CVE-2026-18306

Executive Summary

CVE-2026-18306 is a high-severity remote code execution vulnerability in GIMP's SGI file parsing module, disclosed via the Zero Day Initiative (ZDI-CAN-29396). The flaw stems from an integer overflow condition that occurs when handling SGI image files, allowing an attacker to corrupt memory prior to a write operation and subsequently execute arbitrary code in the context of the GIMP process. Exploitation requires user interaction — the target must open a malicious SGI file. The CVSS v3.0 base score is 7.8, reflecting high confidentiality, integrity, and availability impact but low attack complexity. Patching is confirmed across multiple independent sources (NVD, Debian, RHSA), and while no public PoC or in-the-wild exploitation has been documented to date, the vulnerability class is well understood and actively targeted in image-processing applications.

Technical Analysis

The vulnerability resides in GIMP's parsing routines for the SGI image format (historically associated with Silicon Graphics workstations). The NVD description identifies the core defect as an integer overflow that occurs during memory allocation or size calculation prior to a write operation into a buffer. CWE-190 covers this class of weakness, where an arithmetic operation (addition, multiplication, or size multiplication) on user-supplied values produces a numeric result that exceeds the maximum representable value for the integer type, causing wraparound to a small or negative value.

The attack surface is the SGI file parser. When GIMP loads an SGI image, the parser must interpret dimension fields (width, height, number of channels, and possibly subsampling or compression-related parameters) from the file header. If these values are not properly validated, a crafted SGI file can specify dimension combinations whose product overflows the integer type used for buffer-size computation. The resulting undersized allocation then leads to a heap overflow when pixel data is written into the buffer, because the parser proceeds to write the full declared image size into an insufficiently allocated region.

The specific code path and function names are not disclosed in the public description (ZDI typically withholds details until patches are widely deployed). What is clear from the description is that:

• The flaw occurs before the memory write — it is a pre-write integer overflow, not a post-write corruption of a validly sized buffer.
• User-supplied data from the file controls the arithmetic that overflows — meaning an attacker crafts SGI file headers with extreme dimension values.
• The vulnerability is exploitable for code execution in the context of the current process — GIMP typically runs with the privileges of the user who launched it.

GIMP's SGI loader has historically handled planar and interleaved data, RLE compression, and multi-channel (color-mapped) images, making it a complex parser with multiple size-computation sites. The lack of explicit version ranges in the advisory indicates the issue may exist across a wide swath of GIMP versions, with fixes recently committed to the upstream codebase (the OSV package record notes a patch from commit 476407cd1ba693501c435b6200f877e1ff278627 onward).

Exploitation Scenarios

No public exploit code or PoC is available, and the CVE is not listed in CISA's Known Exploited Vulnerabilities catalog. The attack chain below is therefore inferred from the vulnerability class and the NVD description.

Step 1 — Reconnaissance / Target Identification. An attacker identifies a target running GIMP on Windows, Linux, or macOS. Delivery vectors include phishing emails with malicious SGI attachments, drive-by downloads from compromised websites (when GIMP is configured as a handler for .sgi/.rgb files in browsers), or bundling malicious SGI files inside archives, documents, or containers that encourage extraction and opening.

Step 2 — Triggering the Flaw. The attacker crafts an SGI file with a valid magic header (4749 4d41 — "GIMA" or the 4749 4d41 magic for SGI RGB) and controlled dimensional metadata. The width, height, and channel-count fields are set to values where the product of width × height × bytes_per_pixel × channels overflows a 32-bit signed or unsigned integer. Because GIMP fails to validate these products against practical limits (e.g., sanity checks against file size or maximum dimension constraints), the overflow generates a small allocation size. When the parser subsequently reads pixel data from the file, it writes the declared full image dimensions into the undersized buffer, corrupting adjacent heap metadata or application objects.

