Impact

Wasmtime's native implementation of WASIp1 suffers from a leak in the fdrenumber function where the file descriptor being renumbered to is not properly closed. Wasmtime's implementation erroneously only updated the table of descriptors for WASIp1 and didn't update the underlying table of descriptors used by the host. This behavior means that while fdrenumber works correctly from a guest's perspective it ends up leaking resources in the host that aren't cleaned up until the corresponding Store is destroyed. This means that guests can, in a loop, use fdrenumber to cause hosts to exhaust resources or exhaust file descriptors.

This bug only affects the native implementation of WASIp1. This means that only runtimes which load core wasm modules and expose fdrenumber are affected. Runtimes are additionally only affected if they expose the ability to acquire a file descriptor, such as opening a file. For runtimes that deny access to files they are unaffected.

Patches

Wasmtime 24.0.10, 36.0.11, 44.0.3, and 45.0.2 have been released which fix this issue.

Workarounds

There are no workarounds for this issue and hosts are recommended to update.

Summary

In wasmtime-wasi, when a filesystem preopen is given DirPerms::all() and FilePerms::READ without FilePerms::WRITE, this wasmtime-wasi enforced access control mechanism can be bypassed by using the wasip2 descriptor.open-at or wasip1 pathopen interfaces by opening a file with OpenFlags::TRUNCATE oflag only, for example:

rust dirdescriptor.openat( PathFlags::empty(), FILENAME, OpenFlags::TRUNCATE, DescriptorFlags::READ, )

rust wasip1::pathopen( dirfd, 0, FILENAME, wasip1::OFLAGSTRUNC, wasip1::RIGHTSFDREAD, 0, 0 )

The root cause is that the clause that considered OpenFlags::TRUNCATE did not set openmode |= OpenMode::WRITE;, used later in that function for the access control check against FilePerms for whether opening that file is permitted. With the bug corrected, these calls to open-at and pathopen fail with error-code.not-permitted and ERRNOPERM respectively.

The bug in crates/wasi/src/filesystem.rs, Dir::openat, lines 967–969:

rust if oflags.contains(OpenFlags::TRUNCATE) { opts.truncate(true).write(true); } and the single line fix is: rust if oflags.contains(OpenFlags::TRUNCATE) { opts.truncate(true).write(true); openmode |= OpenMode::WRITE; }

Only wasmtime-wasi embeddings that use a combination of DirPerms::MUTATE with FilePerms::READ are affected by this bug, e.g. those that use in the WasiCtxBuilder: rust builder.preopeneddir("readonly", "readonly", DirPerms::READ | DirPerms::MUTATE, FilePerms::READ);

In particular, the Wasmtime project's wasmtime-cli's use of wasmtime-wasi is not affected, because it always sets FilePerms::all() for all preopens.

Summary

Wasmtime contains a vulnerability where when transcoding a UTF-16 string to the latin1+utf16 component-model encoding it would incorrectly validate the byte length of the input string when performing a bounds check. Specifically the number of code units were checked instead of the byte length, which is twice the size of the code units.

This vulnerability can cause the host to read beyond the end of a WebAssembly's linear memory in an attempt to transcode nonexistent bytes. In Wasmtime's default configuration this will read unmapped memory on a guard page, terminating the process with a segfault. Wasmtime can be configured, however, without guard pages which would mean that host memory beyond the end of linear memory may be read and interpreted as UTF-16.

A host segfault is a denial-of-service vulnerability in Wasmtime, and possibly being able to read beyond the end of linear memory is additionally a vulnerability. Note that reading beyond the end of linear memory requires nonstandard configuration of Wasmtime, specifically with guard pages disabled.

Impact

This is an out-of-bounds memory access. Any user running untrusted wasm components that use cross-component string passing (with UTF-16 source and latin1+utf16 destination encodings) is affected.

- With guard pages: Denial of service. The host process crashes with SIGBUS/SIGSEGV. - Without guard pages: Potential information disclosure. The guest can read host memory beyond its linear memory allocation.

