CVE-2026-26309

An off-by-one write in Envoy::JsonEscaper::escapeString() can corrupt
std::string null-termination, causing undefined behavior and potentially
leading to crashes or out-of-bounds reads when the resulting string is later
treated as a C-string.

Details

The bug is in the control-character escaping path in source/common/common/
json_escape_string.h:67.

• The function pre-sizes result to the final length: std::string
  result(input.size() + required_size, '\');
• For control characters (0x00..0x1f), it emits a JSON escape sequence of
  length 6: \u00XX.
• It uses sprintf(&result[position + 1], "u%04x", ...), which writes 5 chars +
  a trailing NUL (\0) starting at result[position + 1].
• Then it does position += 6; and writes result[position] = '\'; to overwrite
  the NUL.
• If the control character occurs at the end of the output (e.g., the input
  ends with \x01), then after position += 6, position == result.size(), so
  result[position] is one past the end (off-by-one), violating std::string
  bounds/contract.

Concretely, the problematic lines are:

• source/common/common/json_escape_string.h:69 (sprintf(...))
• source/common/common/json_escape_string.h:72 (result[position] = '\';)

Potentially reachable from request-driven paths that escape untrusted data,
e.g. invalid header reporting:

• source/common/http/header_utility.cc:538 ~ source/common/http/
  header_utility.cc:546 (escapes invalid header key for error text)

Even when this doesn’t immediately crash, it can break the std::string
requirement that c_str()[size()] == '\0', which can later trigger UB (e.g., if
passed to strlen, printf("%s"), or any C API that expects NUL termination).

//clang++ -std=c++20 -O0 -g -fsanitize=address -fno-omit-frame-pointer
repro_json_escape_asan.cc -o repro_json_escape_asan
ASAN_OPTIONS=abort_on_error=1 ./repro_json_escape_asan
#include <cstdint>
#include <cstdio>
#include <cstring>
#include <string>
#include <string_view>

static uint64_t extraSpace(std::string_view input) {
  uint64_t result = 0;
  for (unsigned char c : input) {
    switch (c) {
    case '\"':
    case '\\':
    case '\b':
    case '\f':
    case '\n':
    case '\r':
    case '\t':
      result += 1;
      break;
    default:
      if (c == 0x00 || (c > 0x00 && c <= 0x1f)) {
        result += 5;
      }
      break;
    }
  }
  return result;
}

static std::string escapeString(std::string_view input, uint64_t
required_size) {
  std::string result(input.size() + required_size, '\\');
  uint64_t position = 0;

  for (unsigned char character : input) {
    switch (character) {
    case '\"':
      result[position + 1] = '\"';
      position += 2;
      break;
    case '\\':
      position += 2;
      break;
    case '\b':
      result[position + 1] = 'b';
      position += 2;
      break;
    case '\f':
      result[position + 1] = 'f';
      position += 2;
      break;
    case '\n':
      result[position + 1] = 'n';
      position += 2;
      break;
    case '\r':
      result[position + 1] = 'r';
      position += 2;
      break;
    case '\t':
      result[position + 1] = 't';
      position += 2;
      break;
    default:
      if (character == 0x00 || (character > 0x00 && character <= 0x1f)) {
        std::sprintf(&result[position + 1], "u%04x",
static_cast<int>(character));
        position += 6;
        // Off-by-one when this escape is the last output chunk:
        // position can become result.size(), so result[position] is out of
bounds.
        result[position] = '\\';
      } else {
        result[position++] = static_cast<char>(character);
      }
      break;
    }
  }

  return result;
}

int main() {
  std::string input(4096, 'A');
  input.push_back('\x01'); // ends with a control char -> triggers the buggy
path at the end

  const uint64_t required = extraSpace(input);
  std::string escaped = escapeString(input, required);

  std::printf("escaped.size=%zu\n", escaped.size());
  unsigned char terminator = static_cast<unsigned char>(escaped.c_str()
[escaped.size()]);
  std::printf("escaped.c_str()[escaped.size()] = 0x%02x\n", terminator);

  // If NUL termination is corrupted, this can read past the logical end.
  std::printf("strlen(escaped.c_str()) = %zu\n",
std::strlen(escaped.c_str()));
  return 0;
}```

CVE-2026-26309 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.

Kodem intelligence

1. What is CVE-2026-26309? CVE-2026-26309 is a medium-severity security vulnerability in github.com/envoyproxy/envoy (go), affecting versions = 1.37.0. No fixed version is listed yet.
2. How severe is CVE-2026-26309? CVE-2026-26309 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/envoyproxy/envoy are affected by CVE-2026-26309? github.com/envoyproxy/envoy (go) versions = 1.37.0 is affected.
4. Is there a fix for CVE-2026-26309? No fixed version is listed for CVE-2026-26309 yet. Monitor the advisory for updates and apply mitigations in the interim.
5. Is CVE-2026-26309 exploitable, and should I be worried? Whether CVE-2026-26309 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-26309 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.