CVE-2026-34971 [Critical] | Out-of-bounds Read in Wasmtime

Wasmtime is a runtime for WebAssembly. From 32.0.0 to before 36.0.7, 42.0.2, and 43.0.1, Wasmtime's Cranelift compilation backend contains a bug on aarch64 when performing a certain shape of heap accesses which means that the wrong address is accessed. When combined with explicit bounds checks a guest WebAssembly module this can create a situation where there are two diverging computations for the same address: one for the address to bounds-check and one for the address to load. This difference in address being operated on means that a guest module can pass a bounds check but then load a different address. Combined together this enables an arbitrary read/write primitive for guest WebAssembly when accesssing host memory. This is a sandbox escape as guests are able to read/write arbitrary host memory. This vulnerability has a few ingredients, all of which must be met, for this situation to occur and bypass the sandbox restrictions. This miscompiled shape of load only occurs on 64-bit WebAssembly linear memories, or when Config::wasm_memory64 is enabled. 32-bit WebAssembly is not affected. Spectre mitigations or signals-based-traps must be disabled. When spectre mitigations are enabled then the offending shape of load is not generated. When signals-based-traps are disabled then spectre mitigations are also automatically disabled. The specific bug in Cranelift is a miscompile of a load of the shape load(iadd(base, ishl(index, amt))) where amt is a constant. The amt value is masked incorrectly to test if it's a certain value, and this incorrect mask means that Cranelift can pattern-match this lowering rule during instruction selection erroneously, diverging from WebAssembly's and Cranelift's semantics. This incorrect lowering would, for example, load an address much further away than intended as the correct address's computation would have wrapped around to a smaller value insetad. This vulnerability is fixed in 36.0.7, 42.0.2, and 43.0.1.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-06-15	0.01%	0.26%	+0.25%
2	2026-04-10	—	0.01%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-06-15

0.01%

0.26%

+0.25%

2026-04-10

—

0.01%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
9.0	4.0	CRITICAL	`CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X` Click to expand Attack vector (AV:N) Could be attacked over the internet or any normal routed network. Attack complexity (AC:L) Exploitation conditions are straightforward and stable. Attack requirements (AT:P) Additional preconditions must be present for exploitation. Privileges required (PR:L) Low privileges are required. User interaction (UI:N) No user interaction is required. Vulnerable system confidentiality impact (VC:H) High confidentiality impact on the vulnerable system. Vulnerable system integrity impact (VI:H) High integrity impact on the vulnerable system. Vulnerable system availability impact (VA:H) High availability impact on the vulnerable system. Subsequent system confidentiality impact (SC:H) High confidentiality impact on subsequent systems. Subsequent system integrity impact (SI:H) High integrity impact on subsequent systems. Subsequent system availability impact (SA:H) High availability impact on subsequent systems. Exploit maturity (threat) (E:X) Not defined: no reliable threat intelligence; scoring assumes the worst case (equivalent to Attacked). Confidentiality requirement (CR:X) Not defined: insufficient information; scoring treats this like High (worst case). Integrity requirement (IR:X) Not defined: insufficient information; scoring treats this like High (worst case). Availability requirement (AR:X) Not defined: insufficient information; scoring treats this like High (worst case). Modified attack vector (MAV:X) Not defined: scoring uses the Base Attack Vector (AV). Modified attack complexity (MAC:X) Not defined: scoring uses the Base Attack Complexity (AC). Modified attack requirements (MAT:X) Not defined: scoring uses the Base Attack Requirements (AT). Modified privileges required (MPR:X) Not defined: scoring uses the Base Privileges Required (PR). Modified user interaction (MUI:X) Not defined: scoring uses the Base User Interaction (UI). Modified vulnerable system confidentiality impact (MVC:X) Not defined: scoring uses the Base VC metric. Modified vulnerable system integrity impact (MVI:X) Not defined: scoring uses the Base VI metric. Modified vulnerable system availability impact (MVA:X) Not defined: scoring uses the Base VA metric. Modified subsequent system confidentiality impact (MSC:X) Not defined: scoring uses the Base SC metric. Modified subsequent system integrity impact (MSI:X) Not defined: scoring uses the Base SI metric. Modified subsequent system availability impact (MSA:X) Not defined: scoring uses the Base SA metric. Safety (supplemental) (S:X) Not evaluated. Automatable (supplemental) (AU:X) Not evaluated. Recovery (supplemental) (R:X) Not evaluated. Value density (supplemental) (V:X) Not evaluated. Vulnerability response effort (supplemental) (RE:X) Not evaluated. Provider urgency (supplemental) (U:X) Not evaluated.	—	—	[email protected]
7.8	3.1	HIGH	`CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H` Click to expand Attack vector (AV:L) They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by. Attack complexity (AC:H) Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work. Privileges required (PR:L) A normal user session is enough; they don’t have to be admin. User interaction (UI:N) Nobody has to click “OK” or open a trap file; it can work without a victim helping. Scope (S:C) Breaking this can reach past the original component and bite other resources—bigger blast radius. Confidentiality (C:H) Serious risk that confidential data gets exposed in a big way. Integrity (I:H) They could widely tamper with or forge data—trust in the data is badly hurt. Availability (A:H) Could take the service down hard or make it unusable for people who depend on it.	1.1	6.0	134c704f-9b21-4f2e-91b3-4a467353bcc0

