nltk PorterStemmer in versions <= 3.10.2 (fixed in 3.10.3) contains an inefficient-algorithmic-complexity denial of service in PorterStemmer.stem(). The _is_consonant() helper walks backward over the entire run of trailing 'y' characters on every call, and _measure() invokes it for each stem position, causing O(n^2) behavior. A single ~20-50 KB untrusted token consisting of a long run of the letter 'y' followed by a matching suffix (e.g., 'ness') can pin a CPU core for seconds to minutes, causing availability impact.

CVE-2026-81722 affects the NLTK library's PorterStemmer component in versions 3.10.2 and earlier. The vulnerability stems from an inefficient O(n^2) algorithmic complexity in the PorterStemmer.stem() function. Specifically, the _is_consonant() helper method walks backward over the entire run of trailing 'y' characters on every call, while _measure() invokes it for each stem position, compounding the inefficiency. An attacker can exploit this by submitting a single crafted token of approximately 20-50 KB consisting of a long run of the letter 'y' followed by a matching suffix such as 'ness'. This can cause a CPU core to be pinned for seconds to minutes, resulting in a denial of service condition. The vulnerability has been fixed in NLTK version 3.10.3. The primary impact is on availability, particularly in applications that process untrusted user-supplied text through the PorterStemmer interface.