RUSTSEC-2026-0195: Unbounded namespace-declaration allocation in `NsReader` enables memory-exhaustion denial of service

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Unbounded namespace-declaration allocation in NsReader enables memory-exhaustion denial of service

Details
Package	quick-xml
Version	0.39.4
URL	tafia/quick-xml#970
Date	2026-06-29
Patched versions	>=0.41.0

NsReader resolves namespaces by calling NamespaceResolver::push for every
Start/Empty event before the event is returned to the caller. push
iterated all xmlns / xmlns:* attributes on the start tag and, for each one,
appended the prefix bytes to an internal buffer and pushed a NamespaceBinding
(32 bytes on 64-bit) to an internal Vec, with no upper bound on the number of
declarations.

Impact

A start tag with N namespace declarations drove roughly 3× the tag's byte
size in NamespaceResolver heap, allocated inside quick-xml before the
NsReader consumer ever received the event and could inspect or reject it. A
consumer that bounds its input size therefore still cannot bound this
allocation: an M-byte start tag yields on the order of 3 × M bytes of
resolver heap the caller never sees.

On untrusted XML this lets a remote, unauthenticated attacker force large heap
allocations with a single start tag. With several NsReaders running
concurrently on independent inputs (a common server pattern), the allocations
stack and can exhaust process memory, causing the operating system to kill the
process (OOM). This was confirmed against a real-world RPKI relying party (NLnet
Labs Routinator), where concurrent RRDP validation workers parsing a crafted
snapshot.xml exceeded the memory limit and the process was OOM-killed.

Affected code paths

Consumers using NsReader (which always calls NamespaceResolver::push before
yielding Start/Empty), or calling NamespaceResolver::push directly. A plain
Reader that does not perform namespace resolution is not affected.

Remediation

Upgrade to quick-xml >= 0.41.0. NamespaceResolver::push now rejects a start
tag that declares more than DEFAULT_MAX_DECLARATIONS_PER_ELEMENT (256)
namespace bindings, returning the new NamespaceError::TooManyDeclarations
instead of allocating without limit. The limit is configurable via
NamespaceResolver::set_max_declarations_per_element (use usize::MAX to
restore the previous unbounded behavior), and NsReader::resolver_mut() is
provided to reach it.

There is no clean workaround for NsReader consumers before 0.41.0, as the
allocation happens inside the reader with no configuration knob to cap it.

See advisory page for additional details.