CVE-2026-18246 Overview
CVE-2026-18246 affects IBM i versions 7.6, 7.5, 7.4, and 7.3. The vulnerability stems from an interpretation conflict in the multipart parser, classified under [CWE-436] Interpretation Conflict. A remote authenticated attacker with high privileges can bypass security restrictions by exploiting differences in how the multipart parser and downstream components interpret the same input.
The flaw permits limited integrity impact without affecting confidentiality or availability. Exploitation requires network access, high attack complexity, and existing high-privilege authentication. IBM published an advisory documenting the affected releases and remediation guidance.
Critical Impact
Authenticated attackers can bypass security restrictions on IBM i systems by exploiting parser interpretation differences in multipart-encoded HTTP requests.
Affected Products
- IBM i 7.6
- IBM i 7.5
- IBM i 7.4
- IBM i 7.3
Discovery Timeline
- 2026-08-12 - CVE CVE-2026-18246 published to NVD
- 2026-08-12 - Last updated in NVD database
Technical Details for CVE-2026-18246
Vulnerability Analysis
The vulnerability resides in the multipart parser used by IBM i. Multipart parsers process HTTP requests that carry multipart/form-data payloads, splitting them into discrete parts based on boundary markers and headers. When two components interpret the same input differently, an attacker can craft requests that appear benign to one layer but carry a different meaning to another.
An authenticated attacker sends a specially crafted multipart request. Security controls parsing the request perceive one structure, while the processing component perceives another. This mismatch enables the attacker to bypass restrictions the security layer intended to enforce.
The scope change reflected in the impact metrics indicates the flaw affects resources beyond the vulnerable component. Integrity is affected because the bypass allows the attacker to submit content that would otherwise be rejected.
Root Cause
The root cause is an interpretation conflict [CWE-436] between the multipart parser and adjacent components handling the same request. Differences in boundary handling, header parsing, or part delimitation cause the two layers to derive divergent representations of the input.
Attack Vector
Exploitation requires network reachability to the IBM i HTTP interface and valid credentials with high privileges. The attacker submits a multipart request engineered to trigger the parsing divergence. High attack complexity indicates the attacker must control specific request conditions for reliable exploitation. Refer to the IBM Support Page Advisory for technical specifics.
Detection Methods for CVE-2026-18246
Indicators of Compromise
- Multipart HTTP requests with malformed or ambiguous boundary declarations reaching IBM i endpoints
- Requests containing duplicate Content-Type or Content-Disposition headers within a single part
- Successful actions by authenticated users that should have been blocked by application-layer restrictions
Detection Strategies
- Inspect HTTP request logs for multipart payloads containing nested boundaries, unusual whitespace, or non-standard header ordering
- Correlate authenticated user actions with expected authorization outcomes to identify bypass attempts
- Enable verbose logging on IBM i HTTP servers to capture full request headers and body structure for later analysis
Monitoring Recommendations
- Forward IBM i HTTP and application audit logs to a centralized SIEM for continuous review
- Alert on repeated 4xx responses followed by successful 2xx responses from the same authenticated principal against the same endpoint
- Track privileged account activity against IBM i web-facing services for anomalous request patterns
How to Mitigate CVE-2026-18246
Immediate Actions Required
- Apply the IBM-provided fix documented in the IBM Support Page Advisory for IBM i 7.6, 7.5, 7.4, and 7.3
- Audit accounts with high privileges on IBM i systems and revoke unnecessary access
- Restrict network access to IBM i HTTP services to trusted management networks
Patch Information
IBM has published remediation guidance covering IBM i 7.6, 7.5, 7.4, and 7.3. Administrators should consult the IBM Support Page Advisory for the specific PTF (Program Temporary Fix) identifiers applicable to each release and install them following standard IBM i patch procedures.
Workarounds
- Place a web application firewall in front of IBM i HTTP services and enforce strict multipart request validation
- Reject requests with malformed multipart boundaries or duplicated part headers at the network edge
- Limit which authenticated users can submit multipart requests to affected endpoints until patches are deployed
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
