CVE-2026-77658 in Dia
Summary
by MITRE • 08/26/2026
A stack-based buffer overflow vulnerability exists in the Dia diagram editor when processing Network Bus objects from Dia XML project files.
In objects/network/bus.c, bus_load() reads the number of bus handles from the file attribute "bus_handles" using attribute_num_data() without validating an upper bound:
bus->num_handles = attribute_num_data(attr);
When a bus handle is subsequently moved, bus_handle_moved() allocates two temporary arrays on the stack:
parallel = (real *)g_alloca(num_handles * sizeof(real)); perp = (real *)g_alloca(num_handles * sizeof(real));
Because num_handles is fully attacker-controlled via the project file, sufficiently large values (for example 262144 or higher) cause g_alloca() to consume more stack space than the default thread stack limit (typically 8 MB on Linux), resulting in stack overflow, SIGSEGV, and potential stack frame / return-address corruption.
An attacker can embed a Bus object with an excessive bus_handles count in a malicious .dia file. Exploitation requires the victim to open the file in Dia (file dialog, command line, or file association) and trigger handle manipulation (moving a bus handle), which exercises the vulnerable code path.
The identical g_alloca pattern is present in objects/Misc/tree.c (copied from bus.c) and is likely vulnerable to the same class of attack via Tree objects.
Affected versions: Dia 0.98.0 and earlier versions containing this code; issue confirmed on upstream master as of 2026-08-21. Upstream report: https://gitlab.gnome.org/GNOME/dia/-/issues/581
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Analysis
by VulDB Data Team • 08/26/2026
The Dia diagram editor contains a critical stack-based buffer overflow vulnerability within its XML project file parsing logic, specifically affecting the handling of Network Bus objects. This flaw originates in the bus_load function located in the source file objects/network/bus.c, where the application reads the number of bus handles from an attribute named "bus_handles" using the attribute_num_data routine. The core technical deficiency lies in the complete absence of input validation for this numeric value; the software fails to enforce any upper bound on the quantity of handles specified by the attacker-controlled file content. This lack of sanitization allows a malicious actor to inject arbitrarily large integer values into the project structure, which are subsequently used as sizing parameters for memory allocation operations without regard for system resource limits or safe thresholds.
The operational impact manifests when a user interacts with the loaded diagram, specifically during the manipulation of bus handles such as moving them across the canvas. This action triggers the bus_handle_moved function, which attempts to allocate two temporary arrays on the stack using g_alloca based on the previously unsanitized num_handles value. The calculation involves multiplying this untrusted count by the size of a real number type. When an attacker provides a sufficiently large value, such as 262144 or higher, the resulting memory allocation request exceeds the default thread stack limit, which is typically capped at eight megabytes on Linux systems and similar operating environments. This excessive consumption leads directly to a stack overflow condition, causing immediate application termination via SIGSEGV signals due to segmentation faults. Beyond simple denial of service, this corruption poses severe security risks as it can overwrite adjacent stack frames and potentially corrupt return addresses, creating opportunities for arbitrary code execution if the attacker can precisely control the overwritten memory regions.
The vulnerability is classified under CWE-121, which denotes a stack-based buffer overflow resulting from unbounded copy operations or allocations based on external input. In terms of attack vectors, this aligns with ATT&CK techniques involving file manipulation and local exploitation through trusted application execution. The threat model requires the victim to open a malicious .dia file via standard means such as a file dialog, command line invocation, or automatic association handlers. Once opened, the vulnerability is not immediately triggered but remains latent until specific user interaction occurs, namely moving a bus handle within the diagram interface. This two-stage nature allows the payload to persist in the file structure without immediate detection by static analysis tools that do not simulate runtime execution paths involving object manipulation.
Furthermore, this architectural flaw is not isolated to the Network Bus module alone. A code review reveals an identical g_alloca pattern present in objects/Misc/tree.c, which appears to have been copied from the bus implementation. This indicates a systemic design issue where Tree objects are likely vulnerable to the same class of stack-based buffer overflow attacks via excessive handle counts embedded within their respective XML structures. The persistence of this vulnerability into upstream master branches as of August 2026 suggests that remediation efforts may be delayed or overlooked in favor of other development priorities, leaving users of Dia version 0.98.0 and earlier versions exposed to these risks indefinitely unless manual patches are applied.
Mitigation strategies must address both immediate user safety and long-term code integrity. Users should exercise extreme caution when opening diagram files from untrusted sources, ideally utilizing sandboxed environments or virtual machines to isolate potential exploitation attempts. For administrators and developers, the primary remediation involves implementing strict input validation within attribute_num_data calls or immediately after retrieval of numeric attributes like bus_handles. This includes enforcing reasonable upper bounds consistent with typical system stack sizes and application memory constraints before passing values to allocation functions such as g_alloca. Additionally, replacing stack-based allocations with heap-allocated buffers using g_malloc can significantly reduce the risk of stack corruption and simplify boundary checking procedures. Code audits should be extended to all modules utilizing similar dynamic array patterns on the stack to ensure comprehensive coverage against this class of memory safety violations.