6.4
MEDIUM CVSS 3.1
CVE-2026-12520
Stack buffer overflow and off-by-one writes in Zephyr HL7800 modem AT response handlers
Description

The Sierra Wireless HL7800 cellular modem driver (drivers/modem/vendor_standalone/hl7800.c, located at drivers/modem/hl7800.c in v4.4.0 and earlier) parses AT responses with roughly twenty handlers that call net_buf_linearize(value, sizeof(value), *buf, 0, len) into a 128-byte stack buffer and then write value[out_len] = 0. Because net_buf_linearize() (lib/net_buf/buf.c) can return a count equal to its destination-length argument, a field that exactly fills the buffer makes the terminating NUL land one byte past the end, a single-byte out-of-bounds write into adjacent stack memory. The +KCELLMEAS cell-measurement handler on_cmd_atcmdinfo_rssi() is worse: it passed the wire length len as the destination size (net_buf_linearize(value, len, *buf, 0, len)), so a response line longer than 128 bytes overflows the value stack buffer with attacker-influenceable content. The line length comes from net_buf_findcrlf(), which accumulates bytes across the whole net_buf fragment chain and is not bounded to 128, so an over-long line reaches the defect. The data originates from the cellular modem over UART, driven by the network: operator-scan results, +CGCONTRDP IP/DNS info, socket indications, and +KCELLMEAS neighbour-cell reports. An attacker able to shape what the modem emits — a rogue base station, a compromised modem baseband, or a remote peer feeding oversized response framing — can drive a line past 128 bytes. The handlers run in the driver's RX thread in kernel context, so the corruption is kernel-side. The +KCELLMEAS path is a full stack buffer overflow whose worst case is code execution in kernel context and whose floor is a reliable crash; the remaining sites are single-byte NUL out-of-bounds writes. Exploitation requires the modem to emit an over-long AT response line, giving high attack complexity over an adjacent (cellular radio) vector. The fix passes sizeof(dst) - 1 (and correct explicit bounds for the IMSI and +KCELLMEAS sites) so the terminator always stays in bounds.

INFO

Published Date :

Aug. 18, 2026, 8:17 p.m.

Last Modified :

Aug. 26, 2026, 4:59 p.m.

Remotely Exploit :

No
Affected Products

The following products are affected by CVE-2026-12520 vulnerability. Even if cvefeed.io is aware of the exact versions of the products that are affected, the information is not represented in the table below.

ID Vendor Product Action
1 Zephyrproject zephyr
2 Zephyrproject zephyr
CVSS Scores
The Common Vulnerability Scoring System is a standardized framework for assessing the severity of vulnerabilities in software and systems. We collect and displays CVSS scores from various sources for each CVE.
Score Version Severity Vector Exploitability Score Impact Score Source
CVSS 134c704f-9b21-4f2e-91b3-4a467353bcc0
CVSS 3.1 MEDIUM e2e69745-5e70-4e92-8431-deb5529a81ad
CVSS 3.1 MEDIUM [email protected]
CVSS 3.1 MEDIUM MITRE-CVE
Solution
Fix buffer overflows in AT response handlers by ensuring null termination stays within bounds.
  • Apply vendor patches to the Sierra Wireless modem driver.
  • Ensure AT response handlers respect buffer size limits.
  • Validate input lengths before writing to buffers.
  • Update driver code to prevent out-of-bounds writes.
References to Advisories, Solutions, and Tools

Here, you will find a curated list of external links that provide in-depth information, practical solutions, and valuable tools related to CVE-2026-12520.

URL Resource
https://github.com/zephyrproject-rtos/zephyr/commit/ea91f9375677aa4268e4044e096902dbe789f101
https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-9xc4-j5x8-v6jx
CWE - Common Weakness Enumeration

While CVE identifies specific instances of vulnerabilities, CWE categorizes the common flaws or weaknesses that can lead to vulnerabilities. CVE-2026-12520 is associated with the following CWEs:

Common Attack Pattern Enumeration and Classification (CAPEC)

Common Attack Pattern Enumeration and Classification (CAPEC) stores attack patterns, which are descriptions of the common attributes and approaches employed by adversaries to exploit the CVE-2026-12520 weaknesses.

We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).

Results are limited to the first 15 repositories due to potential performance issues.

The following list is the news that have been mention CVE-2026-12520 vulnerability anywhere in the article.

The following table lists the changes that have been made to the CVE-2026-12520 vulnerability over time.

Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.

  • CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0

    Aug. 19, 2026

    Action Type Old Value New Value
    Added SSVC {'id': 'CVE-2026-12520', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-08-19T13:08:28.141064Z'}
  • New CVE Received by [email protected]

    Aug. 18, 2026

    Action Type Old Value New Value
    Added Affected [{'vendor': 'zephyrproject', 'product': 'zephyr', 'versions': [{'status': 'affected', 'version': '2.4.0', 'lessThan': '4.4.2', 'versionType': 'semver'}], 'packageName': 'zephyr', 'programFiles': ['drivers/modem/vendor_standalone/hl7800.c'], 'collectionURL': 'https://github.com/zephyrproject-rtos/zephyr', 'defaultStatus': 'unaffected'}]
    Added Description The Sierra Wireless HL7800 cellular modem driver (drivers/modem/vendor_standalone/hl7800.c, located at drivers/modem/hl7800.c in v4.4.0 and earlier) parses AT responses with roughly twenty handlers that call net_buf_linearize(value, sizeof(value), *buf, 0, len) into a 128-byte stack buffer and then write value[out_len] = 0. Because net_buf_linearize() (lib/net_buf/buf.c) can return a count equal to its destination-length argument, a field that exactly fills the buffer makes the terminating NUL land one byte past the end, a single-byte out-of-bounds write into adjacent stack memory. The +KCELLMEAS cell-measurement handler on_cmd_atcmdinfo_rssi() is worse: it passed the wire length len as the destination size (net_buf_linearize(value, len, *buf, 0, len)), so a response line longer than 128 bytes overflows the value stack buffer with attacker-influenceable content. The line length comes from net_buf_findcrlf(), which accumulates bytes across the whole net_buf fragment chain and is not bounded to 128, so an over-long line reaches the defect. The data originates from the cellular modem over UART, driven by the network: operator-scan results, +CGCONTRDP IP/DNS info, socket indications, and +KCELLMEAS neighbour-cell reports. An attacker able to shape what the modem emits — a rogue base station, a compromised modem baseband, or a remote peer feeding oversized response framing — can drive a line past 128 bytes. The handlers run in the driver's RX thread in kernel context, so the corruption is kernel-side. The +KCELLMEAS path is a full stack buffer overflow whose worst case is code execution in kernel context and whose floor is a reliable crash; the remaining sites are single-byte NUL out-of-bounds writes. Exploitation requires the modem to emit an over-long AT response line, giving high attack complexity over an adjacent (cellular radio) vector. The fix passes sizeof(dst) - 1 (and correct explicit bounds for the IMSI and +KCELLMEAS sites) so the terminator always stays in bounds.
    Added CVSS V3.1 AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
    Added CWE CWE-787
    Added Reference https://github.com/zephyrproject-rtos/zephyr/commit/ea91f9375677aa4268e4044e096902dbe789f101
    Added Reference https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-9xc4-j5x8-v6jx
EPSS is a daily estimate of the probability of exploitation activity being observed over the next 30 days. Following chart shows the EPSS score history of the vulnerability.