VU#536588: Multiple Heap Buffer Overflows in Orthanc DICOM Server

Overview

Multiple vulnerabilities have been identified in Orthanc DICOM Server version, 1.12.10 and earlier, that affect image decoding and HTTP request handling components. These vulnerabilities include heap buffer overflows, out-of-bounds reads, and resource exhaustion vulnerabilities that may allow attackers to crash the server, leak memory contents, or potentially execute arbitrary code.

Description

Orthanc is an open-source lightweight Digital Imaging and Communications in Medicine (DICOM) server used to store, process, and retrieve medical imaging data in healthcare environments. The following nine vulnerabilities identified in Orthanc primarily stem from unsafe arithmetic operations, missing bounds checks, and insufficient validation of attacker-controlled metadata in DICOM files and HTTP requests.

CVE-2026-5437 An out-of-bounds read vulnerability exists in DicomStreamReader during DICOM meta-header parsing. When processing malformed metadata structures, the parser may read beyond the bounds of the allocated metadata buffer. Although this issue does not typically crash the server or expose data directly to the attacker, it reflects insufficient input validation in the parsing logic.

CVE-2026-5438 A gzip decompression bomb vulnerability exists when Orthanc processes an HTTP request with Content-Encoding: gzip. The server does not enforce limits on decompressed size and allocates memory based on attacker-controlled compression metadata. A specially crafted gzip payload can trigger excessive memory allocation and exhaust system memory.

CVE-2026-5439 A memory exhaustion vulnerability exists in ZIP archive processing. Orthanc automatically extracts ZIP archives uploaded to certain endpoints and trusts metadata fields describing the uncompressed size of archived files. An attacker can craft a small ZIP archive containing a forged size value, causing the server to allocate extremely large buffers during extraction.

CVE-2026-5440 A memory exhaustion vulnerability exists in the HTTP server due to unbounded use of the Content-Length header. The server allocates memory directly based on the attacker-supplied header value without enforcing an upper limit. A crafted HTTP request containing an extremely large Content-Length value, such as approximately 4 GB, can trigger excessive memory allocation and server termination, even without sending a request body.

CVE-2026-5441 An out-of-bounds read vulnerability exists in the DecodePsmctRle1 function of DicomImageDecoder.cpp. The PMSCT_RLE1 decompression routine, which decodes the proprietary Philips Compression format, does not properly validate escape markers placed near the end of the compressed data stream. A crafted sequence at the end of the buffer can cause the decoder to read beyond the allocated memory region and leak heap data into the rendered image output.

CVE-2026-5442 A heap buffer overflow vulnerability exists in the DICOM image decoder. Dimension fields are encoded using Value Representation (VR) Unsigned Long (UL), instead of the expected VR Unsigned Short (US), which allows extremely large dimensions to be processed. This causes an integer overflow during frame size calculation and results in out-of-bounds memory access during image decoding.

CVE-2026-5443 A heap buffer overflow vulnerability exists during the decoding of PALETTE COLOR DICOM images. Pixel length validation uses 32-bit multiplication for width and height calculations. If these values overflow, the validation check incorrectly succeeds, allowing the decoder to read and write to memory beyond allocated buffers.

CVE-2026-5444 A heap buffer overflow vulnerability exists in the PAM ( https://netpbm.sourceforge.net/doc/pam.html) image parsing logic. When Orthanc processes a crafted PAM image embedded in a DICOM file, image dimensions are multiplied using 32-bit unsigned arithmetic. Specially chosen values can cause an integer overflow during buffer size calculation, resulting in the allocation of a small buffer followed by a much larger write operation during pixel processing.

CVE-2026-5445 An out-of-bounds read vulnerability exists in the DecodeLookupTable function within DicomImageDecoder.cpp. The lookup-table decoding logic used for PALETTE COLOR images does not validate pixel indices against the lookup table size. Crafted images containing indices larger than the palette size cause the decoder to read beyond allocated lookup table memory and expose heap contents in the output image.

Impact

The vulnerabilities in Orthan DICOM Server 1.20.10 allow attackers to trigger heap memory corruption, out-of-bounds read, information disclosure, and denial-of-service conditions through crafted DICOM files and HTTP requests. The most severe issues are heap-based buffer overflows in image parsing and decoding logic, which can crash the Orthanc process and may, under certain conditions, provide a pathway to remote code execution (RCE). Several additional flaws permit out-of-bounds reads that can expose heap-resident data, including allocator metadata, internal identifiers, points, and portions of adjacent DICOM content through rendered image output.
In addition, multiple vulnerabilities allow resource exhaustion by causing Orthanc to allocate excessive amounts of memory based on attacker-controlled metadata such as Content-Length, ZIP archive size fields, and gzip decompression size values. These conditions can reliably result in process termination and denial of service, often with only a small, crafted payload. Some of the affected code paths may also allow malicious DICOM content to be stored and later re-triggered during normal processing, increasing the persistence and operational impact of exploitation.

Solution

Orthanc has released version 1.12.11 to address these vulnerabilities, and users are strongly encouraged to upgrade as soon as possible. Administrators should review deployment configurations to limit exposure of upload and image processing functionality to trusted users and networks wherever possible. Refer to Orthanc documentation and release notes for patching and deployment guidance.

Acknowledgements

Thanks to Dr. Simon Weber and Volker Schönefeld of Machine Spirits UG (https://machinespirits.com) for the disclosure of these vulnerabilities. This document was written by Michael Bragg.