CVE-2026-74751 riscv: lib: Fix ZBB strnlen reading past count boundary
A boundary check error in the RISC-V ZBB strnlen implementation can cause a kernel fault when reading near page boundaries, leading to denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-74751?
CVE-2026-74751 is a critical denial of service vulnerability in Linux, published on 26 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
riscv: lib: Fix ZBB strnlen reading past count boundary
The ZBB-optimized strnlen loop loads one word ahead before checking the aligned boundary:
REG_L t1, SZREG(t0) // load next word addi t0, t0, SZREG // advance orc.b t1, t1 bgeu t0, t4, 4f // boundary check AFTER load
where t4 = (s + count) & -SZREG. When s is aligned and count is a multiple of SZREG, t4 equals s + count and the loop loads a full word starting at exactly s + count. If s + count falls on a page boundary with the next page unmapped, this faults.
Fix by computing the aligned boundary from the last valid byte (s + count - 1) instead of s + count. This makes the loop stop at the word containing the last valid byte rather than potentially loading the word after it. The count == 0 case is already handled by the beqz early exit.
Also add a pre-loop guard (bgeu t0, t4) for the case where all valid bytes fit within the first word. With the adjusted boundary, t4 can equal t0, and entering the loop with stale register state from the first-word processing would produce incorrect results.
The final minu clamp ensures the result is still correct when the last loaded word extends past s + count - 1 within the same aligned word.
Which versions are affected by CVE-2026-74751?
How do I fix CVE-2026-74751?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-74751 being exploited?
There is no known exploitation of CVE-2026-74751 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-74751, GHSA-ghc8-jmvh-q86v, UBUNTU-CVE-2026-74751.