GNU C Library (glibc) EUVD-2026-23978

Exploitation requires target applications using scanf family functions (scanf, fscanf, sscanf, vscanf, vfscanf, vsscanf) with the %mc format specifier (malloc'd character match POSIX extension) AND accepting attacker-controlled input that influences either the format string itself or the data being parsed AND allowing format width specifiers exceeding 1024 characters. The %mc extension is less commonly used than standard %s string matching, significantly limiting vulnerable code surface area. Most security-conscious applications avoid scanf for untrusted input due to historical vulnerabilities, preferring getline(), fgets(), or custom parsers with explicit bounds checking. Exploitation is further constrained by modern heap protections (ASLR, heap canaries, metadata integrity checks) that make reliable single-byte overflow exploitation challenging compared to arbitrary-length overflows. Applications using hardcoded format strings without dynamic construction or those validating width parameters below 1024 are NOT vulnerable despite using affected glibc versions. Network exposure (AV:N in CVSS) assumes the vulnerable parsing code is reachable via network protocols rather than local file processing, which may not reflect actual deployment for many scanf-using applications.

CVSS 9.8 (AV:N/AC:L/PR:N/UI:N) indicates theoretical maximum severity with network attack vector, low complexity, and no authentication required, suggesting remote unauthenticated exploitation with high confidentiality, integrity, and availability impact. However, this severity rating assumes direct network exposure of vulnerable scanf parsing, which conflicts with typical real-world deployment patterns. EPSS score of 0.02% (5th percentile) strongly contradicts the CVSS severity, indicating security researchers assess actual exploitation probability as negligible. This disparity exists because successful exploitation requires: (1) application code using scanf family functions with %mc format specifier, (2) attacker-controlled format width parameter >1024, and (3) ability to supply malicious input to the parsing function. Most modern applications avoid scanf for untrusted input due to known security weaknesses, use safer alternatives like getline(), or sanitize format strings. The one-byte overflow provides limited exploitation primitives compared to arbitrary-length overflows. Absence from CISA KEV despite glibc's ubiquity confirms no widespread exploitation campaigns exist. Priority should be MODERATE for comprehensive patching, but this is not an emergency 'drop everything' vulnerability despite the high CVSS score. Focus remediation on internet-facing services processing untrusted input through scanf functions rather than blanket infrastructure updates.

An attacker identifies a network service (e.g., custom protocol parser, log ingestion daemon, IoT device management interface) that accepts user-supplied input and processes it using sscanf() with a %mc format specifier. The attacker crafts a malicious input string containing a character match pattern with an explicit width specifier of 1025 or greater (e.g., '%1025mc'). When the vulnerable glibc scanf implementation processes this input, the heap memory allocator writes one byte beyond the allocated buffer boundary, corrupting adjacent heap metadata. The attacker leverages heap feng shui techniques to position critical data structures (function pointers, vtables, or authentication tokens) adjacent to the overflow location, allowing the single-byte overflow to modify control flow or security-critical values. Successful exploitation yields arbitrary code execution with the privileges of the vulnerable service process, potentially root/system level for privileged daemons.