VulnerabilityPublished 20 Mar 2026

CVE-2026-33128: h3js h3 · CRLF Injection

Affects h3js h3Source CVE Database V5

Compare

Not known to be exploited · CVSS 7.5 High : upgrade to 2.0.1-rc.15 or 1.15.6 soon.

Data as of 1 Nov 2026, 08:20 UTC

CVSS 3.1 base score

7.5High

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

EPSS: 0.5% chance of exploitation in 30 days – higher than 47% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.0.1-rc.15, 1.15.6

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

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

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: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
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: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
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 impacted component.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 20 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 31 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

Upgrade h3 to 2.0.1-rc.15 or 1.15.6; h3js/h3 to 1.15.6.

Affected software

Product	Affected	Fixed in
h3jsh3	Affected >=2.0.0 <2.0.1-rc.15	Fixed in 2.0.1-rc.15
h3jsh3	Affected <1.15.6	Fixed in 1.15.6
GitHubh3js/h3`pkg:github/h3js/h3`Other threats	Affected <1.15.6	Fixed in 1.15.6

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

AI-suggested

To mitigate this vulnerability, organizations should upgrade all affected h3js instances to version 1.15.6 or later, or 2.0.1-rc.15 or later, where the issue is fixed. If immediate upgrading is not feasible, implement input validation and sanitization on all SSE message fields (id, event, data, comment) to neutralize CRLF sequences before they reach the createEventStream functions. Employ web application firewalls (WAFs) with custom rules to detect and block suspicious SSE payloads containing CRLF injection patterns. Restrict network access to SSE endpoints to trusted clients and internal networks to reduce exposure. Monitor SSE traffic for anomalies that could indicate injection attempts. Additionally, review client-side SSE event handlers for robustness against malformed or unexpected events to minimize impact if injection occurs. Maintain up-to-date inventory of h3js versions in use and apply security patches promptly. Finally, conduct security testing focused on SSE injection vectors during development and deployment cycles.

Description

H3 is a minimal H(TTP) framework. In versions prior to 1.15.6 and between 2.0.0 through 2.0.1-rc.14, createEventStream is vulnerable to Server-Sent Events (SSE) injection due to missing newline sanitization in formatEventStreamMessage() and formatEventStreamComment(). An attacker who controls any part of an SSE message field (id, event, data, or comment) can inject arbitrary SSE events to connected clients. This issue is fixed in versions 1.15.6 and 2.0.1-rc.15.

Impact

The primary impact of this vulnerability is the compromise of the integrity of Server-Sent Events streams delivered by vulnerable h3js servers. Attackers able to inject arbitrary SSE events can manipulate client-side event handling, potentially causing clients to execute malicious scripts or process falsified data. This could lead to unauthorized actions within client applications relying on SSE for real-time updates, such as injecting misleading information, triggering unintended behaviors, or bypassing client-side logic. While confidentiality and availability impacts are minimal, the integrity breach can undermine trust in real-time data streams and disrupt application workflows. Organizations using h3js in environments where SSE is critical—such as live dashboards, notifications, or real-time monitoring—face risks of data manipulation and client-side exploitation. The attack requires network access to the vulnerable server but no authentication or user interaction, increasing the risk in exposed deployments. Although no exploits are currently known in the wild, the high severity and ease of exploitation warrant prompt remediation to prevent potential attacks.

CWE-93Improper Neutralization of CRLF Sequences (CRLF Injection) (opens in a new tab)
Base weaknessOWASP Top 10 A05:2025: Injection (opens in a new tab)
The product uses CRLF (carriage return line feeds) as a special element, e.g. to separate lines or records, but it does not neutralize or incorrectly neutralizes CRLF sequences from inputs.
Implementation
Avoid using CRLF as a special sequence.
Implementation
Appropriately filter or quote CRLF sequences in user-controlled input.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
Deserialization (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
V1.5.1 (opens in a new tab)
Attack patterns
Command Delimiters (opens in a new tab)
Web Server Logs Tampering (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 20 Mar 2026, 10:08 UTC

WeaknessesCWE-93

Technical details

References

Community reviews

Related threats