CVE-2026-18220: Binutils: binutils: out-of-bounds write in bfd dlx elf backend relocation processing

This is a vulnerability report sent to us through https://issues.redhat.com/browse/PSIRTSUPT-18741 and was created using the PoC auto-triage agents. It contains hints on the vulnerability extracted by the IA and the full report. ALWAYS review it before any action. Once working on this, don't forget to also update the JSM ticket.

An out-of-bounds write vulnerability was found in the BFD library's DLX ELF backend (bfd/elf32-dlx.c) in GNU binutils. The dlxrtypetohowto() function maps ELF relocation types to internal howto structures but fails to perform adequate bounds checking on attacker-controlled relocation type values (via ELF32RTYPE(rinfo)) before indexing into the dlxelfhowtotable[] array. The DLX relocation type number space is non-contiguous (basic types 0-6, extended types at 0x10000+), but the default case in the switch statement allows arbitrary index values to reach the array access.

A specially crafted ELF/DLX object file can trigger this out-of-bounds write when processed by any BFD-consuming tool (objdump, readelf, strip, ld, nm, objcopy). The vulnerability has been demonstrated to achieve arbitrary code execution via a File Stream Oriented Programming (FSOP) attack against glibc FILE structures (stderr), redirecting control flow to system().

Attack scenarios include CI/CD pipelines performing automated binary analysis, developer workstations running objdump/readelf on untrusted binaries, automated security scanning or malware analysis tools invoking binutils, and package build systems processing third-party code.

Note: This vulnerability is only exploitable when binutils is built with the DLX backend enabled (typically via --enable-targets=all). Manual verification of Red Hat package build configurations is required to confirm affected product scope.