How severe is CVE-2026-0621?

No CVSS score has been assigned to CVE-2026-0621 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-0621?

CVE-2026-0621 affects the following packages: @modelcontextprotocol/sdk (npm). Ecosystems affected: npm.

How do I fix CVE-2026-0621?

Update @modelcontextprotocol/sdk to 1.25.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-0621 is resolved across your whole dependency graph.

How do I detect CVE-2026-0621 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @modelcontextprotocol/sdk, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-0621 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2026-0621 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-0621 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-0621?

CVE-2026-0621 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 38th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-0621?

CVE-2026-0621 is classified as CWE-1333 (CWE-1333). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-0621 published, and has it been updated?

CVE-2026-0621 was published on January 5, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦 npm

Not in CISA KEV

CVE-2026-0621 — @modelcontextprotocol/sdk

Fix: modelcontextprotocol/typescript-sdk@7f0cf73

CVE-2026-0621 is a CWE-1333 vulnerability in @modelcontextprotocol/sdk. A fix is available for @modelcontextprotocol/sdk — see the affected versions and patch details below.

MCP TypeScript SDK UriTemplate Exploded Array Pattern ReDoS

Also known asGHSA-8r9q-7v3j-jr4g

Published

Jan 5, 2026

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 2, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-0621.

EPSS Exploitation Probability

via FIRST.org ↗

0.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs38th percentile — riskier than 38% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

77Kother npm packages depend on this — each one inherits the vulnerability until it's patched upstream

@modelcontextprotocol/sdknpm

87.3Mdownloads / week

Description

Impact

A ReDoS vulnerability in the UriTemplate class allows attackers to cause denial of service. The partToRegExp() function generates a regex pattern with nested quantifiers (([^/]+(?:,[^/]+)*)) for exploded template variables (e.g., {/id*}, {?tags*}), causing catastrophic backtracking on malicious input.

Who is affected: MCP servers that register resource templates with exploded array patterns and accept requests from untrusted clients.

Attack result: An attacker sends a crafted URI via resources/read request, causing 100% CPU utilization, server hang/crash, and denial of service for all clients.

Affected Versions

All versions of @modelcontextprotocol/sdk prior to the patched release.

Patches

v1.25.2 contains b392f02ffcf37c088dbd114fedf25026ec3913d3 the fix modifies the regex pattern to prevent backtracking.

Workarounds

Avoid using exploded patterns ({/id*}, {?tags*}) in resource templates
Implement request timeouts and rate limiting
Validate URIs before processing to reject suspicious patterns

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`@modelcontextprotocol/sdk`	≥ 1.3.0&&< 1.25.2	1.25.2`npm install @modelcontextprotocol/sdk@1.25.2`

Affected Products

1 product · 1 configurations

Application

mcp typescript sdklfprojects

≤ 1.25.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @modelcontextprotocol/sdk, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @modelcontextprotocol/sdk to 1.25.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-0621 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

### Impact A ReDoS vulnerability in the `UriTemplate` class allows attackers to cause denial of service. The `partToRegExp()` function generates a regex pattern with nested quantifiers (`([^/]+(?:,[^/]+)*)`) for exploded template variables (e.g., `{/id*}`, `{?tags*}`), causing catastrophic backtracking on malicious input. **Who is affected:** MCP servers that register resource templates with exploded array patterns and accept requests from untrusted clients. **Attack result:** An attacker sends a crafted URI via `resources/read` request, causing 100% CPU utilization, server hang/crash, and

O3 Security · Impact-Aware SCA

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