayState value, the IdP identifier, and the presence of a target URL held in the session. It never compares SubjectConfirmationData/@InResponseTo against the ID of the AuthnRequest the service provider issued, and that request ID is never persisted, so no comparison is possible. SAML 2.0 Core section 4.1.4.3 requires a service provider to reject a response whose InResponseTo does not match a request it made. The underlying esaml library checks status, signature, recipient, audience, and staleness, but likewise never inspects InResponseTo, so nothing else closes the gap. Exploitation requires a validly signed assertion from the trusted IdP, which an attacker can obtain for their own account, and a RelayState matching the victim's session; the assertion signature itself remains intact, so this is not a signature-forgery issue. This issue affects samly: from 0.3.0 onward.

Risk And Classification

Primary CVSS: v4.0 7.6 HIGH from 6b3ad84c-e1a6-4bf7-a703-f496b71e49db

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/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

Problem Types: CWE-345 | CWE-345 CWE-345 Insufficient Verification of Data Authenticity

Version	Source	Type	Score	Severity	Vector
4.0	6b3ad84c-e1a6-4bf7-a703-f496b71e49db	Secondary	7.6	HIGH	`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/C...`
4.0	CNA	CVSS	7.6	HIGH	`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N`

CVSS v4.0 Breakdown

Attack Vector

Network

Attack Complexity

Low

Attack Requirements

Present

Privileges Required

None

User Interaction

Passive

Confidentiality

High

Integrity

High

Availability

None

Sub Conf.

None

Sub Integrity

None

Sub Availability

None

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/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

Vendor Declared Affected Products

Source	Vendor	Product	Version	Platforms
CNA	Dropbox	Samly	affected 0.3.0 * semver	Not specified
CNA	Dropbox	Samly	affected 8a5bb1b4a4753d05470da2036323477f63cfdf4c * git	Not specified
CNA	Handnot2	Samly	affected 8a5bb1b4a4753d05470da2036323477f63cfdf4c * git	Not specified

References

Reference	Source	Link	Tags
osv.dev/vulnerability/EEF-CVE-2026-53425	6b3ad84c-e1a6-4bf7-a703-f496b71e49db	osv.dev
cna.erlef.org/cves/CVE-2026-53425.html	6b3ad84c-e1a6-4bf7-a703-f496b71e49db	cna.erlef.org
CVE Program record	CVE.ORG	www.cve.org	canonical
NVD vulnerability detail	NVD	nvd.nist.gov	canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: Mark Madsen (en)

CNA: Jonatan Männchen / EEF (en)

There are currently no legacy QID mappings associated with this CVE.