Step 3 — Payload/Execution Method. At the vulnerability-class level, the heap overflow is used to overwrite a function pointer, a vtable entry, or adjacent heap chunk metadata. Classic techniques include: (a) overwriting GIMP's object headers to redirect a subsequent virtual call, (b) corrupting a linked-list pointer to achieve an arbitrary write primitive, or (c) hijacking the free() function pointer via chunk metadata corruption (house-of-force or similar heap feng shui). With modern exploitation mitigations (ASLR, DEP, and heap hardening), an attacker would likely chain this overflow with a separate information-disclosure bug or use partial pointer overwrites. The payload is delivered as bytes embedded in the SGI pixel data region, which are read by the parser during the overflow write.

The user interaction requirement is satisfied when the victim opens the malicious file via GIMP's File → Open dialog, double-clicks the file in a file manager where GIMP is the registered handler, or drags the file onto the GIMP window.

Step 4 — Post-Exploitation Impact. Code executes with the privileges of the user running GIMP. On enterprise workstations this typically means a standard domain user, enabling lateral movement via credential theft, keylogging, or deploying secondary malware. The CVSS impact subscores (C:H/I:H/A:H — high confidentiality, integrity, and availability impact) confirm full compromise of the victim session with no meaningful sandbox boundary around GIMP.

Impact Assessment

CVSS v3.0: 7.8 (High). Vector CVSS:3.0/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H reflects: local attack vector (file must be parsed locally), low attack complexity (no exotic conditions beyond opening the file), no privileges required, and high impact across all three security objectives. The requirement for user interaction is the sole mitigating factor that keeps this below a 9.x score.

Business Impact. GIMP is widely deployed in graphic design studios, media-production pipelines, scientific imaging workflows, and as a free alternative to commercial editors in enterprise environments. A successful exploit compromises the user's workstation and provides a foothold for lateral movement. The attacker gains access to whatever the designer or analyst can access — source assets, client files, credentials in password managers, email, and internal web applications.

Blast Radius. The vulnerability affects all platforms where GIMP runs (Windows, Linux, macOS). The lack of a sandbox means the impact is bounded only by the user's privilege level. The EPSS score of 0.00339 (26.6th percentile) suggests low near-term exploitation probability relative to other known CVEs, but the ZDI disclosure and the confirmed patch availability will likely drive targeted exploitation as reverse engineers compare the pre- and post-patch binary diffs to reconstruct the vulnerable code path.

Detection Guidance

Sigma Rule

title: GIMP SGI File Parsing Integer Overflow Exploitation Attempt
id: 3f9d55cb-2a1e-4f8a-9b0c-8e3a1d2f6a4b
status: experimental
description: Detects GIMP processes crashing or exhibiting abnormal memory behavior when parsing SGI files, possibly indicating exploitation of CVE-2026-18306.
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-18306
    - https://www.zerodayinitiative.com/advisories/ZDI-CAN-29396
author: Valters IT Security Team
date: 2026/05/15
logsource:
    category: process_creation
    product: windows
detection:
    selection_gimp:
        Image|endswith:
            - '\gimp-2.10.exe'
            - '\gimp-2.99.exe'
            - '\gimp.exe'
    selection_crash:
        CommandLine|contains:
            - '.sgi'
            - '.rgb'
            - '.icon'
    condition: selection_gimp and selection_crash
level: medium
falsepositives:
    - Legitimate opening of SGI images by users/designers
fields:
    - Image
    - CommandLine
    - ParentImage
tags:
    - attack.t1203
    - attack.execution
    - cve.2026.18306

Note: This rule flags suspicious SGI file openings. For stronger detection, pair with crash-dump monitoring (Event ID 1000/1001 in Windows Application log) showing access violations in gimp.exe immediately after opening a file.

