xrdp is an open source RDP server. xrdp before v0.10.5 contains an unauthenticated stack-based buffer overflow vulnerability. The issue stems from improper bounds checking when processing user domain information during the connection sequence. If exploited, the vulnerability could allow remote attackers to execute arbitrary code on the target system. The vulnerability allows an attacker to overwrite the stack buffer and the return address, which could theoretically be used to redirect the execution flow. The impact of this vulnerability is lessened if a compiler flag has been used to build the xrdp executable with stack canary protection. If this is the case, a second vulnerability would need to be used to leak the stack canary value. Upgrade to version 0.10.5 to receive a patch. Additionally, do not rely on stack canary protection on production systems.
Weakness
A stack-based buffer overflow condition is a condition where the buffer being overwritten is allocated on the stack (i.e., is a local variable or, rarely, a parameter to a function).
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].