CVE-2026-68278 in Linux
Summary
by MITRE • 08/10/2026
In the Linux kernel, the following vulnerability has been resolved:
drm/dp/mst: fix buffer overflows in sideband chunk accumulation
drm_dp_sideband_append_payload() has three related bugs when processing device-provided sideband reply data:
1. Zero-length curchunk_len underflow: msg_len is a 6-bit field taken directly from the DP sideband header. If a device sends msg_len=0, curchunk_len is set to zero. The condition (curchunk_idx >= curchunk_len) is immediately true, and curchunk_len-1 wraps to 255 (u8 underflow). drm_dp_msg_data_crc4() reads 255 bytes from chunk[48], then memcpy()
writes 255 bytes into msg[], both far out of bounds.
2. chunk[48] overflow: curchunk_len can reach 63 (6-bit field). chunk[] is
only 48 bytes. Multi-iteration payload assembly appends 16-byte blocks until curchunk_idx reaches curchunk_len, writing up to 15 bytes past the end of chunk[] into msg[].
3. msg[256] overflow: each chunk contributes (curchunk_len-1) bytes to
msg[]. No check ensures curlen + (curchunk_len-1) stays within msg[256],
so the memcpy can spill into adjacent struct fields.
All three are reachable from any DP MST device that can forge sideband reply messages on a physical connection.
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Analysis
by VulDB Data Team • 08/10/2026
The vulnerability resides in the linux kernel's displayport multi-stream transport subsystem where improper bounds checking leads to multiple buffer overflow conditions during sideband message processing. This flaw affects the drm_dp_sideband_append_payload() function which handles device-provided sideband reply data in displayport mst environments. The issue stems from inadequate validation of 6-bit message length fields extracted directly from dp sideband headers without proper range constraints or overflow protection mechanisms.
The first vulnerability manifests as a zero-length curchunk_len underflow where msg_len field values of zero cause immediate logical failure conditions. When msg_len equals zero, curchunk_len becomes zero and the condition curchunk_idx >= curchunk_len evaluates to true immediately, resulting in u8 underflow that sets curchunk_len to 255. This creates a critical memory access issue where drm_dp_msg_data_crc4() attempts to read 255 bytes from chunk[48] while memcpy() writes 255 bytes into msg[], both operations occurring far beyond their allocated buffer boundaries. This represents a classic integer underflow vulnerability that maps to cwe-190 and cwe-129 within the common weakness enumeration framework.
The second overflow condition occurs when curchunk_len reaches maximum value of 63 due to the 6-bit field limitation, while chunk[] array size remains only 48 bytes. During multi-iteration payload assembly, 16-byte blocks are appended until curchunk_idx reaches curchunk_len, causing writes that extend 15 bytes beyond the end of the chunk[] buffer into msg[]. This memory corruption scenario represents a direct buffer overflow vulnerability classified under cwe-786 and cwe-122 according to industry standards, where insufficient bounds checking allows arbitrary data overwrites.
The third vulnerability involves potential message buffer overflow within the 256-byte msg[] array where each chunk contributes (curchunk_len-1) bytes without any verification that curlen + (curchunk_len-1) remains within acceptable limits. This unchecked accumulation leads to memcpy operations that spill into adjacent struct fields, creating a memory corruption scenario that can potentially corrupt program state or enable privilege escalation attacks. The vulnerability directly maps to cwe-121 and cwe-125 in the weakness enumeration framework, representing improper bounds checking leading to memory access violations.
This vulnerability impacts any system running linux kernel with displayport mst support where malicious devices can forge sideband reply messages on physical connections. Attackers exploiting this flaw could potentially achieve privilege escalation or denial of service conditions through controlled memory corruption. The attack surface extends beyond simple local exploitation since the vulnerability requires only a device capable of sending malformed dp sideband responses, making it particularly dangerous in environments with untrusted displayport devices. According to mitre att&ck framework, this vulnerability maps to techniques involving privilege escalation and execution of malicious code through memory corruption attacks.
Mitigation strategies include implementing strict bounds checking for all 6-bit message length fields before processing, ensuring proper buffer boundary validation during multi-iteration payload assembly operations, and adding comprehensive input validation for sideband reply data. Kernel updates should enforce maximum limits on curchunk_len values, implement proper overflow detection mechanisms, and add memory protection checks that prevent writes beyond allocated buffer boundaries. Additionally, all device-provided sideband data should undergo rigorous validation before processing to ensure compliance with expected message format specifications and prevent exploitation through crafted malicious payloads.