CVE-2021-28846 in TEW-755AP
Summary
by MITRE • 08/11/2021
A Format String vulnerablity exists in TRENDnet TEW-755AP 1.11B03, TEW-755AP2KAC 1.11B03, TEW-821DAP2KAC 1.11B03, and TEW-825DAP 1.11B03, which could let a remote malicious user cause a denial of service due to a logic bug at address 0x40dcd0 when calling fprintf with "%s: key len = %d, too long\n" format. The two variables seem to be put in the wrong order. The vulnerability could be triggered by sending the POST request to apply_cgi with a long and unknown key in the request body.
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Analysis
by VulDB Data Team • 08/15/2021
The CVE-2021-28846 vulnerability represents a classic format string vulnerability affecting multiple TRENDnet wireless access point models including TEW-755AP, TEW-755AP2KAC, TEW-821DAP2KAC, and TEW-825DAP versions 1.11B03. This type of vulnerability falls under CWE-134 which specifically addresses the use of untrusted data in format string operations. The flaw manifests in the firmware's handling of user-supplied input within the apply_cgi endpoint, where the application processes POST requests containing configuration parameters without proper validation or sanitization of the input data.
The technical implementation of this vulnerability occurs at memory address 0x40dcd0 where the fprintf function is called with a format string containing "%s: key len = %d, too long" but the variables are passed in incorrect order. This misordering creates a situation where the application attempts to read from memory locations that do not correspond to the intended parameters, leading to unpredictable behavior. The vulnerability is triggered when a malicious user sends a POST request to the apply_cgi endpoint containing an excessively long unknown key in the request body, causing the application to process the malformed input through the vulnerable printf-like function call.
From an operational perspective, this vulnerability creates a significant risk of denial of service attacks that can completely disrupt network access for end users. The remote exploitation capability means attackers do not need physical access to the devices, making this a particularly dangerous vulnerability in enterprise environments. The impact extends beyond simple service interruption as the improper variable ordering can potentially lead to information disclosure or even arbitrary code execution depending on the specific memory layout and exploitation conditions. Network administrators face the challenge of securing devices that may be deployed in remote locations or within environments where physical access is limited, making this vulnerability particularly concerning for large-scale deployments.
The exploitation of this vulnerability aligns with techniques described in the MITRE ATT&CK framework under the T1211 technique for exploitation of remote services and T1499 for network disruption. Organizations should implement immediate mitigations including firmware updates from TRENDnet, network segmentation to limit access to these devices, and monitoring for unusual POST requests to the apply_cgi endpoint. Additionally, the vulnerability highlights the importance of input validation and proper parameter handling in embedded systems, with recommendations to implement strict bounds checking and parameter validation for all user-supplied inputs. The issue demonstrates how seemingly minor coding errors in embedded firmware can create significant security risks, emphasizing the need for comprehensive security testing of network infrastructure devices.