Patches

Wasmtime 24.0.7, 36.0.7, 42.0.2, and 43.0.1 have been issued to fix this bug. Users are recommended to update to these patched versions of Wasmtime. Workarounds

There is no workaround for this bug. Hosts are recommended to updated to a patched version of Wasmtime.

Impact

Wasmtime's implementation of transcoding strings into the Component Model's utf16 or latin1+utf16 encodings improperly verified the alignment of reallocated strings. This meant that unaligned pointers could be passed to the host for transcoding which would trigger a host panic. This panic is possible to trigger from malicious guests which transfer very specific strings across components with specific addresses.

Host panics are considered a DoS vector in Wasmtime as the panic conditions are controlled by the guest in this situation.

Patches

Wasmtime 24.0.7, 36.0.7, 42.0.2, and 43.0.1 have been issued to fix this bug. Users are recommended to update to these patched versions of Wasmtime.

Workarounds

There is no workaround for this bug. Hosts are recommended to updated to a patched version of Wasmtime.

Impact

Wasmtime contains a possible panic which can happen when a flags-typed component model value is lifted with the Val type. If bits are set outside of the set of flags the component model specifies that these bits should be ignored but Wasmtime will panic when this value is lifted. This panic only affects wasmtime's implementation of lifting into Val, not when using the flags! macro. This additionally only affects flags-typed values which are part of a WIT interface.

This has the risk of being a guest-controlled panic within the host which Wasmtime considers a DoS vector.

Patches

Wasmtime 24.0.7, 36.0.7, 42.0.2, and 43.0.1 have been issued to fix this bug. Users are recommended to update to these patched versions of Wasmtime.

Workarounds

There is no workaround for this bug if a host meets the criteria to be affected. To be affected a host must be using wasmtime::component::Val and possibly work with a flags type in the component model.

On x86-64 platforms with SSE3 disabled Wasmtime's compilation of the f64x2.splat WebAssembly instruction with Cranelift may load 8 more bytes than is necessary. When signals-based-traps are disabled this can result in a uncaught segfault due to loading from unmapped guard pages. With guard pages disabled it's possible for out-of-sandbox data to be loaded, but this data is not visible to WebAssembly guests.

Details

The f64x2.splat operator, when operating on a value loaded from a memory (for example with f64.load), compiles with Cranelift to code on x86-64 without SSE3 that loads 128 bits (16 bytes) rather than the expected 64 bits (8 bytes) from memory. When the address is in-bounds for a (correct) 8-byte load but not an (incorrect) 16-byte load, this can load beyond memory by up to 8 bytes. This can result in three different behaviors depending on Wasmtime's configuration:

1. If guard pages are disabled then this extra data will be loaded. The extra data is present in the upper bits of a register, but the upper bits are not visible to WebAssembly guests. Actually witnessing this data would require a different bug in Cranelift, of which none are known. Thus in this situation while it's something we're patching in Cranelift it's not a security issue. 2. If guard pages are enabled, and signals-based-traps are enabled, then this operation will result in a safe WebAssembly trap. The trap is incorrect because the load is not out-of-bounds as defined by WebAssembly, but this mistakenly widened load will load bytes from an unmapped guard page, causing a segfault which is caught and handled as a Wasm trap. In this situation this is not a security issue, but we're patching Cranelift to fix the WebAssembly behavior. 3. If guard pages are enabled, and signals-based-traps are disabled, then this operation results in an uncaught segfault. Like the previous case with guard pages enabled this will load from an unmapped guard page. Unlike before, however, signals-based-traps are disabled meaning that signal handlers aren't configured. The resulting segfault will, by default, terminate the process. This is a security issue from a DoS perspective, but does not represent an arbitrary read or write from WebAssembly, for example.

Wasmtime's default configuration is case (2) in this case. That means that Wasmtime, by default, incorrectly executes this WebAssembly instruction but does not have insecure behavior.

Impact

If signals-based-traps are disabled and guard pages are enabled then guests can trigger an uncaught segfault in the host, likely aborting the host process. This represents, for example, a DoS vector for WebAssembly guests.

