CVE-2019-25656 Overview
CVE-2019-25656 is a local stack-based buffer overflow in R i386 version 3.5.0 on Windows. The flaw resides in the GUI Preferences dialog, where the Language for menus and messages field fails to validate input length. A local attacker who supplies an oversized payload can overwrite a Structured Exception Handler (SEH) record and redirect execution to attacker-controlled shellcode. Public exploit material demonstrates spawning calc.exe, but arbitrary shellcode execution is possible in the context of the user running R. The issue is tracked under CWE-787: Out-of-bounds Write.
Critical Impact
Local attackers can achieve arbitrary code execution in the user's context by overwriting SEH records through the R GUI Preferences dialog.
Affected Products
- R for Windows 3.5.0 (i386 build)
- R GUI Preferences component (Rgui.exe)
- Distributions installed via the R Project Windows Installer
Discovery Timeline
- 2026-04-05 - CVE-2019-25656 published to NVD
- 2026-04-16 - Last updated in NVD database
Technical Details for CVE-2019-25656
Vulnerability Analysis
The vulnerability is a classic stack-based buffer overflow exposed through the R i386 graphical interface. When a user opens the Preferences dialog in Rgui.exe, the application accepts user input into the Language for menus and messages field without enforcing a bounded copy. Supplying a string substantially longer than the destination buffer corrupts adjacent stack memory, including the SEH chain.
Because the binary is compiled as 32-bit (i386) and does not consistently enforce SafeSEH or other mitigations on this code path, an attacker can overwrite the Next SEH pointer and the SEH handler address. Triggering an exception then transfers control to attacker-supplied shellcode using a standard POP POP RET gadget. Public proof-of-concept material on Exploit-DB #46288 demonstrates this technique.
Root Cause
The root cause is missing length validation on input written into a fixed-size stack buffer in the Preferences dialog handler. The code performs an unchecked string copy, allowing the source to exceed the destination capacity and overwrite the SEH frame stored on the stack.
Attack Vector
Exploitation requires local access. An attacker with the ability to run Rgui.exe or to coerce a user into loading a crafted configuration containing the malicious string can trigger the overflow. The attacker pastes or loads the oversized payload into the language field, applies settings, and the resulting exception dispatches into shellcode. See the VulnCheck Advisory on Buffer Overflow for technical context.
Detection Methods for CVE-2019-25656
Indicators of Compromise
- Unexpected child processes spawned from Rgui.exe, such as calc.exe, cmd.exe, or powershell.exe.
- Crash dumps or Windows Error Reporting events referencing access violations in Rgui.exe version 3.5.0.
- Modifications to R configuration files containing unusually long values in the language setting.
Detection Strategies
- Monitor process creation events where the parent process is Rgui.exe and the child is an interpreter or LOLBin.
- Alert on Windows exception events (Event ID 1000) tied to the Rgui.exe image with faulting modules indicating stack corruption.
- Inspect R configuration and .Renviron files for payload-like strings exceeding expected length thresholds.
Monitoring Recommendations
- Maintain an inventory of hosts running R 3.5.0 i386 to prioritize containment.
- Capture command-line telemetry for all processes descending from R-related binaries.
- Forward endpoint exception and process telemetry to a centralized analytics platform for retrospective hunting.
How to Mitigate CVE-2019-25656
Immediate Actions Required
- Upgrade R for Windows to a current supported release from the R Project Homepage and remove R 3.5.0 i386 installations.
- Restrict execution of Rgui.exe to trusted users through application control policies.
- Block import of untrusted R configuration files and user profiles from external sources.
Patch Information
No vendor-specific patch advisory is referenced in the NVD entry for R 3.5.0. The supported remediation is to upgrade to a current R release, which supersedes the vulnerable 3.5.0 i386 build. Where upgrade is not feasible, prefer the 64-bit build and apply the workarounds below.
Workarounds
- Avoid entering untrusted input into the GUI Preferences dialog and do not load configurations from untrusted sources.
- Run R under standard user accounts with no administrative privileges to limit post-exploitation impact.
- Where possible, use the command-line R interpreter (Rscript.exe) instead of Rgui.exe on Windows hosts.
# Verify installed R version on Windows and remove the vulnerable build
reg query "HKLM\SOFTWARE\R-core\R" /s | findstr /i "InstallPath Current Version"
# Uninstall R 3.5.0 i386 silently if present
"C:\Program Files\R\R-3.5.0\unins000.exe" /SILENT
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
