VulnerabilityPublished 17 Mar 2026Updated 25 Mar 2026

CVE-2026-4349: Duende IdentityServer4 · Improper Authentication

Affects Duende IdentityServer4Source CVE Database V5

Not known to be exploited · CVSS 6.3 Medium · end of life, no fix coming : replace or isolate Duende IdentityServer4; no fix will be released.

Data as of 29 Oct 2026, 19:04 UTC

CVSS 4.0 base score

6.3Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.4% chance of exploitation in 30 days – higher than 36% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: None coming – the product is end of life

CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:L/VI:L/VA:L/SC:N/SI:N/SA:N/E:X

Attack Vector: Network
Attack Complexity: High
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: Low
Vulnerable system integrity: Low
Vulnerable system availability: Low
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: The successful attack depends on the evasion or circumvention of security-enhancing techniques in place that would otherwise hinder the attack. These include: Evasion of exploit mitigation techniques, for example, circumvention of address space randomization (ASLR) or data execution prevention (DEP) must be performed for the attack to be successful; Obtaining target-specific secrets. The attacker must gather some target-specific secret before the attack can be successful. A secret is any piece of information that cannot be obtained through any amount of reconnaissance. To obtain the secret the attacker must perform additional attacks or break otherwise secure measures (e.g. knowledge of a secret key may be needed to break a crypto channel). This operation must be performed for each attacked target.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the Vulnerable System.
Vulnerable system integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact to the Vulnerable System.
Vulnerable system availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the Vulnerable System are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 17 Mar 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

No fix will be released: Duende IdentityServer4 is end of life. Replace it, or isolate it from untrusted networks until you can.

Affected software

Product	Affected	Fixed in
DuendeIdentityServer4	Affected =4.1.0 =4.1.1 =4.1.2	Fixed in–
NuGetDuende/identityserver4`pkg:nuget/Duende/identityserver4`Other threats	Affected =4.1.0 =4.1.1 =4.1.2	Fixed in–

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

To mitigate CVE-2026-4349, organizations should first monitor for any official patches or updates from Duende and apply them promptly once available. In the absence of vendor patches, organizations should conduct a thorough review of their IdentityServer 4 deployment, focusing on the /connect/authorize endpoint and the handling of the id_token_hint parameter. Implement additional validation and sanitization of input parameters to prevent manipulation. Employ strict access controls and monitoring on token renewal endpoints to detect anomalous requests. Consider deploying Web Application Firewalls (WAFs) with custom rules to block suspicious token renewal attempts. Additionally, enforce multi-factor authentication (MFA) and session management best practices to reduce the risk of token misuse. Regularly audit authentication logs for unusual activity and prepare incident response plans specific to authentication bypass scenarios. Finally, evaluate upgrading to newer versions or alternative identity providers if Duende IdentityServer 4 remains unsupported.

Description

A vulnerability was determined in Duende IdentityServer4 up to 4.1.2. The affected element is an unknown function of the file /connect/authorize of the component Token Renewal Endpoint. This manipulation of the argument id_token_hint causes improper authentication. It is possible to initiate the attack remotely. The attack is considered to have high complexity. The exploitability is described as difficult. This vulnerability only affects products that are no longer supported by the maintainer.

Impact

Organizations using Duende IdentityServer 4 for authentication and token management face risks of unauthorized access or token misuse due to this vulnerability. Attackers exploiting the improper authentication flaw could potentially renew tokens or bypass authentication checks, leading to unauthorized access to protected resources or services. Although the exploit complexity is high and no known exploits exist, the impact on confidentiality and integrity could be significant if exploited, especially in environments relying heavily on IdentityServer for secure authentication. This could lead to data breaches, privilege escalation, or unauthorized transactions. The vulnerability does not directly affect availability but could undermine trust in authentication processes. Enterprises in sectors such as finance, healthcare, government, and technology that deploy IdentityServer 4 are particularly at risk, as compromised authentication can lead to regulatory violations and reputational damage.

Summary

CVE-2026-4349 identifies an improper authentication vulnerability in Duende IdentityServer version 4, specifically within an unspecified function of the /connect/authorize endpoint related to token renewal. The vulnerability arises from improper handling or validation of the id_token_hint argument, which is used during token renewal flows to authenticate or validate the user session. By manipulating this parameter, an attacker can bypass intended authentication mechanisms, potentially gaining unauthorized access or renewing tokens without proper validation. The attack can be initiated remotely without requiring user interaction or prior authentication, but it is characterized by high complexity and difficult exploitability, indicating that successful exploitation demands advanced skills or specific conditions. The vulnerability has a CVSS 4.0 base score of 6.3 (medium severity), reflecting limited impact on confidentiality, integrity, and availability due to low scope and vector complexity. The vendor Duende was contacted but did not respond, and no patches or known exploits are currently available. This vulnerability affects only IdentityServer version 4, a widely used OpenID Connect and OAuth 2.0 framework for .NET applications, often deployed in enterprise environments for identity and access management.

References

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