CVE-2026-73649: Velocity.js: Remote Code Execution via property-read to Function constructor (bypass of GHSA-j658-c2gf-x6pq fix)

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.7%, percentile 54: FIRST’s model estimates a 0.7% chance of exploitation activity in the next 30 days, higher than 54% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes velocityjs (npm) <= 2.1.6, fixed in 2.1.7. Check your lockfile, including transitive dependencies.

What it is

Velocity.js is a JavaScript implementation of the Apache Velocity template engine. Prior to 2.1.7, the earlier fix for CVE-2026-44966 filtered constructor, proto, and prototype only in the #set assignment handler in src/compile/set.ts, while property-read expressions in src/compile/references.ts remained unfiltered. The getReferences() flow called getAttributes(), whose property access allowed an attacker-controlled template to traverse constructor.constructor to the JavaScript Function constructor. The #set handler validated only the assignment target and did not inspect the right-hand property-read expression, allowing arbitrary shell commands, environment-variable access, cloud-credential access, and internal-network access in the server process. This issue is fixed in version 2.1.7.

From the GitHub advisory

Summary

Remote Code Execution (RCE) in velocityjs v2.1.6 via property-read to the Function constructor. This bypasses the fix for GHSA-j658-c2gf-x6pq ("Prototype Pollution in #set path assignment") — that advisory blocked constructor/proto/prototype only in the #set assignment handler (set.cjs), but property read expressions are unfiltered. Any application rendering attacker-controlled Velocity templates is vulnerable to arbitrary code execution on the server.

Details

GHSA-j658-c2gf-x6pq added isBlockedPathKey() to dist/cjs/compile/set.cjs:35-43, which blocks proto, constructor, and prototype keys. However, this check only runs when the #set directive assigns a value — it validates the assignment target path, not the value expression being evaluated. The value expression is evaluated via getReferences() in dist/cjs/compile/references.cjs:16, which calls getAttributes() at line 81. The property access at line 88-89 has no filtering: // references.cjs:81-91 getAttributes(property, baseRef, ast) { if (property.type === "property") { return baseRef[property.id]; // ← NO BLOCK on "constructor", "prototype", etc. } ... } Meanwhile, set.cjs:35-43 properly blocks these keys, but only for the #set target: // set.cjs:35-43 isBlockedPathKey(baseRef, key, isEnd) { if (key === PROTOKEY) return true; // "proto" if (key === "prototype" && typeof baseRef === "function") return true; return !isEnd && PROTOTYPECHAINKEYS.has(key) && !hasOwnProperty(baseRef, key); } The exploit chain: 1. $x.constructor → getAttributes() → {}["constructor"] → Object 2. .constructor → getAttributes() → Object["constructor"] → Function 3. ("return process.mainModule.require('childprocess').execSync('whoami')") → calls Function(...) → creates a function 4. The #set assigns the result to $f, which is then rendered as $r The #set handler validates $f as the assignment target (which passes — f is not blocked), but never inspects the right-hand expression for prototype chain traversal.

PoC

Weakness type: CWE-94.

Product: velocityjs (npm) · Assigned by: GitHub_M · Published: 2026-08-13

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.73% probability of exploitation activity in the next 30 days; percentile 54.

Timeline

• 2026-07-24 — GitHub-reviewed advisory GHSA-7gfh-x38p-prh3 published (critical)
• 2026-08-13 — CVE record published by GitHub_M
• 2026-08-14 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-7gfh-x38p-prh3 github.com
2. CVE.org record CVE-2026-73649 cve.org
3. NVD entry CVE-2026-73649 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com

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