CVE-2026-19508: RDK-B WebUI Buffer Overflow Vulnerability

CVE-2026-19508 Overview

CVE-2026-19508 is a heap-based buffer overflow in the multipart form-data parser located in jst_post.c within the RDK-B WebUI component. The affected build is rdkb-2025q4-kirkstone.04.10.26. A remote, unauthenticated attacker can trigger memory corruption by sending a crafted multipart/form-data request to the web interface. Successful exploitation results in denial of service and may allow arbitrary code execution on the affected broadband device. Because RDK-B (Reference Design Kit for Broadband) powers a wide range of customer-premises equipment (CPE), the vulnerability directly exposes gateway and router administrative interfaces.

Critical Impact
Remote unauthenticated attackers can corrupt heap memory in the RDK-B WebUI and potentially execute arbitrary code on broadband gateway devices.

Affected Products

RDK-B WebUI build rdkb-2025q4-kirkstone.04.10.26
Broadband gateway devices running the affected RDK-B WebUI component
Deployments that expose the WebUI jst_post.c handler to untrusted networks

Discovery Timeline

2026-08-19 - CVE-2026-19508 published to the National Vulnerability Database (NVD)
2026-08-19 - Last updated in NVD database

Technical Details for CVE-2026-19508

Vulnerability Analysis

The flaw resides in the multipart form-data parsing logic implemented in jst_post.c, part of the RDK-B WebUI. The parser handles HTTP POST requests with Content-Type: multipart/form-data, which typically deliver file uploads and form field values delimited by boundary strings. The parser fails to correctly bound the length of attacker-controlled data copied into a heap-allocated buffer. As a result, an attacker who submits a crafted multipart request can overwrite adjacent heap memory. Depending on the runtime layout, corruption can crash the WebUI process, corrupt control structures, or redirect execution flow to attacker-supplied data.

Root Cause

The root cause is missing or insufficient length validation when copying multipart segments—boundary tokens, header fields, or body content—into a fixed heap buffer inside jst_post.c. Classifies as a heap-based buffer overflow [CWE-122]. The parser trusts the size of incoming multipart fields rather than enforcing a strict maximum before the copy operation.

Attack Vector

Exploitation requires network reachability to the WebUI listener and no authentication. An attacker crafts a multipart/form-data HTTP request with oversized or malformed part headers, boundary markers, or body content designed to overflow the vulnerable heap buffer. The vulnerability manifests during request parsing, before any authentication check completes, which broadens the exposure. See the PWR.edu.pl vulnerability blog post for the technical write-up and the RDK Central WebUI documentation for component context.

No verified proof-of-concept code is published. Refer to the PWR.edu.pl advisory for technical details of the crafted multipart/form-data request.

Detection Methods for CVE-2026-19508

Indicators of Compromise

Unexpected crashes, restarts, or watchdog resets of the RDK-B WebUI process on broadband gateways
Inbound HTTP POST requests with Content-Type: multipart/form-data containing abnormally long boundary strings, part headers, or field names
WebUI log entries showing malformed multipart requests originating from untrusted or WAN-facing sources

Detection Strategies

Inspect HTTP request bodies at network chokepoints and flag multipart requests where individual part headers exceed conventional limits
Alert on repeated POST requests to WebUI endpoints from a single source followed by service restarts
Correlate WebUI process crash telemetry with concurrent inbound HTTP traffic to identify likely exploitation attempts

Monitoring Recommendations

Enable verbose logging on the WebUI HTTP frontend to record request paths, sizes, and Content-Type headers
Monitor device management planes for anomalous outbound connections that could indicate post-exploitation activity
Baseline normal WebUI request patterns and alert on statistical deviations in request length or frequency

How to Mitigate CVE-2026-19508

Immediate Actions Required

Restrict access to the RDK-B WebUI to trusted management networks and disable WAN-side administration
Place affected devices behind a network filter that rejects oversized or malformed multipart/form-data requests
Audit deployed firmware inventory to identify devices running rdkb-2025q4-kirkstone.04.10.26 or related builds

Patch Information

At publication, no vendor patch identifier is listed in the NVD entry for CVE-2026-19508. Operators should monitor the RDK Central WebUI documentation and downstream device vendor advisories for a fixed release of jst_post.c and deploy firmware updates as soon as they are available.

Workarounds

Block external access to the WebUI TCP port at the network edge until a firmware fix is applied
Enforce a request-size limit on inbound HTTP traffic destined for the WebUI using an upstream reverse proxy or firewall
Disable remote management features on affected gateways where operationally acceptable

bash

# Example iptables rule restricting WebUI access to a management subnet
iptables -A INPUT -p tcp --dport 443 -s 10.10.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP