CVE-2025-37749 Overview
CVE-2025-37749 is an out-of-bounds read vulnerability [CWE-125] in the Linux kernel's Point-to-Point Protocol (PPP) subsystem. The flaw resides in the ppp_sync_txmung function, which fails to validate the size of the linear buffer in a socket buffer (skb) before accessing its initial bytes. When the function receives a packet with an empty payload, accessing data[2] reads memory beyond the allocated buffer boundary. The issue affects multiple Linux kernel versions including releases up to 6.15-rc1 and Debian Linux 11.0.
Critical Impact
A local authenticated user can trigger an out-of-bounds read in the PPP synchronous transmit path, leading to memory disclosure or kernel panic resulting in denial of service.
Affected Products
- Linux Kernel (multiple versions through 6.15-rc1)
- Linux Kernel 2.6.12 (including release candidates rc2–rc5)
- Debian Linux 11.0
Discovery Timeline
- 2025-05-01 - CVE-2025-37749 published to NVD
- 2025-11-04 - Last updated in NVD database
Technical Details for CVE-2025-37749
Vulnerability Analysis
The vulnerability exists in the ppp_sync_txmung function within the Linux kernel's PPP synchronous tty driver. This function processes outbound PPP frames before transmission. The code accesses the first few bytes of the skb linear buffer to inspect protocol fields without first verifying that the buffer contains sufficient data.
When a crafted PPP over Ethernet (PPPoE) header arrives with a zero-length payload, the socket buffer contains len = 0x0 and data_len = 0x0. The function then dereferences data[2] to read protocol bytes, but the linear portion of the skb holds no payload data at that offset. This produces an out-of-bounds read past the valid skb data region.
The upstream patch adds explicit bound checking on the skb linear buffer length before accessing protocol bytes, returning early when insufficient data is present.
Root Cause
The root cause is missing input validation on attacker-controlled packet data. The kernel assumed PPP frames reaching ppp_sync_txmung contain at least three bytes of header data. No length check guarded the indexed access into the skb data pointer, violating defensive programming practices for kernel-mode packet processing.
Attack Vector
Exploitation requires local access with privileges sufficient to send PPP frames through a PPP synchronous tty channel. An attacker submits a specially crafted short packet with an empty or truncated payload. The kernel then reads beyond the allocated buffer when processing the frame. The vulnerability does not require user interaction and impacts confidentiality and availability of the host system.
The vulnerability manifests when ppp_sync_txmung accesses data[2] on an skb where len = 0 and tail - data does not cover three bytes. No public proof-of-concept exploit is currently available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. See the kernel commit fix for technical details of the patched code path.
Detection Methods for CVE-2025-37749
Indicators of Compromise
- Unexpected kernel oops or panic messages referencing ppp_sync_txmung in /var/log/kern.log or dmesg output.
- KASAN (Kernel Address Sanitizer) reports flagging out-of-bounds reads in the PPP synchronous tty driver.
- Anomalous PPP interface activity or repeated short-frame transmissions from non-administrative user contexts.
Detection Strategies
- Inventory kernel versions across Linux endpoints and servers to identify hosts running affected kernel builds.
- Monitor kernel ring buffer logs for crash signatures involving ppp_synctty.c or ppp_generic.c symbols.
- Audit which local accounts hold capabilities to open PPP tty devices or load the ppp_synctty module.
Monitoring Recommendations
- Forward kernel logs to a centralized logging or SIEM platform to detect repeated PPP-related faults across the fleet.
- Track loading of the ppp_synctty and ppp_generic kernel modules on hosts where PPP is not operationally required.
- Establish baselines for legitimate PPP usage and alert on deviations such as unprivileged process activity targeting PPP devices.
How to Mitigate CVE-2025-37749
Immediate Actions Required
- Apply the upstream Linux kernel patch from the stable tree commits listed in the vendor advisories.
- Update Debian systems using the fixes shipped in the Debian LTS Announcement (msg00030) and Debian LTS Announcement (msg00045).
- Restrict access to PPP tty devices and limit which users can load PPP-related kernel modules.
Patch Information
The fix has been merged across multiple stable kernel branches. Reference commits include 1f6eb9fa87a7, 529401c8f12e, 6e8a6bf43cea, and additional backports listed in the kernel.org stable tree. The patch adds bound checking on the skb linear buffer before accessing initial bytes in ppp_sync_txmung.
Workarounds
- Blacklist the ppp_synctty and ppp_generic modules on systems that do not require PPP connectivity.
- Remove CAP_NET_ADMIN and related capabilities from unprivileged user accounts to prevent PPP interface manipulation.
- Apply mandatory access control policies (SELinux, AppArmor) restricting which processes may interact with /dev/ppp and PPP tty devices.
# Configuration example
# Blacklist PPP synchronous tty module if not required
echo "blacklist ppp_synctty" | sudo tee /etc/modprobe.d/blacklist-ppp.conf
echo "blacklist ppp_generic" | sudo tee -a /etc/modprobe.d/blacklist-ppp.conf
sudo update-initramfs -u
# Verify current kernel version against patched releases
uname -r
# Check whether the module is currently loaded
lsmod | grep ppp
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
