critical CVSS 9 EPSS 0.50%

CVE-2026-56846·Node.js vulnerability

A flaw in Node.js HTTP/2 handling can cause HTTP/2 retained header blocks evade maxSessionMemory and enable remote memory exhaustion. This vulnerability affects Node.js **24.x** and **22.x**.

Severity
critical
Software
Node.js
Fixed in
18.20.4+dfsg-1~deb12u3
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 0.50% — higher than 41% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2026-56846 scores 9 on CVSS, so a successful attack would be serious — but at 0.50%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

Until: 18.20.4+dfsg-1~deb12u3

Fixed in: 18.20.4+dfsg-1~deb12u3

How to fix this CVE

Update Node.js to version 18.20.4+dfsg-1~deb12u3 or later to patch the HTTP/2 header memory exhaustion vulnerability. This critical issue allows remote attackers to bypass memory limits and trigger denial-of-service conditions by sending specially crafted HTTP/2 requests with retained header blocks. Prioritize this update immediately for all production systems running affected Node.js versions.

sudo dnf update nodejs

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

HTTP/2 SETTINGS frames with excessive header block fragments; memory growth correlated with HEADERS frames containing large or repeated pseudo-header values; pattern: multiple CONTINUATION frames with crafted :authority or :path headers exceeding normal sizes

How to check if you are affected

  1. Step 1: Check your Node.js version by running: node --version and compare against 18.20.4+dfsg-1~deb12u3
  2. Step 2: Verify HTTP/2 is enabled in your application with: grep -r 'spdy\|http2' /path/to/app or check Node process flags with ps aux | grep node
  3. Step 3: Monitor system memory usage during HTTP/2 traffic with: watch -n 1 'free -h && ps aux | grep node' to detect abnormal consumption patterns
  4. Step 4: After patching, confirm the updated version is running by executing: node --version and restart all Node.js services with: sudo systemctl restart nodejs or equivalent service manager command

FAQ

What is CVE-2026-56846?

CVE-2026-56846 is a critical HTTP/2 memory exhaustion flaw in Node.js where malformed header blocks can circumvent the maxSessionMemory limit, allowing remote attackers to exhaust server memory without authentication and cause denial-of-service.

Is CVE-2026-56846 being actively exploited?

No active exploitation has been reported in the wild according to CISA KEV data, and no public exploits are currently available. However, the critical CVSS 9.0 score warrants immediate patching.

What versions of Node.js are affected by CVE-2026-56846?

Node.js versions 24.x and 22.x are confirmed affected. The fix is available in version 18.20.4+dfsg-1~deb12u3 and later releases on maintained branches.

How do I check if my server is vulnerable to CVE-2026-56846?

Run 'node --version' and compare the output against 18.20.4+dfsg-1~deb12u3. If your version is earlier and you are running Node.js 22.x or 24.x, your system is vulnerable. Additionally, verify HTTP/2 support is enabled in your application configuration.

Does Defensia detect CVE-2026-56846?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Node.js is installed on a monitored server, CVE-2026-56846 will appear in your dashboard with remediation steps.

Related Node.js CVEs

CVE-2026-48930CVSS 10A flaw in Node.js TLS hostname handling can cause Embedded-nul hostnames can lead to silent authority rebinding due to c-string truncation in resolver bindings. This vulnerability affects all supported release lines: **Node.js 22**, **Node.js 24**, and **Node.js 26**.
CVE-2026-21636CVSS 10A flaw in Node.js's permission model allows Unix Domain Socket (UDS) connections to bypass network restrictions when `--permission` is enabled. Even without `--allow-net`, attacker-controlled inputs (such as URLs or socketPath options) can connect to arbitrary local sockets via net, tls, or undici/fetch. This breaks the intended security boundary of the permission model and enables access to privileged local services, potentially leading to privilege escalation, data exposure, or local code execution. * The issue affects users of the Node.js permission model on version v25. In the moment of this vulnerability, network permissions (`--allow-net`) are still in the experimental phase.
CVE-2025-55130CVSS 9.1A flaw in Node.js’s Permissions model allows attackers to bypass `--allow-fs-read` and `--allow-fs-write` restrictions using crafted relative symlink paths. By chaining directories and symlinks, a script granted access only to the current directory can escape the allowed path and read sensitive files. This breaks the expected isolation guarantees and enables arbitrary file read/write, leading to potential system compromise. This vulnerability affects users of the permission model on Node.js v20, v22, v24, and v25.
CVE-2026-48619CVSS 9A flaw in Node.js HTTP/2 client allows a server to send an unlimited number of ORIGIN frames, which could lead to an Out of Memory error on the client. This vulnerability affects all supported release lines: **Node.js 22**, **Node.js 24**, and **Node.js 26**.
CVE-2026-56848CVSS 9A flaw in Node.js HTTP/2 handling allows `nghttp2_session_mem_send()` to be called re-entrantly while `nghttp2_session_mem_recv()` is executing, resulting in a heap-use-after-free. This vulnerability affects Node.js **26.x**, **24.x**, and **22.x**.

References

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