This bug does not affect Wasmtime's default configuration and requires signals-based-traps to be disabled. This bug only affects the x86-64 target with the SSE3 feature disabled and the Cranelift backend (Wasmtime's default backend).

Patches

Wasmtime 24.0.7, 36.0.7, 42.0.2, and 43.0.1 have been issued to fix this bug. Users are recommended to update to these patched versions of Wasmtime.

Workarounds

This bug only affects x86-64 hosts where SSE3 is disabled. If SSE3 is enabled or if a non-x86-64 host is used then hosts are not affect. Otherwise there are no known workarounds to this issue.

Impact

Wasmtime's Winch compiler contains a bug where a 64-bit table, part of the memory64 proposal of WebAssembly, incorrectly translated the table.size instruction. This bug could lead to disclosing data on the host's stack to WebAssembly guests. The host's stack can possibly contain sensitive data related to other host-originating operations which is not intended to be disclosed to guests.

This bug specifically arose from a mistake where the return value of table.size was statically typed as a 32-bit integer, as opposed to consulting the table's index type to see how large the returned register could be. When combined with details about Wnich's ABI, such as multi-value returns, this can be combined to read stack data from the host, within a guest. This information disclosure should not be possible in WebAssembly, violates spec semantics, and is a vulnerability in Wasmtime.

Patches

Wasmtime 36.0.7, 42.0.2, and 43.0.1 have been issued to fix this bug. Users are recommended to update to these patched versions of Wasmtime.

Workarounds

Users of Cranelift are not affected by this issue, but users of Winch have no workarounds other than disabling the Config::wasmmemory64 proposal.

Impact

Wasmtime's Winch compiler contains a vulnerability where the compilation of the table.fill instruction can result in a host panic. This means that a valid guest can be compiled with Winch, on any architecture, and cause the host to panic. This represents a denial-of-service vulnerability in Wasmtime due to guests being able to trigger a panic.

The specific issue is that a historical refactoring, #11254, changed how compiled code referenced tables within the table. instructions. This refactoring forgot to update the Winch code paths associated as well, meaning that Winch was using the wrong indexing scheme. Due to the feature support of Winch the only problem that can result is tables being mixed up or nonexistent tables being used, meaning that the guest is limited to panicking the host (using a nonexistent table), or executing spec-incorrect behavior and modifying the wrong table.

Patches

Wasmtime 36.0.7, 42.0.2, and 43.0.1 have been issued to fix this bug. Users are recommended to update to these patched versions of Wasmtime.

Workarounds

Users of Cranelift are not affected by this issue, but for users of Winch there is no workaround for this bug. Hosts are recommended to updated to a patched version of Wasmtime.

Impact

Wasmtime with its Winch (baseline) non-default compiler backend may allow properly constructed guest Wasm to access host memory outside of its linear-memory sandbox.

This vulnerability requires use of the Winch compiler (-Ccompiler=winch). By default, Wasmtime uses its Cranelift backend, not Winch. With Winch, the same incorrect assumption is present in theory on both aarch64 and x86-64. The aarch64 case has an observed-working proof of concept, while the x86-64 case is theoretical and may not be reachable in practice.

This Winch compiler bug can allow the Wasm guest to access memory before or after the linear-memory region, independently of whether pre- or post-guard regions are configured. The accessible range in the initial bug proof-of-concept is up to 32KiB before the start of memory, or ~4GiB after the start of memory, independently of the size of pre- or post-guard regions or the use of explicit or guard-region-based bounds checking. However, the underlying bug assumes a 32-bit memory offset stored in a 64-bit register has its upper bits cleared when it may not, and so closely related variants of the initial proof-of-concept may be able to access truly arbitrary memory in-process. This could result in a host process segmentation fault (DoS), an arbitrary data leak from the host process, or with a write, potentially an arbitrary RCE.

Patches

Wasmtime 43.0.1, 42.0.2, and 36.0.7 have been released with fixes for this issue.

