7. <del> TBD.  Also see Section 7.1 of RFC7541. </del> <ins> This section describes potential areas of security concern with QPACK: Use of compression as a length-based oracle for verifying guesses about secrets that are compressed into a shared compression context. Denial of service resulting from exhausting processing or memory capacity at a decoder. 7.1. QPACK reduces the length of header field encodings by exploiting the redundancy inherent in protocols like HTTP.  The ultimate goal of this is to reduce the amount of data that is required to send HTTP requests or responses. The compression context used to encode header fields can be probed by an attacker who can both define header fields to be encoded and transmitted and observe the length of those fields once they are encoded.  When an attacker can do both, they can adaptively modify requests in order to confirm guesses about the dynamic table state. If a guess is compressed into a shorter length, the attacker can observe the encoded length and infer that the guess was correct. This is possible even over the Transport Layer Security Protocol (TLS, see RFC5246), because while TLS provides confidentiality protection for content, it only provides a limited amount of protection for the length of that content. Padding schemes only provide limited protection against an attacker with these capabilities, potentially only forcing an increased number of guesses to learn the length associated with a given guess.  Padding schemes also work directly against compression by increasing the number of bits that are transmitted. Attacks like CRIME CRIME demonstrated the existence of these general attacker capabilities.  The specific attack exploited the fact that DEFLATE RFC1951 removes redundancy based on prefix matching.  This permitted the attacker to confirm guesses a character at a time, reducing an exponential-time attack into a linear-time attack. 7.2. QPACK mitigates but does not completely prevent attacks modeled on CRIME CRIME by forcing a guess to match an entire header field value, rather than individual characters.  An attacker can only learn whether a guess is correct or not, so is reduced to a brute force guess for the header field values. The viability of recovering specific header field values therefore depends on the entropy of values.  As a result, values with high entropy are unlikely to be recovered successfully.  However, values with low entropy remain vulnerable. Attacks of this nature are possible any time that two mutually distrustful entities control requests or responses that are placed onto a single HTTP/3 connection.  If the shared QPACK compressor permits one entity to add entries to the dynamic table, and the other to access those entries, then the state of the table can be learned. Having requests or responses from mutually distrustful entities occurs when an intermediary either: sends requests from multiple clients on a single connection toward an origin server, or takes responses from multiple origin servers and places them on a shared connection toward a client. Web browsers also need to assume that requests made on the same connection by different web origins RFC6454 are made by mutually distrustful entities. 7.3. Users of HTTP that require confidentiality for header fields can use values with entropy sufficient to make guessing infeasible.  However, this is impractical as a general solution because it forces all users of HTTP to take steps to mitigate attacks.  It would impose new constraints on how HTTP is used. Rather than impose constraints on users of HTTP, an implementation of QPACK can instead constrain how compression is applied in order to limit the potential for dynamic table probing. An ideal solution segregates access to the dynamic table based on the entity that is constructing header fields.  Header field values that are added to the table are attributed to an entity, and only the entity that created a particular value can extract that value. To improve compression performance of this option, certain entries might be tagged as being public.  For example, a web browser might make the values of the Accept-Encoding header field available in all requests. An encoder without good knowledge of the provenance of header fields might instead introduce a penalty for a header field with many different values, such that a large number of attempts to guess a header field value results in the header field not being compared to the dynamic table entries in future messages, effectively preventing further guesses. Simply removing entries corresponding to the header field from the dynamic table can be ineffectual if the attacker has a reliable way of causing values to be reinstalled.  For example, a request to load an image in a web browser typically includes the Cookie header field (a potentially highly valued target for this sort of attack), and web sites can easily force an image to be loaded, thereby refreshing the entry in the dynamic table. This response might be made inversely proportional to the length of the header field value.  Disabling access to the dynamic table for a header field might occur for shorter values more quickly or with higher probability than for longer values. 7.4. Implementations can also choose to protect sensitive header fields by not compressing them and instead encoding their value as literals. Refusing to insert a header field into the dynamic table is only effective if doing so is avoided on all hops.  The never indexed literal bit (see literal-name-reference) can be used to signal to intermediaries that a particular value was intentionally sent as a literal. An intermediary MUST NOT re-encode a value that uses a literal representation with the 'N' bit set with another representation that would index it.  If QPACK is used for re-encoding, a literal representation with the 'N' bit set MUST be used.  If HPACK is used for re-encoding, the never indexed literal representation (see Section 6.2.3 of RFC7541) MUST be used. The choice to mark that a header field should never be indexed depends on several factors.  Since QPACK doesn't protect against guessing an entire header field value, short or low-entropy values are more readily recovered by an adversary.  Therefore, an encoder might choose not to index values with low entropy. An encoder might also choose not to index values for header fields that are considered to be highly valuable or sensitive to recovery, such as the Cookie or Authorization header fields. On the contrary, an encoder might prefer indexing values for header fields that have little or no value if they were exposed.  For instance, a User-Agent header field does not commonly vary between requests and is sent to any server.  In that case, confirmation that a particular User-Agent value has been used provides little value. Note that these criteria for deciding to use a never indexed literal representation will evolve over time as new attacks are discovered. 7.5. There is no currently known attack against a static Huffman encoding. A study has shown that using a static Huffman encoding table created an information leakage, however this same study concluded that an attacker could not take advantage of this information leakage to recover any meaningful amount of information (see PETAL). 7.6. An attacker can try to cause an endpoint to exhaust its memory. QPACK is designed to limit both the peak and stable amounts of memory allocated by an endpoint. The amount of memory used by the encoder is limited by the protocol using QPACK through the definition of the maximum size of the dynamic table, and the maximum number of blocking streams.  In HTTP/3, these values are controlled by the decoder through the settings parameters SETTINGS_QPACK_MAX_TABLE_CAPACITY and SETTINGS_QPACK_BLOCKED_STREAMS, respectively (see maximum-dynamic-table-capacity and blocked- streams).  The limit on the size of the dynamic table takes into account the size of the data stored in the dynamic table, plus a small allowance for overhead.  The limit on the number of blocked streams is only a proxy for the maximum amount of memory required by the decoder.  The actual maximum amount of memory will depend on how much memory the decoder uses to track each blocked stream. A decoder can limit the amount of state memory used for the dynamic table by setting an appropriate value for the maximum size of the dynamic table.  In HTTP/3, this is realized by setting an appropriate value for the SETTINGS_QPACK_MAX_TABLE_CAPACITY parameter.  An encoder can limit the amount of state memory it uses by signaling a lower dynamic table size than the decoder allows (see eviction). A decoder can limit the amount of state memory used for blocked streams by setting an appropriate value for the maximum number of blocked streams.  In HTTP/3, this is realized by setting an appropriate value for the QPACK_BLOCKED_STREAMS parameter.  An encoder can limit the amount of state memory by only using as many blocked streams as it wishes to support; no signaling to the decoder is requred. The amount of temporary memory consumed by an encoder or decoder can be limited by processing header fields sequentially.  A decoder implementation does not need to retain a complete list of header fields while decoding a header block.  An encoder implementation does not need to retain a complete list of header fields while encoding a header block if it is using a single-pass algorithm.  Note that it might be necessary for an application to retain a complete header list for other reasons; even if QPACK does not force this to occur, application constraints might make this necessary. </ins> While the negotiated limit on the dynamic table size accounts for much of the memory that can be consumed by a QPACK implementation,