CVE-2026-12220 Overview
CVE-2026-12220 is a stack-based buffer overflow in the Yealink SIP-T46U IP phone running firmware version 108.86.0.118. The flaw resides in the mod_upgrade.SparePartsUpload function exposed through the /api/upgrade/accupgradebychunk endpoint of the Firmware Chunk Upload handler. Attackers manipulate the uid argument to corrupt the stack and influence program execution. The issue is classified under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer). Exploitation requires adjacent network access and low-level privileges. The exploit details have been published, and the vendor did not respond to disclosure attempts.
Critical Impact
An adjacent-network attacker with low privileges can corrupt stack memory on the device, potentially leading to denial of service or arbitrary code execution on the affected phone.
Affected Products
- Yealink SIP-T46U IP phone
- Firmware version 108.86.0.118
- Firmware Chunk Upload handler component (mod_upgrade.SparePartsUpload)
Discovery Timeline
- 2026-06-15 - CVE-2026-12220 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2026-12220
Vulnerability Analysis
The vulnerability is a stack-based buffer overflow in the firmware upgrade subsystem of the Yealink SIP-T46U. The affected code path handles chunked firmware uploads through the HTTP endpoint /api/upgrade/accupgradebychunk. The SparePartsUpload handler within the mod_upgrade module processes the uid parameter without enforcing length boundaries before copying it into a fixed-size stack buffer.
Writing beyond the buffer overwrites saved registers, return addresses, and adjacent stack data. Depending on the device's memory protections, exploitation can produce service crashes or hijacked control flow. Because the SIP-T46U is a desk phone deployed in enterprise voice environments, a compromised device can be used to pivot, capture call audio, or persist on the internal LAN.
Root Cause
The root cause is missing input validation on the uid argument before it is copied into a stack-allocated buffer. The handler does not check the parameter length against the destination buffer size, and no stack canaries or length-aware copy primitives intercept the overflow. This pattern is consistent with [CWE-119] memory boundary violations common in embedded HTTP services written in C.
Attack Vector
Exploitation requires the attacker to reach the device's management HTTP interface from the adjacent network, typically the same Layer 2 segment as the phone. The attacker must also possess valid low-privilege credentials to invoke the upgrade endpoint. The attacker then sends a crafted POST request to /api/upgrade/accupgradebychunk with an oversized uid value, triggering the overflow inside mod_upgrade.SparePartsUpload.
A public exploit archive has been distributed referencing the T46U_mod_upgrade_SparePartsUpload_stack_overflow payload. No verified code example is reproduced here; refer to the VulDB CVE-2026-12220 record and the VulDB Vulnerability #370863 entry for technical artifacts.
Detection Methods for CVE-2026-12220
Indicators of Compromise
- HTTP POST requests to /api/upgrade/accupgradebychunk containing unusually long uid parameter values.
- Unexpected reboots, crashes, or service restarts on Yealink SIP-T46U devices following management traffic.
- Authentication attempts to the phone's web interface from non-administrative hosts on the same VLAN.
- Outbound connections from the phone to attacker infrastructure following an upload request.
Detection Strategies
- Inspect HTTP request bodies destined for the phone's management API and alert on uid field lengths exceeding expected bounds.
- Correlate device crash or watchdog reset events with preceding HTTP traffic to the upgrade endpoint.
- Monitor for anomalous SIP phone behavior such as new listening ports, unexpected DNS queries, or lateral SMB/SSH activity.
Monitoring Recommendations
- Centralize Yealink phone syslog output and alert on firmware-related error messages or repeated upgrade attempts.
- Capture NetFlow data on VLANs hosting IP phones and flag administrative access from non-management subnets.
- Track firmware version inventory across the fleet to identify devices still running 108.86.0.118.
How to Mitigate CVE-2026-12220
Immediate Actions Required
- Restrict access to the phone management interface to a dedicated administration VLAN using ACLs or 802.1X segmentation.
- Rotate and strengthen credentials on all SIP-T46U devices to limit access to the upgrade endpoint.
- Disable remote firmware upload functionality where operationally feasible until a patch is available.
- Audit existing devices running firmware 108.86.0.118 and prioritize them for compensating controls.
Patch Information
No vendor patch is referenced in the published CVE data. The vendor was contacted prior to disclosure but did not respond. Monitor the VulDB CVE-2026-12220 record and Yealink security advisories for updated firmware availability.
Workarounds
- Place IP phones on an isolated voice VLAN unreachable from user workstations and guest networks.
- Block inbound HTTP and HTTPS to phone management ports from all sources except authorized provisioning servers.
- Disable the web management interface on phones that are centrally provisioned and do not require local administration.
- Enforce strong, unique passwords on every phone and remove default or shared accounts.
# Example switch ACL restricting management access to provisioning server only
ip access-list extended VOICE-MGMT
permit tcp host 10.10.10.5 10.20.0.0 0.0.255.255 eq 443
deny tcp any 10.20.0.0 0.0.255.255 eq 443
deny tcp any 10.20.0.0 0.0.255.255 eq 80
permit ip any any
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
