fs_read_glyphs() in the libXfont2 font-server client (src/fc/fserve.c) copies each glyphs bitmap into a single buffer. Existing checks validates only that the source slice (position, length) lies within the source bitmap buffer. It does not check whether the running destination cursor has exceeded the allocation.
A malicious font server can send overlapping source offsets – for example 1000 glyphs each referencing {position:0, length:64} with nbytes=64. Each individual source range passes the existing validation, but the cumulative writes total 64000 bytes into a 64-byte destination buffer. This is a heap buffer overflow with attacker-controlled content.
Weakness
A heap overflow condition is a buffer overflow, where the buffer that can be overwritten is allocated in the heap portion of memory, generally meaning that the buffer was allocated using a routine such as malloc().
Affected Software
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Red Hat Enterprise Linux 10 | RedHat | libXfont2-0:2.0.6-5.el10_2.3 | * |
| Red Hat Enterprise Linux 10.0 Extended Update Support | RedHat | libXfont2-0:2.0.6-5.el10_0.2 | * |
| Red Hat Enterprise Linux 7 Extended Lifecycle Support | RedHat | libXfont2-0:2.0.3-3.el7_9 | * |
| Red Hat Enterprise Linux 8 | RedHat | libXfont2-0:2.0.3-2.el8_10.3 | * |
| Red Hat Enterprise Linux 8.4 Advanced Mission Critical Update Support | RedHat | libXfont2-0:2.0.3-2.el8_4.2 | * |
| Red Hat Enterprise Linux 8.4 Extended Update Support Long-Life Add-On | RedHat | libXfont2-0:2.0.3-2.el8_4.2 | * |
| Red Hat Enterprise Linux 8.6 Advanced Mission Critical Update Support | RedHat | libXfont2-0:2.0.3-2.el8_6.2 | * |
| Red Hat Enterprise Linux 8.6 Extended Update Support Long-Life Add-On | RedHat | libXfont2-0:2.0.3-2.el8_6.2 | * |
| Red Hat Enterprise Linux 8.8 Telecommunications Update Service | RedHat | libXfont2-0:2.0.3-2.el8_8.2 | * |
| Red Hat Enterprise Linux 8.8 Update Services for SAP Solutions | RedHat | libXfont2-0:2.0.3-2.el8_8.2 | * |
| Red Hat Enterprise Linux 9 | RedHat | libXfont2-0:2.0.3-12.el9_8.3 | * |
| Red Hat Enterprise Linux 9.2 Update Services for SAP Solutions | RedHat | libXfont2-0:2.0.3-12.el9_2.3 | * |
| Red Hat Enterprise Linux 9.4 Update Services for SAP Solutions | RedHat | libXfont2-0:2.0.3-12.el9_4.3 | * |
| Red Hat Enterprise Linux 9.6 Extended Update Support | RedHat | libXfont2-0:2.0.3-12.el9_6.3 | * |
| Libxfont | Ubuntu | upstream | * |
Potential Mitigations
- Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking.
- D3-SFCV (Stack Frame Canary Validation) from D3FEND [REF-1334] discusses canary-based detection in detail.
- Run or compile the software using features or extensions that randomly arrange the positions of a program’s executable and libraries in memory. Because this makes the addresses unpredictable, it can prevent an attacker from reliably jumping to exploitable code.
- Examples include Address Space Layout Randomization (ASLR) [REF-58] [REF-60] and Position-Independent Executables (PIE) [REF-64]. Imported modules may be similarly realigned if their default memory addresses conflict with other modules, in a process known as “rebasing” (for Windows) and “prelinking” (for Linux) [REF-1332] using randomly generated addresses. ASLR for libraries cannot be used in conjunction with prelink since it would require relocating the libraries at run-time, defeating the whole purpose of prelinking.
- For more information on these techniques see D3-SAOR (Segment Address Offset Randomization) from D3FEND [REF-1335].