Svelte devalue EUVD-2026-119452

Exploitation requires two conditions, both tied to the consuming application, not devalue alone: (1) the application must pass attacker-controlled serialized input to devalue.parse or devalue.unflatten, and (2) downstream code must merge the returned object into a target using Object.assign (or an equivalent own-property copy) so that an emitted __proto__ own property pollutes the prototype. Devalue emitting the __proto__ property is explicitly stated by the vendor to not be a vulnerability on its own. Limiting factors: affects only devalue versions >= 4.0.0 and < 5.6.4 (fixed in 5.6.4); impact is confined to prototype/integrity corruption (no direct confidentiality or availability loss per VC:N/VA:N); applications that never merge parsed output unsafely are not exploitable, which is why the CVSS 4.0 vector carries AT:P.

This is a moderate, conditional-risk issue rather than a top-tier emergency. The CVSS 4.0 vector (6.3, AV:N/AC:L/AT:P/PR:N/UI:N) shows a network-reachable, unauthenticated flaw with only low integrity impact (VI:L) and no confidentiality or availability impact - consistent with prototype pollution that corrupts object state but does not directly leak data or crash the service. The AT:P (attack requirement present) metric aligns precisely with the advisory's own caveat: devalue emitting a __proto__ own property is NOT by itself a vulnerability (JSON.parse can do the same) and only becomes prototype injection when downstream application code merges the parsed result via Object.assign or similar. The vuln.today decision label is 'this_month' (not act_now/emergency), there is no CISA KEV listing, and no EPSS score was supplied - so no signal here indicates active or widespread exploitation. No KEV and no EPSS means exploitation likelihood cannot be quantified from the input; treat the missing EPSS as unknown, not as low. Net: a real but defense-in-depth-tier issue whose actual impact hinges on whether consuming applications handle devalue output unsafely.

An attacker submits crafted serialized input to an application endpoint that deserializes it with devalue.parse or devalue.unflatten; the returned object carries a __proto__ own property. If the application then merges that object into another via Object.assign (e.g., Object.assign(target, result)), Object.prototype is polluted, letting the attacker influence downstream object behavior. No confirmed working public exploit was identified; the GHSA advisory supplies only illustrative code showing the unsafe downstream pattern.