CVE-2025-61684VulnerabilityPublished 19 Jan 2026Updated 20 Jan 2026

CVE-2025-61684: h2o quicly · Improper Input Validation

Affects h2o quiclySource CVE Database V5

Not known to be exploited · CVSS 7.5 High.

Data as of 26 Oct 2026, 06:26 UTC

CVSS 3.1 base score

7.5High

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

EPSS: 0.3% chance of exploitation in 30 days – higher than 33% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

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

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, 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: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 19 Jan 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: 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 2026-10-26.

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.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC 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

Affected software

Product	Affected	Fixed in
h2oquicly	Affected =< d9d3df6	Fixed in–

AI-suggested

European organizations should immediately identify any deployments of h2o’s quicly implementation and verify the version in use. The primary mitigation is to upgrade to the fixed version including commit d9d3df6a8530a102b57d840e39b0311ce5c9e14e or later. If immediate patching is not feasible, organizations should consider network-level mitigations such as filtering or rate-limiting QUIC traffic from untrusted sources to reduce exposure. Deploying Web Application Firewalls (WAFs) or Intrusion Prevention Systems (IPS) with signatures targeting malformed QUIC packets may help detect or block exploit attempts. Monitoring for unusual crashes or service restarts in quicly-based components can provide early warning of exploitation attempts. Additionally, organizations should review their incident response plans to handle potential denial-of-service events and ensure redundancy and failover mechanisms are in place to maintain service continuity. Collaboration with vendors and upstream projects for timely updates is essential.

Description

CVE-2025-61684 is a high-severity vulnerability in quicly, an IETF QUIC protocol implementation used by the h2o project. The flaw stems from improper input validation (CWE-20) that allows a remote attacker to cause an assertion failure, crashing the process and resulting in a denial-of-service (DoS) condition. This vulnerability affects all versions of quicly prior to commit d9d3df6. Exploitation requires no authentication or user interaction and can be triggered remotely over the network. While no known exploits are currently observed in the wild, the CVSS score of 7.5 reflects the ease of exploitation and significant impact on availability. The issue was fixed in commit d9d3df6a8530a102b57d840e39b0311ce5c9e14e. European organizations using h2o’s quicly implementation in their infrastructure, especially those relying on QUIC for web or application traffic, should prioritize patching to prevent potential service disruptions. Countries with high adoption of modern web infrastructure and significant internet-facing services are most at risk.

Impact

For European organizations, the primary impact is denial of service caused by remote attackers crashing quicly-based services. This can lead to service outages, degraded user experience, and potential cascading failures if the affected service is critical infrastructure such as web servers, API gateways, or load balancers using QUIC. Organizations relying on h2o’s quicly for HTTP/3 or other QUIC-based communications face increased risk of network-level disruption. This could affect sectors with high availability requirements like finance, telecommunications, government, and e-commerce. Additionally, denial-of-service attacks can be leveraged as part of larger multi-vector attacks, amplifying operational risks. The lack of confidentiality or integrity impact limits data breach concerns, but availability loss can still cause significant operational and reputational damage. The vulnerability’s ease of exploitation and remote attack vector make it a pressing concern for network perimeter defenses and internet-facing services.

CWE-20Improper Input Validation (opens in a new tab)
Class weaknessLikelihood of exploit: HighCWE Top 25 (2025): #18OWASP Top 10 A05:2025: Injection (opens in a new tab)
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.
MITRE discourages mapping vulnerabilities to this broad entry; the root cause is usually a more specific weakness.
Architecture and Design · Attack Surface Reduction
Consider using language-theoretic security (LangSec) techniques that characterize inputs using a formal language and build "recognizers" for that language. This effectively requires parsing to be a distinct layer that effectively enforces a boundary between raw input and internal data representations, instead of…
Architecture and Design · Libraries or Frameworks
Use an input validation framework such as Struts or the OWASP ESAPI Validation API. Note that using a framework does not automatically address all input validation problems; be mindful of weaknesses that could arise from misusing the framework itself (CWE-1173).
Architecture and Design · Implementation · Attack Surface Reduction
Understand all the potential areas where untrusted inputs can enter the product, including but not limited to: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any…
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
Injection Prevention (opens in a new tab)
SQL Injection Prevention (opens in a new tab)
Injection Prevention in Java (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (opens in a new tab)
Attack patterns
Buffer Overflow via Environment Variables (opens in a new tab)
Server Side Include (SSI) Injection (opens in a new tab)
Cross Zone Scripting (opens in a new tab)
Command Line Execution through SQL Injection (opens in a new tab)
Object Relational Mapping Injection (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 26 Jan 2026, 20:04 UTC

WeaknessesCWE-20

Technical details

References

Community reviews

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