devalue CVE-2026-123561

Exploitation requires that the application pass attacker-controlled serialized data to devalue's parse function in a vulnerable version (<= 5.9.2). The attacker must craft a payload with a malformed null-prototype object key that is coerced during property-key handling to bypass the library's __proto__ rejection. Key limiting factors: the flaw by the vendor's own statement does NOT by itself produce prototype pollution - it only creates an object with a __proto__ own property (behavior analogous to native JSON.parse), so any actual security impact depends on insecure downstream handling of the parsed object. There is no confidentiality or availability impact (VC:N/VA:N) and only partial integrity impact (VI:L). Deployments that never feed untrusted input to parse, or that have upgraded to 5.9.3, are not exposed.

The input signals point to a modest, defense-in-depth issue rather than an urgent one. The vendor-supplied CVSS 4.0 vector (AV:N/AC:L/AT:P/PR:N/UI:N) yields 6.3 (Medium) and, critically, records only VC:N/VI:L/VA:N - no confidentiality or availability impact and merely LOW integrity impact - with no scope change to subsequent systems. This is consistent with the advisory's own framing: the bypass causes parse to create an object with a __proto__ own property, which the vendor explicitly states 'on its own is not enough to cause prototype pollution' and is 'very unlikely' to cause issues. The AT:P metric signals that a specific attack requirement (a crafted malformed null-prototype key payload) must be met. No EPSS score was provided, the CVE is NOT in CISA KEV, and no confirmed public exploit code is listed - only the GHSA and VulnCheck advisories. The vuln.today decision label is 'this_month' (not act_now/emergency), aligning with the medium severity. Net assessment: this is a high-value library getting a defensive hardening fix for an incomplete prior patch (GHSA-mwv9-gp5h-frr4), not a high-CVSS/high-real-risk emergency. Note the CWE-1321 (prototype pollution) classification overstates standalone impact, since the vendor confirms no pollution results from this flaw alone; real-world danger depends entirely on how downstream application code handles the resulting object.

An application deserializes attacker-controlled data using devalue's parse (version <= 5.9.2). The attacker submits a crafted payload whose malformed null-prototype object key is coerced so that it bypasses devalue's __proto__ rejection, producing an object that carries a __proto__ own property. By itself this yields no prototype pollution, but it defeats the library's intended defensive check and could compound with insecure downstream handling of the parsed object. No public exploit code is identified in the provided sources.