CVE-2026-77219 in Emacs
Summary
by MITRE • 08/21/2026
GNU Emacs before 31.0.91 contains an integer overflow in the PBM/PPM/PGM image loader that allows an attacker to leak heap memory contents by supplying a crafted image with large dimensions and an elevated max color index. The image loader multiplies image dimensions and channel count using signed integer arithmetic; for sufficiently large values, the result wraps to a negative number, bypassing the bounds check and causing the pixel reader to access heap memory past the end of the allocated buffer. The over-read contents are interpreted as pixel color values and rendered on screen.
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Analysis
by VulDB Data Team • 08/21/2026
The vulnerability identified in GNU Emacs prior to version 31.0.91 represents a critical security flaw within the image processing subsystem, specifically affecting the loaders for PBM, PPM, and PGM formats. These are common portable bitmap graphics formats often used in technical documentation and legacy systems. The core of the issue lies in how the application handles arithmetic operations during the initialization phase of loading these images. When a user opens an image file, Emacs calculates the total size required to store pixel data by multiplying the horizontal dimensions, vertical dimensions, and the number of color channels together. This calculation is performed using signed integer arithmetic rather than unsigned or checked arithmetic types that would prevent overflow conditions.
In standard two's complement representation used by most modern processors, signed integers have a fixed range determined by their bit width. When the product of image dimensions and channel counts exceeds this maximum positive value, the result wraps around to become a negative number due to integer overflow. This behavior is classified under CWE-190: Integer Overflow or Wraparound. The vulnerability arises because subsequent memory allocation routines typically rely on the magnitude of this calculated size rather than checking for sign bits or overflow conditions explicitly before proceeding. Consequently, when the computed value becomes negative, it may be interpreted as a very large unsigned integer by certain system calls, or more commonly in this context, it bypasses logical bounds checks that expect positive values representing valid memory sizes.
The operational impact of this flaw is severe because it leads to an out-of-bounds read operation on the heap. After the flawed size calculation occurs, the application allocates a buffer based on the erroneous value and proceeds to populate it with pixel data from the crafted image file. Because the bounds check was bypassed due to the negative result wrapping or being misinterpreted, the pixel reader continues accessing memory locations beyond the end of the allocated heap buffer. This results in an arbitrary read primitive where sensitive information stored elsewhere in the process's heap memory is exposed. The leaked data includes whatever bytes reside immediately after the image buffer, which can contain pointers, object metadata, credentials, or other confidential user data depending on what else was loaded into that region of memory at runtime.
This vulnerability aligns with CWE-125: Out-of-bounds Read, as it involves reading beyond allocated boundaries without proper validation. From a threat modeling perspective, this behavior is consistent with ATT&CK technique T1083: File and Directory Discovery or potentially data exfiltration vectors if the leaked memory contents are subsequently transmitted over a network by malware exploiting Emacs for initial access. The attack vector requires only that an attacker can supply a crafted image file to be opened within GNU Emacs, which could occur through email attachments, web browsing via plugins, or opening files from untrusted directories.
Mitigation strategies primarily involve upgrading to version 31.0.91 or later of GNU Emacs, where the developers have implemented proper integer overflow checks and switched to safer arithmetic operations that validate bounds before allocation. For environments unable to upgrade immediately, restricting image file sources to trusted origins and disabling automatic loading of images in untrusted contexts can reduce exposure. Additionally, employing sandboxing mechanisms such as SELinux or AppArmor profiles for Emacs processes can limit the impact of any successful exploitation by preventing access to sensitive memory regions outside the application's designated scope. Security teams should also monitor for unusual heap activity patterns that might indicate attempted exploitation of this integer overflow condition in legacy systems still running older versions of the editor.