Impact

When user-provided input is provided to any type that parses with the RFC 2822 format, a denial of service attack via stack exhaustion is possible. The attack relies on formally deprecated and rarely-used features that are part of the RFC 2822 format used in a malicious manner. Ordinary, non-malicious input will never encounter this scenario.

Patches

A limit to the depth of recursion was added in v0.3.47. From this version, an error will be returned rather than exhausting the stack.

Workarounds

Limiting the length of user input is the simplest way to avoid stack exhaustion, as the amount of the stack consumed would be at most a factor of the length of the input.

BytesStart::attributes() returns an Attributes iterator which, by default (with_checks(true)), rejects a start tag that repeats an attribute name. For each attribute yielded, the iterator compared the new name against every name seen so far in the same tag using a linear scan, so a start tag with N distinct attribute names cost O(N²) byte comparisons. There was no bound on N other than the size of the buffered start tag.

Impact

Any code that parses untrusted XML and iterates a start tag's attributes with the default duplicate check enabled can be made to spend CPU time quadratic in the number of attributes on a single tag. Because the check is pure computation with no .await/I/O, an I/O-based timeout on the consumer (for example a read or request timeout) cannot interrupt it while it runs.

Measured cost of a single start tag, release build:

| Attributes on one tag | Time | |---|---| | 80,000 | ~6 s | | 800,000 | ~10 min |

The cost grows with the square of the attribute count, so a start tag of a few tens of megabytes can stall a parsing thread for hours. No memory is exhausted and the parser does not crash; the effect is CPU exhaustion on the thread doing the parsing: a single crafted start tag can pin a CPU core for minutes to hours, denying service to that worker. A deployment that places a wall-clock bound on parsing, or confines it to a non-critical thread, may consider the availability impact lower.

Affected code paths

• BytesStart::attributes() / Attributes iterated with checks enabled (the default), and BytesStart::try_get_attribute.
• NsReader, which resolves namespaces by iterating a tag's attributes and so reaches the same check internally.

Consumers that iterate attributes with .attributes().with_checks(false) and do not use NsReader are not affected.

This was reported as reachable by a remote, unauthenticated attacker in a real-world RPKI relying party (NLnet Labs Routinator) via a crafted RRDP snapshot.xml.

Remediation

Upgrade to quick-xml >= 0.41.0, where the duplicate check keeps the linear scan for start tags with a small number of attributes and switches to an O(1) hash pre-filter above a threshold, making the whole tag O(N). The reported AttrError::Duplicated positions are unchanged.

If upgrading is not possible and duplicate-name detection is not required, disable it with .attributes().with_checks(false) (this does not help NsReader consumers, which have no equivalent opt-out before 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.