Workaround

There are no workarounds within the Winch compiler backend while using the affected versions. Users of Wasmtime are encouraged either to upgrade to patched versions or, if that is not possible, use the Cranelift compiler backend.

Impact

Wasmtime's Winch compiler backend contains a bug where translating the table.grow operator causes the result to be incorrectly typed. For 32-bit tables this means that the result of the operator, internally in Winch, is tagged as a 64-bit value instead of a 32-bit value. This invalid internal representation of Winch's compiler state compounds into further issues depending on how the value is consumed.

One example can be seen when the result of table.grow is used as the address of a load operation. The load operation is tricked into thinking the address is a 64-bit value, not a 32-bit value, which means that the final address to load from is calculated incorrectly. This can lead to a situation where the bytes before the start of linear memory can be loaded/stored to.

The primary consequence of this bug is that bytes in the host's address space can be stored/read from. This is only applicable to the 16 bytes before linear memory, however, as the only significant return value of table.grow that can be misinterpreted is -1. The bytes before linear memory are, by default, unmapped memory. Wasmtime will detect this fault and abort the process, however, because wasm should not be able to access these bytes.

Overall this this bug in Winch represents a DoS vector by crashing the host process, a correctness issue within Winch, and a possible leak of up to 16-bytes before linear memory. Wasmtime's default compiler is Cranelift, not Winch, and Wasmtime's default settings are to place guard pages before linear memory. This means that Wasmtime's default configuration is not affected by this issue, and when explicitly choosing Winch Wasmtime's otherwise default configuration leads to a DoS. Disabling guard pages before linear memory is required to possibly leak up to 16-bytes of host data.

Patches

Wasmtime 43.0.1, 42.0.2, and 36.0.7 have been released with fixes for this issue.

Workaround

There are no workarounds within the Winch compiler backend while using the affected versions. Users of Wasmtime are encouraged either to upgrade to patched versions or, if that is not possible, use the Cranelift compiler backend.

Impact

Wasmtime's implementation of transcoding strings between components contains a bug where the return value of a guest component's realloc is not validated before the host attempts to write through the pointer. This enables a guest to cause the host to write arbitrary transcoded string bytes to an arbitrary location up to 4GiB away from the base of linear memory. These writes on the host could hit unmapped memory or could corrupt host data structures depending on Wasmtime's configuration.

Wasmtime by default reserves 4GiB of virtual memory for a guest's linear memory meaning that this bug will by default on hosts cause the host to hit unmapped memory and abort the process due to an unhandled fault. Wasmtime can be configured, however, to reserve less memory for a guest and to remove all guard pages, so some configurations of Wasmtime may lead to corruption of data outside of a guest's linear memory, such as host data structures or other guests's linear memories.

Patches

Wasmtime 24.0.7, 36.0.7, 42.0.2, and 43.0.1 have been issued to fix this bug. Users are recommended to update to these patched versions of Wasmtime.

Workarounds

There is no known workaround for this issue and affected hosts/embeddings are recommended to upgrade.

Impact

Wasmtime's implementation of the wasi:http/types.fields resource is susceptible to panics when too many fields are added to the set of headers. Wasmtime's implementation in the wasmtime-wasi-http crate is backed by a data structure which panics when it reaches excessive capacity and this condition was not handled gracefully in Wasmtime. Panicking in a WASI implementation is a Denial of Service vector for embedders and is treated as a security vulnerability in Wasmtime.

Patches

Wasmtime 24.0.6, 36.0.6, 40.0.4, 41.0.4, and 42.0.0 patch this vulnerability and return a trap to the guest instead of panicking.

Workarounds

There are no known workarounds at this time, embedders are encouraged to update to a patched version of Wasmtime.

Resources

Limitations of http::HeaderMap

Impact

Wasmtime's implementation of WASI host interfaces are susceptible to guest-controlled resource exhaustion on the host. Wasmtime did not appropriately place limits on resource allocations requested by the guests. This serves as a Denial of Service vector where a guest can induce a range of crashing behaviors on the host such as:

Allocating arbitrarily large amounts of host memory. Causing an allocation failure on the host, which in Rust defaults to aborting the process. Causing a panic on the host due to over-large allocations being performed. Cause degredation in performance of the host by holding excessive host memory alive.

