CVE-2026-71392 in Emacs
Summary
by MITRE • 08/10/2026
GNU Emacs for Android is vulnerable to an integer overflow in the sfnt_read_cmap_format_12() function in src/sfnt.c. When processing a crafted TrueType font file, an unguarded addition in the xmalloc allocation call wraps around on 32-bit builds, causing a heap buffer overflow write. An attacker can deliver a malicious font file via email, EWW (Emacs Web Wowser), or documents with custom faces, causing Emacs to load it. This results in heap memory corruption that can lead to code execution.
This issue was fixed in commit c4e20777c26548722a37b03db93243e83a0d6188
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 08/10/2026
The vulnerability identified in GNU Emacs for Android represents a critical integer overflow condition within the sfnt_read_cmap_format_12() function located in src/sfnt.c. This flaw manifests specifically during the processing of crafted TrueType font files, where the application fails to properly validate input data before performing memory allocation operations. The core technical issue stems from an unguarded arithmetic operation that causes integer wraparound behavior on 32-bit system architectures, leading to a heap buffer overflow condition that can be exploited for remote code execution.
The vulnerability operates through a precise sequence of events where malicious font files containing specially crafted data structures trigger the problematic code path. When Emacs encounters such a file through normal operations like email attachments, web browsing via EWW (Emacs Web Wowser), or document rendering with custom faces, it attempts to parse the TrueType font metadata using the vulnerable function. The integer overflow occurs during the calculation of memory allocation size required for the cmap format 12 table processing, where an addition operation produces a result that exceeds the maximum value representable by a 32-bit signed integer, causing the subsequent xmalloc call to allocate insufficient memory.
This heap buffer overflow creates a condition where arbitrary data can be written beyond the bounds of allocated memory regions, potentially corrupting adjacent heap allocations and creating opportunities for exploitation. The attack vector is particularly concerning as it requires no special privileges or user interaction beyond normal document processing operations, making it suitable for drive-by attacks through email attachments or web content. The vulnerability's impact extends to all Android builds of GNU Emacs that execute on 32-bit architectures, though the risk may be mitigated on 64-bit systems due to different memory management characteristics.
The fix implemented in commit c4e20777c26548722a37b03db93243e83a0d6188 addresses this issue by introducing proper integer overflow checks before the memory allocation occurs. This solution follows established security practices for preventing arithmetic overflows and aligns with common mitigations recommended in industry standards such as those outlined in CWE-190, which specifically addresses integer overflow conditions. The fix ensures that input validation occurs prior to any memory allocation operations, preventing the wraparound behavior that previously enabled the heap corruption. This remediation approach directly addresses the root cause rather than attempting to patch symptoms, providing a robust solution that eliminates the attack surface while maintaining the application's core functionality for legitimate font processing operations.
The vulnerability demonstrates characteristics consistent with ATT&CK technique T1203, which involves exploitation of software vulnerabilities through memory corruption attacks, and represents a classic example of how seemingly benign input processing can become an attack vector when proper bounds checking is absent. The security implications extend beyond immediate code execution to potential privilege escalation scenarios, as successful exploitation could allow attackers to gain unauthorized access to the device or execute arbitrary commands within the application's security context. This makes the vulnerability particularly dangerous in mobile environments where applications have access to user data and system resources.
The remediation approach taken by the GNU Emacs development team demonstrates adherence to secure coding practices that emphasize prevention over detection, implementing checks that would be classified under CWE-191, which addresses integer underflow conditions, and CWE-192, which covers integer overflow in multiple contexts. By incorporating proper validation before memory allocation, the fix reduces the attack surface significantly while maintaining backward compatibility with legitimate font files. This type of vulnerability highlights the importance of thorough input validation in graphics processing libraries and font rendering engines, where malformed input can often be processed without proper safety checks, creating dangerous conditions for heap manipulation attacks.