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

9.0

4.0

CRITICAL

CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Click to expand

Attack vector (AV:N): Could be attacked over the internet or any normal routed network.
Attack complexity (AC:L): Exploitation conditions are straightforward and stable.
Attack requirements (AT:P): Additional preconditions must be present for exploitation.
Privileges required (PR:L): Low privileges are required.
User interaction (UI:N): No user interaction is required.
Vulnerable system confidentiality impact (VC:H): High confidentiality impact on the vulnerable system.
Vulnerable system integrity impact (VI:H): High integrity impact on the vulnerable system.
Vulnerable system availability impact (VA:H): High availability impact on the vulnerable system.
Subsequent system confidentiality impact (SC:H): High confidentiality impact on subsequent systems.
Subsequent system integrity impact (SI:H): High integrity impact on subsequent systems.
Subsequent system availability impact (SA:H): High availability impact on subsequent systems.
Exploit maturity (threat) (E:X): Not defined: no reliable threat intelligence; scoring assumes the worst case (equivalent to Attacked).
Confidentiality requirement (CR:X): Not defined: insufficient information; scoring treats this like High (worst case).
Integrity requirement (IR:X): Not defined: insufficient information; scoring treats this like High (worst case).
Availability requirement (AR:X): Not defined: insufficient information; scoring treats this like High (worst case).
Modified attack vector (MAV:X): Not defined: scoring uses the Base Attack Vector (AV).
Modified attack complexity (MAC:X): Not defined: scoring uses the Base Attack Complexity (AC).
Modified attack requirements (MAT:X): Not defined: scoring uses the Base Attack Requirements (AT).
Modified privileges required (MPR:X): Not defined: scoring uses the Base Privileges Required (PR).
Modified user interaction (MUI:X): Not defined: scoring uses the Base User Interaction (UI).
Modified vulnerable system confidentiality impact (MVC:X): Not defined: scoring uses the Base VC metric.
Modified vulnerable system integrity impact (MVI:X): Not defined: scoring uses the Base VI metric.
Modified vulnerable system availability impact (MVA:X): Not defined: scoring uses the Base VA metric.
Modified subsequent system confidentiality impact (MSC:X): Not defined: scoring uses the Base SC metric.
Modified subsequent system integrity impact (MSI:X): Not defined: scoring uses the Base SI metric.
Modified subsequent system availability impact (MSA:X): Not defined: scoring uses the Base SA metric.
Safety (supplemental) (S:X): Not evaluated.
Automatable (supplemental) (AU:X): Not evaluated.
Recovery (supplemental) (R:X): Not evaluated.
Value density (supplemental) (V:X): Not evaluated.
Vulnerability response effort (supplemental) (RE:X): Not evaluated.
Provider urgency (supplemental) (U:X): Not evaluated.

—

[email protected]

7.8

3.1

HIGH

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H Click to expand

Attack vector (AV:L): They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by.
Attack complexity (AC:H): Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:L): A normal user session is enough; they don’t have to be admin.
User interaction (UI:N): Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C): Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:H): Serious risk that confidential data gets exposed in a big way.
Integrity (I:H): They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H): Could take the service down hard or make it unusable for people who depend on it.

1.1

6.0

134c704f-9b21-4f2e-91b3-4a467353bcc0

OS Trackers for CVE-2026-34971

vendor	priority	summary	link
`debian`	not yet assigned	CVE-2026-34971 not yet assigned priority: Debian including 1 source packages (rust-wasmtime), 3 status rows across 3 suites (forky, sid, trixie): resolved 2, open 1.	https://security-tracker.debian.org/tracker/CVE-2026-34971
`redhat`	high	—	https://access.redhat.com/security/cve/CVE-2026-34971
`suse`	high	—	https://www.suse.com/security/cve/CVE-2026-34971/
`ubuntu`	medium	CVE-2026-34971 medium priority: Ubuntu including 1 source packages (rust-wasmtime), 4 status rows across 4 suites (jammy, noble, questing, upstream): needs-triage 3, DNE 1.	https://ubuntu.com/security/CVE-2026-34971

Affected software / configurations for CVE-2026-34971

Vendor	Product	Version	Raw CPE
bytecodealliance	wasmtime	>= 32.0.0, < 36.0.7	cpe:2.3:a:bytecodealliance:wasmtime::::::rust::*
bytecodealliance	wasmtime	>= 37.0.0, < 42.0.2	cpe:2.3:a:bytecodealliance:wasmtime::::::rust::*
bytecodealliance	wasmtime	43.0.0	cpe:2.3:a:bytecodealliance:wasmtime:43.0.0:::::rust::

References for CVE-2026-34971

URL	Tags
https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-jhxm-h53p-jm7w	Vendor Advisory Mitigation

URL

CVE-2026-34971 | Wasmtime miscompiled guest heap access enables sandbox escape on aarch64 Cranelift

Exploit prediction scoring system (EPSS) score for CVE-2026-34971

Common vulnerability scoring system (CVSS) metrics for CVE-2026-34971

Weakness enumeration for CVE-2026-34971

GitHub Security Advisory for CVE-2026-34971

OS Trackers for CVE-2026-34971

Affected software / configurations for CVE-2026-34971

References for CVE-2026-34971