Wasmtime's security bug policy considers all of these behaviors a security vulnerability. Wasmtime's implementation of WASI has a number of different ways that resource exhaustion could happen, and fixing any one of them is insufficient from solving this vulnerability. A number of individual issues are grouped within this advisory and as a whole represent the known ways that guests can exhaust resources on the host.

An example of guest-controlled resource exhaustion within Wasmtime's implementation of WASI is guests could repeatedly allocate handles to themselves without limit. Some APIs also caused the host to perform a guest-controlled-sized allocation of a buffer on the host for I/O operations. Other APIs could force the host to buffer arbitrary amounts of data for the guest. Finally the guest could hand arbitrarily large allocations from itself to the host which could cause the host to perform an arbitrarily sized copy of memory which in some situations could result in quadratically sized allocations.

Wasmtime's implementations of WASIp1 and WASIp2 are affected by this vulnerability. Any host API modeled with the Component Model (or WIT) which operates on a string or list<T> type is also affected. Not all WIT and WASI APIs are affected by this issue, but that's more of an exception so it's recommended for all embedders to consider themselves affected.

To address this issue a number of mitigations are being applied to limit the behavior of a guest in WASI. All of these mitigations manifest in the form of a limit of some kind applied to various situations, and as such all of these mitigations are backwards-incompatible as they run the risk of breaking preexisting programs. To address this all backports to previous stable releases have these limits tuned to overly-large values. This ensures that preexisting guests do not break while still providing embedders the knobs to prevent this DoS vector as well. The limits added to Wasmtime are:

-Smax-resources=N or ResourceTable::setmaxcapacity - the maximum number of resources that a guest is allowed to allocate for itself. -Shostcall-fuel=N or Store::sethostcallfuel - the maximum amount of data that the guest may copy to the host in a single function call. -Smax-random-size=N or WasiCtxBuilder::maxrandomsize - the maximum size of the return value of get-random-bytes and get-insecure-random-bytes in the wasi:random implementations. -Smax-http-fields-size=N or WasiHttpCtx::setmaxfieldssize - the maximum size of headers for an HTTP request/response.

These settings are equally applicable to both WASIp1 and WASIp2. Wasmtime 41.0.x and prior previously did not limit these settings and the knobs being released are set to very large values by default to avoid any breaking behavior. Embedders will need to proactively tune these knobs as appropriate for their embeddings. The default settings in the unreleased Wasmtime 42.0.0 are 1M for max resources, 128MiB for hostcall fuel, 64MiB for max-random-size, and 32KiB for http fields size. Tuning is not expected for Wasmtime 42.0.0+.

Hosts/embedders affected by this issue are encouraged to audit and double-check their own host APIs they have implemented to see whether they are affected by this issue as well. The -Shostcall-fuel setting is intended to be a relatively coarse fix for many possible issues by limiting the amount of data for all host APIs at once, so many embedders may not need to take further action beyond updating Wasmtime and configuring it appropriately (if not updating to 42.0.0). Embedders should audit to see, however, if the guest is able to force the host to allocate on its behalf and ensure that the allocation is limited or tracked somehow.

Patches

Wasmtime 24.0.6, 36.0.6, 40.0.4, 41.0.4, and 42.0.0 have all been released with the fix for this issue. These versions do not prevent this issue in their default configuration to avoid breaking preexisting behaviors. All versions of Wasmtime have appropriate knobs to prevent this behavior, and Wasmtime 42.0.0-and-later will have these knobs tuned by default to prevent this issue from happening.

Workarounds

There are no known workarounds for this issue without upgrading. Embedders are recommended to upgrade and configure their embeddings as necessary to prevent possibly-malicious guests from triggering this issue.

Resources

Store::sethostcallfuel ResourceTable::setmaxcapacity WasiCtxBuilder::maxrandomsize Original PR showing resource exhaustion Issue about limiting max resource handles per-guest