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The CVE-2026-23835 vulnerability in LobeChat stems from inadequate server-side validation of client-supplied parameters during file uploads. When a user uploads a file via the Knowledge Base interface, a POST request is sent to the endpoint /trpc/lambda/file.createFile?batch=1. This request contains JSON data with fields like “name” and “size” that dictate file metadata and storage accounting. Crucially, the server trusts these client-provided values without independent verification. An attacker can intercept this HTTP request using a proxy tool such as Burp Suite during transmission. By modifying the “size” parameter in the intercepted request, the attacker can misrepresent the actual file size to the server. For example, they can set “size” to 10485760 (10 MB) while uploading a file of 1073741824 bytes (1 GB). The server, relying on this manipulated value, updates the user’s storage quota based on the reported 10 MB, allowing the 1 GB upload to proceed unimpeded. Similarly, the “name” field can be altered to include directory traversal sequences (e.g., “../../malicious.exe”), enabling file creation in unauthorized directories. This flaw bypasses the monthly upload limits enforced by subscription plans, as the server uses the client-reported “size” for quota calculations. The absence of integrity checks on these parameters allows arbitrary file uploads beyond intended storage paths and limits. This manipulation can also distort billing metrics, as the server may charge for 10 MB while storing 1 GB, or vice versa. The vulnerability exploits the trust boundary between client and server, a common security misconfiguration. It effectively grants attackers the ability to exhaust storage resources, evade financial controls, and potentially deploy malicious files. The core issue is the server’s failure to validate file metadata against actual upload content or to implement server-side size verification. This lack of validation makes the system susceptible to simple parameter tampering attacks.
Platform: LobeChat
Version: Prior to patch
Vulnerability: File upload manipulation
Severity: Critical
date: 2026-01-01
Prediction: Patch released 2026-01-01
What Undercode Say:
!/bin/bash
curl -X POST https://lobechat.com/trpc/lambda/file.createFile?batch=1 -H “Content-Type: application/json” -d ‘{“name”:”../../evil.txt”,”size”:10485760}’ –data-binary @largefile.bin
Use Burp Suite to intercept and modify POST requests.
Modify ‘size’ field to bypass quota checks.
Change ‘name’ field for path traversal.
How Exploit:
Intercept upload request.
Tamper size parameter.
Manipulate name field.
Bypass quota limits.
Protection from this CVE:
Server-side size validation.
Sanitize file names.
Apply security patches.
Independent metadata verification.
Impact:
Financial loss.
Resource exhaustion.
Service disruption.
Arbitrary file upload.
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Sources:
Reported By: github.com
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