YARA Rule

rule CVE_2026_18306_SGI_Malicious_Header
{
    meta:
        author = "Valters IT Security Team"
        date = "2026-05-15"
        description = "Detects SGI files with header dimensions that could trigger integer overflow in GIMP parser (CVE-2026-18306)"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-18306"
        cve = "CVE-2026-18306"
        severity = "high"
    strings:
        // SGI file magic - "GIMA" at offset 0 (or 0x01DA for big-endian variant)
        $sgi_magic_le = { 47 49 4D 41 }  // GIMA little-endian
        $sgi_magic_be = { 01 DA 00 00 }  // Big-endian start marker

        // Known GIMP-specific SGI storage signatures
        $gimp_note = "GIMP" ascii wide
        $sgi_compression_rle = { 01 00 }  // RLE compression flag
    condition:
        uint32(0) == 0x47494D41 or uint32(0) == 0x01DA0000 and
        // Check width or height field for absurd values (offset 8 = width, offset 10 = height in LE SGI)
        (
            uint16(8) > 32768 or
            uint16(10) > 32768 or
            (uint16(8) > 0 and uint16(10) > 0 and uint16(8) * uint16(10) > 0x7FFFFFFF)
        ) and
        filesize < 10485760  // Suspicious: small file with huge declared dimensions
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-18306 GIMP SGI File Malicious Header Download Attempt"; flow:established,to_server; content:".sgi"; http.uri; nocase; content:"GET"; http.method; pcre:"/\.(sgi|rgb|icon)$/Ui"; classtype:attempted-user; sid:2026183061; rev:1; metadata:cve CVE-2026-18306;)

alert smtp any any -> any any (msg:"CVE-2026-18306 GIMP SGI Malicious Attachment"; flow:established,to_client; content:"filename="; nocase; content:".sgi"; nocase; pcre:"/filename=\"[^\"]*\.(sgi|rgb)/i"; classtype:attempted-user; sid:2026183062; rev:1; metadata:cve CVE-2026-18306;)

# Network-level file inspection of SGI magic in downloads
alert http any any -> any any (msg:"CVE-2026-18306 Suspicious SGI File Content in HTTP Response"; flow:established,from_server; file_data; content:"GIMA"; offset:0; depth:4; classtype:attempted-user; sid:2026183063; rev:1; metadata:cve CVE-2026-18306;)

Note: Since exploitation is local (user opens file), network rules primarily detect delivery (SGI files over HTTP, SMTP, SMB). Detection of the actual memory corruption requires host-level monitoring.

Elastic Detection

// Detect GIMP process terminating abnormally shortly after opening a file
sequence by process.entity_id with maxspan=2m
  [process where process.name : "gimp*.exe" and
     process.args : ("*.sgi", "*.rgb")]
  [process where process.name : "gimp*.exe" and
     event.code : ("3", "5") or
     (process.name : "gimp*.exe" and process.exit_code : (0xC0000005, 0xC0000409))]

// Broader: any GIMP crash in the Windows Application log
sequence with maxspan=10m
  [process where process.name : "gimp*.exe"]
  [file where process.name : "gimp*.exe" and file.name : "*.dmp" and file.size > 100000]

// Detect creation of suspiciously small SGI files matching overflow profile on disk
file where file.name : "*.sgi" and file.size < 100000
  and (
    // Detect by parsing header via ingest pipeline enrichment (pseudo-extension point)
    file.Ext.header.width > 32768 or
    file.Ext.header.height > 32768
  )

Splunk SPL Query

# Detect GIMP crashes following SGI file access (Windows)
index=windows EventCode=1000 OR EventCode=1001
| search "gimp" AND ("0xc0000005" OR "0xc0000409" OR "faulting module")
| rex field=_raw "Faulting application name: (?<app>.*)"
| where app LIKE "%gimp%"
| stats count min(_time) as first_seen max(_time) as last_seen by host, app
| where last_seen <= relative_time(now(), "-1h") OR count > 1

# Detect SGI files opened by GIMP (Process command line)
index=windows sourcetype=WinEventLog:Security EventCode=4688
| where New_Process_Name like "%gimp%" AND Process_Command_Line like "%.sgi"
| stats count by host, user, Process_Command_Line

# Detect Sysmon process creation for GIMP with SGI args
index=windows sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=1
| search Image="*gimp*.exe" AND (CommandLine="*.sgi*" OR CommandLine="*.rgb*")
| stats count by Computer, User, Image, CommandLine
| sort - count

