symfony/ux-live-component: LiveComponentHydrator HMAC checksum lacks component and slot binding
Description
Description
In symfony/ux-live-component, a component's server-side state is exposed to the browser as a set of props (#[LiveProp]-annotated properties). Props marked writable: true can be freely changed by the client. Read-only props are round-tripped to the browser and back, and their integrity is protected by an HMAC so the client cannot tamper with them. Child components additionally receive a propsFromParent blob, also HMAC-signed.
The HMAC computed by Symfony\UX\LiveComponent\LiveComponentHydrator covered only the sorted prop key/value pairs. It didn't include the component name, the slot identifier (props vs propsFromParent), or any request context, and a single application-wide secret is used for every component. A signed blob the server minted for component A is therefore a valid signature for component B if the key names happen to match, and a props blob can be replayed in the propsFromParent slot (or the reverse). An attacker can use this to set a read-only prop on a target component to a value they were only ever allowed to choose as a writable prop on another component.
Resolution
The HMAC is now bound to its context: the component name and a slot identifier are included in the pre-image before hashing. Two constants (CHECKSUM_SLOT_PROPS and CHECKSUM_SLOT_PROPS_FROM_PARENT) name the two slots, and calculateChecksum(), verifyChecksum(), addChecksumToData(), and ChildComponentPartialRenderer thread these values through. Cross-component and cross-slot replays no longer verify.
The patch for this issue is available here for branch 2.x (and forward-ported to 3.x).
Credits
Symfony would like to thank Anthropic (via Project Glasswing) for reporting the issue and Hugo Alliaume for providing the fix.
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Affected products
1Patches
Vulnerability mechanics
Root cause
"The HMAC checksum was computed only over the sorted prop key/value pairs, omitting the component name and slot identifier, enabling cross-component and cross-slot replay attacks."
Attack vector
An attacker can replay a signed HMAC blob that the server minted for one component (component A) as the checksum for a different component (component B) if the key names happen to match, because the HMAC was not bound to the component identity. Similarly, a `props` blob can be replayed in the `propsFromParent` slot (or vice versa). This allows the attacker to set a read-only prop on a target component to a value they were only ever allowed to choose as a writable prop on another component. [ref_id=2]
Affected code
The vulnerability resides in `Symfony\UX\LiveComponent\LiveComponentHydrator`, specifically in the `calculateChecksum()` and `verifyChecksum()` methods. The HMAC was computed solely over the sorted prop key/value pairs without including the component name or slot identifier (`props` vs `propsFromParent`). The `ChildComponentPartialRenderer` and `addChecksumToData()` methods also lacked these context bindings. [patch_id=6625360]
What the fix does
The patch modifies `calculateChecksum()` to include `$componentName` and `$slot` in the pre-image before hashing (`$preImage = $componentName."\0".$slot."\0".json_encode($dehydratedPropsData)`). The `verifyChecksum()` and `addChecksumToData()` methods now accept and pass these parameters. Two public constants (`CHECKSUM_SLOT_PROPS` and `CHECKSUM_SLOT_PROPS_FROM_PARENT`) name the two slots, and `ChildComponentPartialRenderer` passes the correct slot when signing child-component data. Cross-component and cross-slot replays no longer verify because the HMAC is now context-bound. [patch_id=6625360]
Preconditions
- networkThe attacker must be able to observe a signed HMAC blob for a component they interact with (e.g., via the browser's network traffic).
- configThe target component must have a read-only prop whose key name matches a writable prop on another component the attacker can influence.
Generated on Jun 19, 2026. Inputs: CWE entries + fix-commit diffs from this CVE's patches. Citations validated against bundle.
References
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