LibVNCServer 0.9.12 release and earlier contains heap buffer overflow vulnerability within the HandleCursorShape() function in libvncclient/cursor.c. An attacker sends cursor shapes with specially crafted dimensions, which can result in remote code execution.
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 7 | RedHat | libvncserver-0:0.9.9-14.el7_7 | * |
| Red Hat Enterprise Linux 8 | RedHat | libvncserver-0:0.9.11-9.el8_1.2 | * |
| Red Hat Enterprise Linux 8.0 Update Services for SAP Solutions | RedHat | libvncserver-0:0.9.11-9.el8_0.2 | * |
| Libvncserver | Ubuntu | bionic | * |
| Libvncserver | Ubuntu | eoan | * |
| Libvncserver | Ubuntu | esm-infra/bionic | * |
| Libvncserver | Ubuntu | esm-infra/focal | * |
| Libvncserver | Ubuntu | esm-infra/xenial | * |
| Libvncserver | Ubuntu | focal | * |
| Libvncserver | Ubuntu | trusty | * |
| Libvncserver | Ubuntu | xenial | * |
| X11vnc | Ubuntu | bionic | * |
| X11vnc | Ubuntu | eoan | * |
| X11vnc | Ubuntu | focal | * |
| X11vnc | Ubuntu | groovy | * |
| X11vnc | Ubuntu | hirsute | * |
| X11vnc | Ubuntu | impish | * |
| X11vnc | Ubuntu | kinetic | * |
| X11vnc | Ubuntu | lunar | * |
| X11vnc | Ubuntu | mantic | * |
| X11vnc | Ubuntu | trusty | * |
| X11vnc | Ubuntu | trusty/esm | * |
| X11vnc | Ubuntu | xenial | * |
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].