Wazuh OSSEC Rule

<rule id="886745" level="12">
  <if_sid>100002</if_sid>
  <match>gimp</match>
  <regex>faulting application|access violation|unhandled exception</regex>
  <description>GIMP process crashed - possible CVE-2026-18306 SGI file integer overflow exploitation attempt</description>
  <group>gimp,cve_2026_18306,attack_execution,</group>
  <mitre>
    <id>T1203</id>
  </mitre>
  <var name="alert">GIMP crash detected; investigate recent SGI file access</var>
</rule>

<rule id="886746" level="8">
  <if_sid>550</if_sid>
  <field name="win.eventdata.image">\.*gimp.*\.exe</field>
  <field name="win.eventdata.commandLine">\.sgi|\.rgb</field>
  <description>GIMP process opened SGI file - monitor for subsequent crash per CVE-2026-18306</description>
  <group>gimp,cve_2026_18306,attack_execution,</group>
  <mitre>
    <id>T1203</id>
  </mitre>
</rule>

<rule id="886747" level="10" frequency="5" timeframe="300">
  <if_matched_sid>2026184</if_matched_sid>
  <description>Multiple SGI files opened in short window - possible targeted attack/CVE-2026-18306 preparation</description>
  <group>gimp,cve_2026_18306,attack_execution,</group>
</rule>

Mitigation & Patch Guidance

Patch Confidence: 4/4 — CONFIRMED. Multiple independent sources (NVD, Debian security tracker, Red Hat Security Advisories) confirm a fix exists. The OSV record indicates upstream commit 476407cd1ba693501c435b6200f877e1ff278627 introduces the fix.

1) Immediate Mitigations:

• Do not open SGI, RGB, or ICON files from untrusted or unexpected sources.
• Configure email gateways to block .sgi, .rgb, and .icon attachments.
• Restrict browser auto-launching of GIMP for file types (disable "open with GIMP" default handlers in browsers and file managers).
• Consider blocking GIMP from executing in sandboxed/restricted environments for high-risk user populations until patched.

2) Patch/Upgrade Steps:

• GIMP 2.10.x (stable): Upgrade to the latest 2.10.x release that includes the SGI parsing fix (≥ commit 476407cd). Check your Linux distribution package feed or Windows/macOS installer from gimp.org.
• Debian/Ubuntu: apt update && apt upgrade gimp — refer to DSA advisories for your specific release (bookworm, trixie).
• RHEL/CentOS/Fedora: dnf update gimp or yum update gimp per RHSA advisory.
• Windows users: Download the updated installer from the official GIMP website or your software-management tool.
• macOS (Homebrew): brew upgrade gimp

3) Long-Term Hardening:

• Where feasible, run GIMP inside a sandbox (e.g., Firejail on Linux, AppContainer via third-party wrappers on Windows) to limit the blast radius of file-parsing vulnerabilities.
• Subscribe to the GIMP security mailing list for pre-release notifications of security fixes.
• Validate that your enterprise software inventory (e.g., via SCCM, Intune, or Ansible) tracks GIMP versions so you can respond quickly to future advisories.
• Educate design/media teams on phishing risks involving image attachments — file-format bugs in GIMP are a recurring theme (multiple ZDI advisories over the years), not a one-off issue.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-18306
2. ZDI Advisory (ZDI-CAN-29396): https://www.zerodayinitiative.com/advisories/ (refer to ZDI-CAN-29396)
3. OSV Record: https://osv.dev/vulnerability/ (commit 476407cd1ba693501c435b6200f877e1ff278627)
4. Debian Security Tracker: https://security-tracker.debian.org/tracker/CVE-2026-18306
5. Red Hat Security Advisory (RHSA): https://access.redhat.com/security/cve/CVE-2026-18306

Threat Actor Activity / Exploit-in-the-Wild: No known exploitation campaigns, CISA KEV listing, or public exploit code exists as of the document date. The EPSS score (0.00339, 26.6th percentile) indicates limited active exploitation pressure, but the ZDI public disclosure typically triggers reverse-engineering activity within weeks. Security teams should prioritize patching within their standard SLAs for High-severity client-side vulnerabilities.