GNU Aspell 0.60.8.2 (first vulnerable version is not known) contains a heap-based buffer overflow write (CWE-122) in its read-only dictionary loader. String::reserve() in common/string.hpp computes its capacity check with a signed int ((int)s + 1), so a requested size of 0x7FFFFFFF wraps negative and the backing buffer is never grown. ReadOnlyDict::load() in modules/speller/default/readonly_ws.cpp passes the unvalidated 32-bit size fields from a .rws dictionary header into this path, so a crafted .rws file with a valid endian marker (12345678) and an oversized length field (e.g. lang_name_size = 0x7FFFFFFF) causes attacker-controlled data to be written out of bounds past an undersized heap allocation. An attacker who induces a victim to load a malicious dictionary — for example via aspell dump master — can corrupt heap memory, resulting in denial of service and potentially code execution. Fixed in b830b6e and will be fixed in release 0.60.8.3.
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().
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].